A tropical cyclone is a generic term for a low-pressure system that formed over tropical waters (25°S to 25°N) with thunderstorm activity near the center of its closed, cyclonic winds. Tropical cyclones derive their energy from vertical temperature differences, are symmetrical, and have a warm core.

If it lacks a closed circulation it is called a tropical disturbance.  If it has a closed circulation but under 39 mph (34 knots, or 17 meters per second) maximum sustained surface winds, it is called a tropical depression. When winds exceed that threshold, it becomes a tropical storm and is given a name.  Once winds exceed 74 mph (64 knots, 33 meters per second) it will be designated a hurricane (in the Atlantic or East Pacific Oceans) or a typhoon (in the northern West Pacific).

Tropical Disturbances -> Tropical Depressions -> Tropical Storms -> Hurricane or Typhoon.

References:

Holland, G.J. (1993): “Ready Reckoner” – Chapter 9, Global Guide to Tropical Cyclone Forecasting, WMO/TC-No. 560, Report No. TCP-31, World Meteorological Organization; Geneva, Switzerland

Neumann, C.J. (1993): “Global Overview” – Chapter 1″ Global Guide to Tropical Cyclone Forecasting, WMO/TC-No. 560, Report No. TCP-31, World Meteorological Organization; Geneva, Switzerland

The “sub-tropical” in sub-tropical cyclone refers to the latitudes 25°N to 35°N (or °S). However, the term refers to cyclones whose characteristics are neither fully tropical nor extratropical. They are either asymmetrical with a warm core or symmetrical with a cold core. Sub-tropical cyclones can transform into tropical or extra-tropical storms depending on conditions.

The “extra-tropical” in extra-tropical cyclone refers to the latitudes 35°N to 65°N (or °S). However, the term refers to cyclones that get their energy from the horizontal temperature contrasts that exist in the atmosphere. Extra-tropical cyclones are low-pressure systems generally associated with cold fronts, warm fronts, and occluded fronts. They are asymmetrical and have a cold core.

A post-tropical cyclone is a former tropical cyclone that no longer possesses sufficient characteristics to be considered a tropical cyclone, such as convection at its center. Post-tropical cyclones can continue producing heavy rains and high winds. Former tropical cyclones that have become fully extra-tropical, sub-tropical, or remnant lows, are three classes of post-tropical cyclones.

Neutercane is a term no longer in use. It referred to small (<100 miles in diameter) sub-tropical low-pressure systems that are short-lived.

If you’re wondering, “what is UTC time?”, or “what is GMT time?”, or “What is Z time?”, the answer is they are time schemes. Universal Time Coordinated (UTC) used to be Greenwich Mean Time and Zulu Time (Z). This is the time at the Prime Meridian given in hours and minutes on a 24 hour clock. Most satellite pictures will give the time code next to the time taken with a UTC, GMT, or Z, but they are the same time zone. The conversion table for local times can be found below.

On most satellite pictures and radar images the time will be given. If it’s not in local time then it will usually be given as UTC, GMT, or Z time.

To convert this to your local time it is necessary to subtract the appropriate number of hours for the Western Hemisphere or add the correct number of hours for the Eastern Hemisphere. And don’t forget the extra hour adjustment for Daylight Savings Time or Winter Time over Standard Time for your zone.

Central Dense Overcast (CDO) – This is the cirrus cloud shield that results from the thunderstorms in the eyewall of a tropical cyclone and its rainbands. Before the tropical cyclone reaches hurricane strength (33 m/s, 64 kts, 74mph), typically the CDO is uniformly showing the cold cloud tops of the cirrus with no eye apparent. Once the storm reaches the hurricane strength threshold, usually an eye can be seen in either the infrared or visible channels of the satellites. Tropical cyclones that have nearly circular CDO’s are indicative of favorable, low vertical shear environments.

Tropical Upper Tropospheric Trough- A “TUTT” is a Tropical Upper Tropospheric Trough. A TUTT low is a TUTT that has completely cut-off. TUTT lows are more commonly known in the Western Hemisphere as an “upper cold low”. TUTTs are different than mid-latitude troughs in that they are maintained by subsidence warming near the tropopause which balances radiational cooling. TUTTs are important for tropical cyclone forecasting as they can force large amounts of vertical wind shear over tropical disturbances and tropical cyclones which may inhibit their strengthening. There are also suggestions that TUTTs can assist tropical cyclone genesis and intensification by providing additional forced ascent near the storm center and/or by allowing for an efficient outflow channel in the upper troposphere.

Prior to the 20^th century, hurricane names were inspired by everything from saints’ feast days, ship names, to unpopular politicians. In 1950,  the National Hurricane Center officially began designating Atlantic hurricanes with code names and then women’s names. In 1979, naming responsibility was passed to a committee of the World Meteorological Organization who used alternating men and women’s names following the practice adopted by Australia’s Bureau of Meteorology three years earlier in 1975.

Currently, there are six yearly lists used in rotation found here. If a particularly damaging storm occurs, the name of that storm is retired. Storms retired in 2017 include Harvey, Irma, Maria, and Nate. If there are more storms than names on the list in a given season, an auxiliary name list is used. Lastly, if a storm happens to move across basins, it keeps the original name. The only time it is renamed is in the case that it dissipates to a tropical disturbance and then reforms.

In the Atlantic basin, tropical cyclone names are “retired” (not to be used again for a new storm) if it is deemed to be quite noteworthy because of the damage and/or deaths it caused. This is to prevent confusion with a historically well-known cyclone with a current one in the Atlantic basin. Sometimes names are removed for other reasons, such as cultural considerations or politics.

History of Hurricane Naming

For much of history, tropical cyclones were only given designations post facto. After they had come ashore and done much destruction, they would be commemorated by being named either for the Saint’s feast day they happened on (such as the San Felipe hurricanes in 1876 & 1928) or by some characteristic (the Salty hurricane 1810, the Yankee hurricane 1935).

The first use of a proper name for a tropical cyclone was by Clement Wragge, an Australian forecaster late in the 19th century. He first designated tropical cyclones by the letters of the Greek alphabet, then started using South Sea Island girls’ names. When the newly constituted Australian national government failed to create a federal weather bureau and appoint him director, Wragge began naming cyclones “after political figures whom he disliked. By properly naming a hurricane, the weatherman could publicly describe a politician (who perhaps was not too generous with weather-bureau appropriations) as ‘causing great distress’ or ‘wandering aimlessly about the Pacific.’ (Dunn and Miller 1960)

Although Wragge’s naming practice lapsed when his Queensland weather bureau closed in 1903, forty years later the idea inspired author George R. Stewart. In his 1941 novel “Storm”, a junior meteorologist named Pacific extratropical storms after former girlfriends. The novel was widely read, especially by US Army Air Forces and Navy meteorologists during World War II. When Reid Bryson, E.B. Buxton, and Bill Plumley were assigned to a USAAF base on Saipan in 1944 they had to forecast any tropical cyclones affecting operations. They decided (à la Stewart) to name them after their wives. In 1945, the armed services publicly adopted a list of women’s names for typhoons of the western Pacific using the names of officers’ wives assigned to forward forecast centers on Guam and the Philippines. However, the Air Forces were unable to persuade the U.S. Weather Bureau (USWB) to adopt a similar practice for Atlantic hurricanes.

Starting in 1947, the Air Force Hurricane Office in Miami began designating tropical cyclones of the North Atlantic Ocean using the Army/Navy phonetic alphabet (Able-Baker-Charlie-etc.) in internal communications. During the busy 1950 hurricane season there were three hurricanes occurring simultaneously in the Atlantic basin, causing considerable confusion. Grady Norton of the USWB’s Miami Hurricane Warning Center then decided to use the Air Force’s naming system in public bulletins and in his year-end summary. By the next year, these names began appearing in newspaper articles.

This practice proved popular. However, in 1952 a new International phonetic alphabet was adopted (Alpha-Beta-Charlie-etc.) which caused some confusion about which names were to be used. So in 1953, the US Weather Bureau finally acceded to the Armed Services’ practice of using women’s names. This was both controversial and popular. In 1978, under political pressure, the US National Hurricane Center (NHC) requested that the WMO’s Region IV Hurricane Committee (which had just taken control of the list) switch to a hurricane name list that alternated men’s and women’s names following the practice adopted by Australia’s Bureau of Meteorology in 1975. This was first implemented in the eastern Pacific then in 1979 in the Atlantic.

A rare hurricane near Hawaii in 1950 was called Hiki (Hawai’ian for Able). In 1957, three storms were detected in the Central Pacific, and the military forecast centers called them Kanoa, Della and Nina.  In 1959, another hurricane threatened the islands and the Weather Bureau designated it “Dot”. The next year an official name list for tropical cyclones was drawn up for the Northeast Pacific basin. In 1978, both men’s and women’s names were utilized, and in 1979 a separate list was created for the Central Pacific (from 140°W to 180°W) using Hawaiian names.

The Northwest Pacific basin tropical cyclones were given women’s names officially starting in 1945 and men’s names were also included beginning in 1979. As of 1 January 2000, tropical cyclones in the Northwest Pacific basin are now being named from a new and very different list. The new names contributed by all the nations and territories that are members of the WMO’s Typhoon Committee. These newly selected names have two major differences from the rest of the world’s tropical cyclone name rosters.

1. The names by and large are not personal names. There are a few men’s and women’s names, but the majority are names of flowers, animals, birds, trees, or even foods, etc, while some are descriptive adjectives.
2. The names will not be allotted in alphabetical order, but are arranged by contributing nation with the countries being alphabetized.

The Philippine weather service PAGASA maintains their own separate list of names for any tropical system that threatens their archipelago.

For many years the Indian Ocean cyclones were given alphanumeric designators.  The Southwest Indian Ocean tropical cyclones were first named during the 1960/1961 season.  The North Indian Ocean region tropical cyclones were named as of 2006.

The Australian and South Pacific region (east of 90E, south of the equator) started giving women’s names to the storms for the 1964/1965 season and both men’s and women’s names for the 1974/1975 season.  For the 2008/2009 season the three separate name lists of the different BoM forecast centers were consolidated into one list.

A rare South Atlantic storm in 2004 was post facto given the name Catarina. Another such system in 2010 was designated Anita after the fact. Starting in 2011, a name list was begun for the South Atlantic basin using mostly Brazilian designations.

Reference:

Dunn, G.E. and B.I. Miller (1960): Atlantic Hurricanes, Louisiana State Univ. Press, Baton Rouge, Louisiana, 377pp

Skilton, Liz, (2019): Tempest, Louisiana State Univ. Press, Baton Rouge, Louisiana, 306pp

Well, we all found out the answer in 2005 and 2020. In those years,  when they ran through the name list they then use the Greek alphabet : Alpha, Beta, Gamma, Delta, Epsilon,… etc. .  In 2020, they made it to a Iota on the list.  Since several Greek-letter storms that year were damaging enough to have their names retired, it was decided to scrap this scheme and instead come up with an auxiliary name list each year. The same was done for the East Pacific name lists.

In the Central and West Pacific they have a perpetual lists of names, so when one list is through they simply start on the next.

Since 1978, the United Nations’ World Meteorological Organization, a group representing some 120 different countries, has used pre-determined lists of names for tropical storms for each ocean basin of the world. The Atlantic basin, which falls under Regional Association IV, has a six year supply of names with 21 names for each year. Why 21 names? Well, the letters Q, U, X, Y and Z are not used because names beginning with those letters are in short supply (you would need at least 3 male and 3 female names for each letter, plus a backup supply for those retired). Think about it; how many men and women do you know whose names begin with these letters?

When a damage or casualty producing storm like Mitch, Andrew, or Katrina strikes, the country most affected by the storm may recommend to the World Meteorological Organization’s Regional Association that the name be “retired.” Retiring a name is an act of respect for its victims, and reduces confusion in the insurance, legal or scientific literature. A retired name is replaced with a like-gender name beginning with the same letter. For example, Honduras recommended (1998) the name Mitch be retired and proposed the replacement name, Matthew, for consideration (and vote) by the 25-member countries of the Regional Association-IV. Eighty-three names have been retired in the Atlantic basin.

The names used on the list must meet some fundamental criteria. They should be short, and readily understood when broadcast. Further the names must be culturally sensitive and not convey some unintended and potentially inflammatory meaning. The potential for misunderstanding increases when you figure that in the Atlantic basin there are twenty-four countries, reflecting an international mix of English, Spanish and French cultures.

Typically, over the historical record, about one storm each year causes so much death and destruction that its name is considered for retirement. This means that in a “normal” year, the odds are about 1 in 8 of requiring a replacement name, given that over the last 57 years (of reliable record) we’ve averaged slightly over 8 tropical storms and hurricanes per season (actually 8.6). So, it’s more likely that letters/ names toward the front of the alphabet (letters A through H) might be retired.

The Region IV Naming Committee has a rather large file folder of nominated names that have already been submitted. The next time the need arises and it’s a storm affecting mainly the United States, the Committee will be casting about for a replacement tropical cyclone name. They will take out this file to make a selection. But as we say, it’s pure chance from there.

The Automated Tropical Cyclone Forecast (ATCF) system was developed for the Joint Typhoon Warning Center in 1988. It is used by computer software to identify tropical cyclones  and assist in the generation of forecast messages. In order to distinguish different tropical cyclones that might be occurring simultaneously, a distinct alphanumeric code is assigned to each cyclone once it develops  a closed circulation. This code system was adopted by other warning centers in order to facilitate the passing of storm information and reduce confusion.

The code designation consists of two letters designating the oceanic basin (“AL” for Atlantic, “EP” for Eastern Pacific, “CP” for Central Pacific and “WP” for Western Pacific), a two-digit number designating the sequential number of that particular cyclone for that basin in the year, and lastly a four-digit year number. So, the first depression to form in the Atlantic for 2001 would be AL012001, the third depression for the Central Pacific in 1999 would be CP031999.

A cyclone retains its ATCF code designation as long as it remains a distinct tropical vortex. Even if it becomes a named tropical storm or hurricane the software will still track it by its ATCF code.

AL90, AL92, 92L from the Tropical Discussions

Oftentimes, hurricane specialists become curious about disturbances in the tropics long before they form into tropical depressions and are given a tropical cyclone number. In order to alert forecasting centers that they are investigating such a disturbance and that they wish to have it tracked by the various forecast models, the specialist will attach a 9-series number to it. The first such disturbance of the year will be designated 90, the next 91, and so on until 99. After that, they restart the sequence with 90 again. The purpose of these numbers is to clarify which disturbance they are tracking as there are often more than one happening at the same time.

To further clarify matters, each number is accompanied by a two-letter code designating which tropical cyclone basin the disturbance is in. “AL” is used for the Atlantic basin (including the Caribbean Sea and Gulf of Mexico), “EP” for the Eastern Pacific, “CP” for Central Pacific, and “WP” for the Western Pacific.

In discussions, these designations will be shortened to 90L, 91L, and so forth. They may also be referred to as ‘Invest 90L’. However, once a disturbance is designated a tropical depression this 9-series number will be dropped and an ATCF code number will be assigned in its place.

You may also occasionally see an 8-series number, such as AL82. This means that this is a test investigation. There is no particular disturbance that the specialists are interested in, they’re just running a test of the system to make sure communications and software are running properly.

In the Atlantic basin, tropical cyclone names are “retired” (not to be used again for a new storm) if it is deemed to be quite noteworthy because of the damage and/or deaths it caused. This is to prevent confusion with a historically well-known cyclone and a current one in the Atlantic basin. Sometimes names are removed for other reasons, such as cultural considerations or politics. The following list gives the names that have been retired and the year of the storm in question.

Name retired because of previous storm in 1954 with the same name.

Although rarer, some East Pacific names have been retired from the list. The climatology of this basin has most hurricanes moving away from the shore, so chances are rare that these storms would adversely affect people necessitating the name be retired.

Name retired because of political or social considerations

A few Central Pacific names have been retired from their list. Most of them were removed for inflicting damage or adversely affecting the Hawaiian Islands. However, some have moved into the western Pacific to cause destructions, prompting their retirement.

Names retired before the 2000 season come from the name lists used by the Joint Typhoon Warning Center. Since 2000, the names removed come from the name lists used by the Japan Meteorological Agency. Most of the retired names inflicted significant damage to the nations affected.

Bess 1974 was retired after the season and replaced with Bonnie. In 1979, new name lists featuring both sexes were introduced and Bess was added back. In 1982, Bess was again retired and replaced with Brenda.

Throughout history there have been many alternative spellings in different languages: foracan, foracane, furacana, furacane, furicane, furicano, haracana, harauncana, haraucane, haroucana, harrycain, hauracane, haurachana, herican, hericane, hericano, herocane, herricao, herycano, heuricane, hiracano, hirecano, hurac[s]n, huracano, hurican, hurleblast, hurlecan, hurlecano, hurlicano, hurrican, hurricano, hyrracano, urycan, hyrricano, jimmycane, oraucan, uracan, uracano

The ocean’s primary direct response to a hurricane is a cooling of the sea surface temperature (SST). How does this occur? When the strong winds of a hurricane move over the ocean they churn-up much cooler water from below. The net result is that the SST of the ocean after storm passage can be lowered by several degrees Celsius (up to 10° Fahrenheit).

A warmer ocean can have intensifying effects because the warmer an ocean is, the easier it is for the liquid water to become vapor and fuel the storm’s clouds.

Figure 1 shows SSTs ranging between 25-27°C (77-81°F) several days after the passage of Hurricane Georges in 1998. As Figure 1 illustrates, Georges’ post-storm ‘cold wake’ along and to the right of the superimposed track is 3-5°C (6-9°F) cooler than the undisturbed SST to the west and south (i.e. red/orange regions are ~30°’C [86°’F]). The magnitude and distribution of the cooling pattern shown in this illustration is fairly typical for a post-storm SST analysis.

One important caveat to realize however is that most of the 3-5°C (6-9°F) ocean cooling shown in Figure 1 occurs well after the storm has moved away from the region (in this case several days after Georges made landfall). The amount of ocean cooling that occurs directly beneath the hurricane within the high wind region of the storm is a much more important question scientists would like to have answered. Why? Hurricanes get their energy from the warm ocean water beneath them. However, in order to get a more accurate estimate of just how much energy is being transferred from the sea to the storm, scientists need to know ocean temperature conditions directly beneath the hurricane. Unfortunately, with 150kph+ (100mph+) winds, 20m+ (60ft+) seas and heavy cloud cover being the norm in this region of the storm, direct (or even indirect) measurement of SST conditions within the storm’s “inner core” environment are very rare.

Thankfully in this case “very rare” does not mean “once in a lifetime”. Recently, scientists in AOML’s Hurricane Research Division (HRD) were able to get a better idea of how much SST cooling occurs directly under a hurricane by looking at many storms over a 28 year period. By combining these rare events, HRD scientists put together a “composite average” of ocean cooling directly under the storm.

Figure 2 illustrates that, on average, cooling patterns are a lot less than the post storm 3-5°C (6-9°F) cold wake estimates shown in Figure 1. In most cases, the ocean temperature under a hurricane will range somewhere between 0.2 and 1.2°C (0.4 and 2.2°F) cooler that the surrounding ocean environment. Exactly how much depends on many factors including ocean structure beneath the storm (i.e. location), storm speed, time of year and to a lesser extent, storm intensity (Cione and Uhlhorn 2003).

While the estimates in Figure 2 represent a dramatic improvement when it comes to more accurately representing actual SST cooling patterns experienced under a hurricane, even small errors in inner core SST can result in significant miscalculations when it comes to accurately assessing how much energy is transferred from the warm ocean environment directly to the hurricane. With all other factors being equal, being “off” by a mere 0.5°C (1°F) can be the difference between a storm that rapidly intensifies and one that falls apart! With that much at stake, scientists at HRD and other government and academic institutions are working to improve our ability to accurately estimate, observe and predict “under-the-storm” upper ocean conditions. These efforts include statistical studies, modeling efforts and enhanced observational capabilities designed to help scientists better assess upper ocean thermal conditions under the storm. It is believed that future forecasts of tropical cyclone intensity change will be significantly improved.

Reference:
Cione, J. J., and E. W. Uhlhorn, 2003: Sea Surface Temperature Variability in Hurricanes: Implications with Respect to Intensity Change. Monthly Weather Review, 131, 1783-1796.

The Eye is a roughly circular area of fair weather found at the center of a severe tropical storm. The eye is the region of the lowest pressure at the surface and the warmest temperatures at the top. Eye size ranges from 5-120 miles across, but most are 20-40 miles in diameter. Understanding exactly how the eye forms has been controversial. Some scientists believe the radial spreading of the wind creates a warm dry down flow from the upper atmosphere, and this forms the cloud-free eye. Others have think the latent heat release in the eyewall forces the subsidence in the storm center creating the eye.

The Eyewall is a ring of deep convection bordering the eye of the storm. This area has the highest surface winds in the tropical cyclone. Because air in the eye is slowly sinking, it creates an updraft in the eyewall. In particularly strong storms, concentric eyewall circles (or an “eyewall replacement cycle”) can occur. Eyewall replacement happens when a storm reaches its intensity threshold and the eye contracts to a smaller size (5-15 miles). Strong rain bands in the outer storm move inward towards the eye, robbing the inner eyewall of its moisture and momentum and weakening the storm.

Spiral Bands are long, narrow bands of rain and thunderstorms that are oriented in the same direction as the wind movement. They are caused by convection (the vertical movement of air masses) and they spiral into the center of the tropical cyclone. In contrast, the Moat of a storm usually refers to the region between the eyewall and an outer spiral band where rainfall is relatively lighter. Not all hurricanes have moats.

References: Hawkins, H.F., and D.T. Rubsam (1968): “Hurricane Hilda, 1964 : II Structure and budgets of the hurricane on October 1, 1964” Mon. Wea. Rev., 104, pp.418-442

Weatherford, C. and W.M. Gray (1988): “Typhoon structure as revealed by aircraft reconnaissance. Part II: Structural variability” Mon. Wea. Rev., 116, pp.1044-1056

Smith, R.K. (1980): “Tropical Cyclone Eye Dynamics.” J. Atmos. Sci., 37 (6), pp.1227-1232.

Willoughby, H.E. (1979): “Forced secondary circulations in hurricanes” J. Geophys. Res., 84, pp.3173-3183

Shapiro, L.J. and H.E. Willoughby (1982): “The Response of Balanced Hurricanes to Local Sources of Heat and Momentum” J. Atmos. Sci., 39 (2), pp.378-394

Willoughby, H.E. (1990a): “Temporal changes of the primary circulation in tropical cyclones” J. Atmos. Sci., 47, pp.242-264

Willoughby, H.E. (1995): “Mature structure and evolution. Global Perspectives on Tropical Cyclones, R.L. Elsberry (ed.). World Meteorological Organization, Report No. TCP-38; Geneva, Switzerland, 62 pp.

Tropical cyclones tend to be symmetrical. This means the winds should be the same in all quadrants at a given distance from the center.  However, most hurricanes are moving, and the storm’s motion will be added to or subtracted from those winds creating an asymmetric structure.  The side where the motion is added to the winds is called the “dirty side” as the weather is rougher and more dangerous there.

The “right side” is in reference to the storm’s direction of movement in the Northern Hemisphere. If a hurricane is moving to the west, the right side would be to the north of the storm, if it is heading north, then the right side would be to the east of the storm. In the Southern Hemisphere, this is reversed since a tropical cyclone’s winds spiral around its center clockwise there as opposed to counterclockwise in the Northern Hemisphere.  So south of the Equator the “dirty side” is the “left side” of the cyclone.

For example, a hurricane with 90mph winds moving at 10mph would have a 100mph wind speed on the forward-moving side and 80 mph on the side with the backward motion. Weather forecast advisories already take this asymmetry into account and, in this case, would state that the highest winds were 100 mph [160 km/hr].

For a tropical cyclone to form, it needs an energy source, typically heat released by convection. Convection describes the process by which warm air rises and expands in the atmosphere, releasing heat and energy that fuels the storm. When the convection is “disorganized,” the heat is scattered and released sporadically throughout the storm, making it difficult for a cyclone to generate the energy necessary to form. In contrast, when convection is “organized,” the heat and energy are focused, persistent, and steady – concentrating the energy and increasing the likelihood of tropical cyclone formation.

The energy released from a hurricane can be explained in two ways: the total amount of energy released by the condensation of water droplets (latent heat), or the amount of kinetic energy generated to maintain the strong, swirling winds of a hurricane. The vast majority of the latent heat released is used to drive the convection of a storm, but the total energy released from condensation is 200 times the world-wide electrical generating capacity, or 6.0 x 10¹⁴ watts per day.

If you measure the total kinetic energy instead, it comes out to about 1.5 x 10¹² watts per day, or ½ of the world-wide electrical generating capacity. It would seem that although wind energy seems to be the most obvious energetic process, it is actually the latent release of heat that feeds a hurricane’s momentum.

To Calculate:

Method 1 – Total energy released through cloud/rain formation: An average hurricane produces 1.5 cm/day (0.6 inches/day) of rain inside a circle of radius 665 km (360 n.mi) (Gray 1981). (More rain falls in the inner portion of hurricane around the eyewall, less in the outer rainbands.) Converting this to a volume of rain gives 2.1 x 10¹⁶ cm3/day. A cubic cm of rain weighs 1 gm. Using the latent heat of condensation, this amount of rain produced gives 5.2 x 10¹⁹ Joules/day or 6.0 x 10¹⁴ Watts.

Method 2 – Total kinetic energy (wind energy) generated: For a mature hurricane, the amount of kinetic energy generated is equal to that being dissipated due to friction. The dissipation rate per unit area is air density times the drag coefficient times the wind speed cubed (See Emanuel 1999 for details). One could either integrate a typical wind profile over a range of radii from the hurricane’s center to the outer radius encompassing the storm, or assume an average wind speed for the inner core of the hurricane. Doing the latter and using 40 m/s (90 mph) winds on a scale of radius 60 km (40 n.mi.), gets a wind dissipation rate (wind generation rate) of 1.3 x 10¹⁷ Joules/day (1.5 x 10¹²Watts).

Reference: Emanuel, K. A., (1999): “The power of a hurricane: An example of reckless driving on the information superhighway” Weather, 54, 107-108

There are no other planets known to have warm water oceans from which true water cloud hurricanes can form. However, many astronomers and planetary meteorologists believe gas giant planets such as Jupiter and Saturn exhibit similar storms. The principal candidate is the famous Great Red Spot (GRS) on Jupiter, and the numerous whorls that surround it, where ammonia takes the place of water. The GRS exhibits an anticyclonic circulation at its top, just as tropical cyclones do at the top of the troposphere. On Saturn, a polar storm has been spotted by the Cassini spacecraft measuring up to 1,250 miles in diameter, about 20 time larger than an Earthly hurricane with winds four times stronger. On Mars, a large, cyclonic cloud feature forms every year in the northern hemisphere. It forms in the morning and dissipates by the afternoon. This cloud is likely composed of water/ice and is white in appearance. It doesn’t appear to rotate but is about 1000 miles wide with an inner hole or ‘eye’ about 200 miles across.

Over 3,400 extrasolar planets have been found to date, but no others are confirmed to have convectively driven storms. However, there is reason to believe such storms exist on extrasolar planets as well.

The Atlantic hurricane season is June 1st to November 30th. In the East Pacific, it runs from May 15th to November 30th. Hurricane Awareness week runs from May 25th through May 31st and is a great time to get your hurricane kit and plans up to date. NOAA’s seasonal outlook is published here: NOAA Seasonal Outlook

Hurricanes have occurred outside of the official six month season , but these dates were selected to encompass the majority of Atlantic tropical cyclone activity (over 97%). When the Weather Bureau organized its new hurricane warning network in 1935 it scheduled a special telegraph line to connect the various centers to run from June 15th through November 15th. Those remained the start and end dates of the ‘official’ season until 1964, when it was decided to end the season on November 30th, and in 1965, when the start was moved to the beginning of June.  These changes made the Atlantic hurricane season six months long and easier for people to remember.

Atlantic Ocean tropical cyclone activity over a year

The Atlantic basin shows a very peaked season from August through October, with 78% of the tropical storm days, 87% of the minor hurricane days, and 96% of the major hurricane days occurring then (Landsea (NHC) 1993). Maximum activity occurs in early to mid September.  “Out of season” tropical cyclones primarily occur in May or December.

East Pacific Ocean tropical cyclone activity over a year

The Northeast Pacific basin has a broader peak with activity beginning in late May or early June and going until late October or early November with a peak in storminess in late August/early September. The National Hurricane Center’s official dates for this basin are from May 15th to November 30th.

The Northwest Pacific basin has tropical cyclones occurring all year round regularly. There is no official definition of typhoon season for this reason. There is a distinct minimum in February and the first half of March, and the main season goes from July to November with a peak in late August/early September.

The North Indian basin has a double peak of activity in May and November though tropical cyclones are seen from April to December. The severe cyclonic storms (>33 m/s winds [76 mph]) occur almost exclusively from April to June and late September to early December.

The Southwest Indian and Australian/Southeast Indian basins have very similar annual cycles with tropical cyclones beginning in late October/early November, reaching a peak in activity from mid-January to early May. The Australian/Southeast Indian basin February lull in activity is a bit more pronounced than the Southwest Indian basin’s lull.

The Australian/Southwest Pacific basin begin with tropical cyclone activity in late October/early November, reaches a single peak in  March, and then fades out in early May.

Globally, September is the most active month and May is the least active month. (Neumann 1993)

References: Neumann, C.J., B.R. Jarvinen, C.J. McAdie, and J.D. Elms (1993): Tropical Cyclones of the North Atlantic Ocean, 1871-1992, Prepared by the National Climatic Data Center, Asheville, NC, in cooperation with the NHC, Coral Gables, FL, 193pp.

The best time to prepare is before hurricane season begins. Make a plan for you and your family about what to do if a hurricane threatens. Put together a hurricane kit. Ensure your house is up to code, and check for problems, such as overhanging branches or missing roof tiles. Check your shutters and other window and door coverings. Once the season begins, stay informed. Check the outlook every day, and if anything is threatening keep updated on the latest advisories.

For hurricane preparation tips, check out FEMA’s comprehensive downloadable guidebook and visit www.ready.gov/hurricanes for the best information available on hurricane preparedness.

Don’t forget to sign up for wireless emergency alerts. Alternatively, you can get updates from NOAA Radio or Radio Fax (for mariners).

The mean annual damage from hurricanes in the US is 9.5 billion dollars, when we adjust not only for inflation but for the increase in value of real goods in average households. Hurricane damage varies greatly from year to year, depending on the number and strength of hurricanes making landfall, but there does not seem to be a long-term trend in adjusted damage over the last century.

There is very little association between the physical size of a hurricane and its intensity. A big hurricane does not have to be an intense one and vice versa. The damage a hurricane can cause is a function of both its maximum sustained wind and the extent of the hurricane force winds. A broad, weak storm may cause as much damage as a small, strong one.

It is false to think that damage is linear with wind speed, that a 150-mph winds will cause twice the damage as a 75-mph winds. The relationship is exponential, and not linear. A category 5 storm could cause up to 250 times the damage of a category 1 hurricane of the same size.

Mean annual damage in mainland US is $4,900,000,000.

The worst U.S. hurricane damage – after normalizing to today’s population, wealth and dollars – is no longer Hurricane Andrew, but is instead the 1926 Great Miami Hurricane. If this storm hit in the mid-1990s, it is estimated that it would cause over $70 billion in South Florida and then an additional $10 billion in the Florida panhandle and Alabama.

The United States has at least a 1 in 6 chance of experiencing losses related to hurricanes of at least $10 billion on average.

Even though the major hurricanes (the category 3, 4 and 5 storms) comprise only 21% of all US landfalling tropical cyclones, they account for 83% of all of the damage.

Damages have not been on the increase once one normalizes for inflation, wealth, and coastal population changes. Instead one sees that hurricane damages that were fairly low during the first two decades of the 20th Century, are quite high in the 1920s and 1940s to 1960s, and substantially lower in the 1970s and 1980s. Only during the early 1990s does damage approach the high level of impacts seen back in the 1940s through the 1960s. Thus recent hurricane damages are not unprecedented.

References: Weatherford, C. and W.M. Gray (1988): “Typhoon structure as revealed by aircraft reconnaissance. Part II: Structural variability” Mon. Wea. Rev., 116, pp.1044-1056

Pielke, Jr. R. A., and C. W. Landsea, 1998: “Normalized Atlantic hurricane damage 1925-1995” Wea. Forecasting, 13, pp.621-631

Just as every person is an individual, every hurricane is different. So every experience with such a storm will be unique. The summary below is of a general sequence of events one might expect from a Category 2 hurricane approaching a coastal area. What you might experience could be vastly different.

• 96 hours before landfall
  At first there aren’t any apparent signs of a storm. The barometer is steady, winds are light and variable, and fair weather cumulus clouds dot the sky. But the perceptive observer will note a swell on the ocean surface of about a meter (3 feet) in height with a wave coming ashore every ten seconds. These waves race out far ahead of a storm at sea, but could easily be masked by locally wind driven waves.
• 72 hours before landfall
  Little has changed, except that the swell has increased to about 2 meters (6 feet) in height and the waves now come in every nine seconds. This means that the storm, still far over the horizon, is approaching.
• 48 hours before landfall
  If anything, conditions have improved. The sky is now clear of clouds, the barometer is steady, and the wind is almost calm. The swell is now about 3 m (9 feet) and coming in every 8 seconds. A hurricane watch is issued, and areas with long evacuation times are given the order to begin.
• 36 hours before landfall
  The first signs of the storm appear. The barometer is falling slightly, the wind is around 5 m/s (10 kts, 11 mph), and the ocean swell is about 4m (13 feet) in height and coming in 7 seconds apart. On the horizon a large mass of white cirrus clouds appear. As the veil of clouds approaches it covers more of the horizon. A hurricane warning is issued and low lying areas and people living in mobile homes are ordered to evacuate.
• 30 hours before landfall
  The sky is now covered by a high overcast. The barometer is falling at .1 millibar per hour (.003 inches of Hg/hr), and the winds pick up to about 10 m/s (20 kts, 23 mph). The ocean swell, coming in only 5 seconds apart, is beginning to be obscured by wind driven waves, and small whitecaps begin to appear on the ocean surface.
• 24 hours before landfall
  In addition to the overcast, small low clouds streak by overhead. The barometer is falling by .2 mb/hr (.006″Hg/hr), the wind picks up to 15 m/s (30 kts, 34 mph). The wind driven waves are covered in whitecaps and streaks of foam begin to ride over the surface. Evacuations should be completed and final preparations made by this time.
• 18 hours before landfall
  The low clouds are thicker and bring driving rain squalls with gusty winds. The barometer is steadily falling at half a millibar per hour (.015 “Hg/hr), and the winds are whistling by at 20 m/s (40 kts, 46 mph). It is hard to stand against the wind.
• 12 hours before landfall
  The rain squalls are more frequent and the winds don’t diminish after they depart. The cloud ceiling is getting lower, and the barometer is falling at 1 mb/hr (.029 “Hg/hr). The wind is howling at hurricane force at 32 m/s (64 kts, 74 mph), and small, loose objects are flying through the air and branches are stripped from trees. The sea advances with every storm wave that crashes ashore and the surface is covered with white streaks and foam patches.
• 6 hours before landfall
  The rain is constant now and the 40 m/s wind (80 kts, 92 mph) drives it horizontally. The barometer is falling 1.5 mb/hr (.044 “Hg/hr), and the storm surge has advanced above the high tide mark. It is impossible to stand upright outside without bracing yourself, and heavy objects like coconuts and plywood sheets become airborne missiles. The wave tops are cut off and make the sea surface a whitish mass of spray.
• 1 hour before landfall
  It didn’t seem possible, but the rain has become heavier, a torrential downpour. Low areas inland become flooded from the rain. The winds are roaring at 45 m/s (90 kts, 104 mph), and the barometer is free-falling at 2 mb/hr (.058 “Hg/hr). The sea is white with foam and streaks. The storm surge has covered coastal roads and 5 meter (16 foot) waves crash into buildings near the shore.
• The eye
  Just as the storm reaches its peak, the winds begin to slacken, and the sky starts to brighten. The rain ends abruptly and the clouds break and blue sky is seen. However the barometer continues falling at 3 mb/hr (.09 “Hg/hr) and the storm surge reaches the furthest inland. Wild waves crash into anything in the grasp of the surge. Soon the winds fall to near calm, but the air is uncomfortably warm and humid. Looking up you can see huge walls of cloud on every side, brilliant white in the sunlight.
  At this point, the barometer stops falling and in a moment begins to rise, soon as fast as it fell. The winds begin to pick up slightly and the clouds on the far side of the eyewall loom overhead.
• 1 hour after landfall
  The sky darkens and the winds and rain return just a heavy as they were before the eye. The storm surge begins a slow retreat, but the monstrous waves continue to crash ashore. The barometer is now rising at 2 mb/hr (.058 “Hg/hr). The winds top out at 45 m/s (90 kts, 104 mph), and heavy items torn loose by the front side of the storm are thrown about and into sides of buildings that had been in the lee before the eye passed.
• 6 hours after landfall
  The flooding rains continue, but the winds have diminished to a ‘mere’ 40 m/s (80 kts, 92 mph). The storm surge is retreating and pulling debris out to sea or stranding sea borne objects well inland. It is still impossible to go outside.
• 12 hours after landfall
  The rain now comes in squalls and the winds begin to diminish after each squall passes. The cloud ceiling is rising, as is the barometer at 1 mb/hr (.029 “Hg/hr). The wind is still howling at near hurricane force at 30 m/s (60 kts, 69 mph), and the ocean is covered with streaks and foam patches. The sea level returns to the high tide mark.
• 24 hours after landfall
  The low clouds break into smaller fragments and the high overcast is seen again. The barometer is rising by .2 mb/hr (.006″Hg/hr), the wind falls to 15 m/s (30 kts, 34 mph). The surge has fully retreated from land, but the ocean surface is still covered by small whitecaps and large waves.
• 36 hours after landfall
  The overcast has broken and the large mass of white cirrus clouds disappears over the horizon. The sky is clear and the sun seems brilliant. The barometer is rising slightly, the wind are a steady 5 m/s (10 kts, 11 mph). All around are torn trees and battered buildings. The air stinks of dead vegetation and muck that was dredged by the storm from the bottom of the sea to cover the shore. The all clear is given.

Last updated August 13, 2004

Hurricane forecasters estimate tropical cyclone strength from satellite using a method called the Dvorak technique. Vern Dvorak developed the scheme in the early 1970s using a pattern recognition decision tree (Dvorak 1975, 1984). Utilizing the current satellite picture of a tropical cyclone, one matches the image versus a number of possible pattern types: Curved band Pattern, Shear Pattern, Eye Pattern, Central Dense Overcast (CDO) Pattern, Embedded Center Pattern or Central Cold Cover Pattern. If infrared satellite imagery is available for Eye Patterns (generally the pattern seen for hurricanes, severe tropical cyclones and typhoons), then the scheme utilizes the difference between the temperature of the warm eye and the surrounding cold cloud tops. The larger the difference, the more intense the tropical cyclone is estimated to be.

From this one gets a “T-number” and a “Current Intensity (CI) Number”. CI numbers have been calibrated against aircraft measurements of tropical cyclones in the Northwest Pacific and Atlantic basins. On average, the CI numbers correspond to the following intensities:

Note that this estimation of both maximum winds and central pressure assumes that the winds and pressures are always consistent. However, since the winds are really determined by the pressure gradient, small tropical cyclones (like the Atlantic’s Andrew in 1992, for example) can have stronger winds for a given central pressure than a larger tropical cyclone with the same central pressure. Thus caution is urged in not blindly forcing tropical cyclones to “fit” the above pressure- wind relationships. (The reason that lower pressures are given to the Northwest Pacific tropical cyclones in comparison to the higher pressures of the Atlantic basin tropical cyclones is because of the difference in the background climatology. The Northwest Pacific basin has a lower background sea level pressure field. Thus to sustain a given pressure gradient and thus the winds, the central pressure must accordingly be smaller in this basin.)

The errors for using the above Dvorak technique in comparison to aircraft measurements taken in the Northwest Pacific average 10 mb with a standard deviation of 9 mb (Martin and Gray 1993). Atlantic tropical cyclone estimates likely have similar errors. Thus an Atlantic hurricane that is given a CI number of 4.5 (winds of 77 kt and pressure of 979 mb) could in reality be anywhere from winds of 60 to 90 kt and pressures of 989 to 969 mb. These would be typical ranges to be expected; errors could be worse. However, in the absence of other observations, the Dvorak technique does at least provide a consistent estimate of what the true intensity is.

While the Dvorak technique was calibrated for the Atlantic and Northwest Pacific basin because of the aircraft reconnaissance data ground truth, the technique has also been quite useful in other basins that have limited observational platforms. However, at some point it would be preferable to re-derive the Dvorak technique to calibrate tropical cyclones with available data in the other basins.

Lastly, while the Dvorak technique is primarily designed to provide estimates of the current intensity of the storm, a 24 h forecast of the intensity can be obtained also by extrapolating the trend of the CI number. Whether this methodology provides skillful forecasts is unknown.

References: Dvorak, V.F., 1975: “Tropical cyclone intensity analysis and forecasting from satellite imagery” Mon. Wea. Rev., 103, pp.420-430

Dvorak, V.F., 1984: “Tropical cyclone intensity analysis using satellite data” NOAA Tech. Rep. NESDIS 11, 47pp

Fitzpatrick, P.J., J.A. Knaff, C.W. Landsea, and S.V. Finley (1995): “A systematic bias in the Aviation model’s forecast of the Atlantic tropical upper tropospheric trough: Implications for tropical cyclone forecasting” Wea. Forecasting, 10, pp.433-446

Martin, J.D., and W.M. Gray (1993): “Tropical cyclone observation and forecasting with and without aircraft reconnaissance” Wea. Forecasting, 8, pp.519-532

Observations & Measurements

There are several methods used by NOAA, the United States Geological Survey (USGS), and the Federal Emergency Management Agency (FEMA) to measure storm surge. Each method has advantages and draw backs. Post-storm analysis of storm surge requires resolving differences in what each measures in order to find the best approximation of the surge heights.

Tide Stations (NOAA)

Check your coastal marine forecast at NOAA’s Tides & Currents Website.

A network of 175 long-term, continuously operating water level stations located throughout the U.S. serving as the foundation for NOAA’s tide prediction products.

Measures still water (e.g. no waves)

Traditionally the most reliable method

Limited, fixed stations

High Water Marks (USGS / FEMA)
These are the lines left on trees and structures marking the highest (peak) elevation of the water surface from a flood event. They are created by foam, seeds, and other debris. Survey crews deploy after a storm, locate, and record reliable high-water marks. GPS methods are used to determine the location of these marks, which are then mapped relative to a vertical reference datum.

Perishable

Traditionally best method for capturing highest surge level

Subjective and often includes impact of waves

Pressure Sensors (USGS)
These are temporary water-level and barometric-pressure sensors which provide information about storm surge duration, times of surge arrival and retreat, and maximum depths.

Relatively new method

Mobile, deployed in advance of storms at expected location of highest surge

Can contain impact of waves

Forecasting

The Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model is the computer model utilized by the National Oceanic and Atmospheric Administration (NOAA) for coastal inundation risk assessment and the operational prediction of storm surge.

The eastern seaboard and Gulf Coast of the United States, Puerto Rico, the Bahamas, the Virgin Islands, and Hawaii, are subdivided into 39 regions or “basins.” These areas represent sections of the coastline that are centered upon particularly susceptible features: inlets, large coastal centers of population, low-lying topography, and ports. The SLOSH model computes the maximum potential impact of the storm in these “computational domains” based on storm intensity, track, and estimates of storm size provided by hurricane specialists at the National Hurricane Center.

The National Center for Environmental Prediction (NCEP) is the part of NOAA that handles forecasting the weather. The National Hurricane Center (NHC) is the division of NCEP that monitors and forecasts tropical cyclones in the North Atlantic and East Pacific.  The Central Pacific Hurricane Center (CPHC) monitors and forecasts tropical cyclones in the central Pacific from 140°W to 180°W.

NHC and CPHC issue an official forecast, every six hours, of the center position, maximum one-minute surface (10 meter [33 ft] elevation) wind speed (intensity), and radii of the 34 knot (39 mph,63 kph), 50 knot (58 mph,92 kph), and 64 knot (74 mph,117 kph) wind speeds in four quadrants (northeast, southeast, southwest, and northwest) surrounding the cyclone.

NHC’s Track and Intensity forecasts have both improved substantially over the years and continue to improve. Today a 3-day forecast is as accurate as those issued for a 2-day prediction in the late 1980s. However, much work still remains to better understand and predict wind intensity changes in tropical storms and hurricanes.

Read more at the National Hurricane Center’s Forecast Accuracy Page.

These official forecasts are later verified and then consolidated into a “best track” for the storm.  The Best Track has a center position and maximum wind speed value for each six-hour time that represents the official NHC estimate of the location and intensity of a tropical cyclone.  Values of central pressure and the radii of hurricane-force and gale-force winds may also be included as well as other significant events, such as landfall or peak intensity, especially if they occur other than the six-hourly times.

The Best Tracks are included in the Tropical Cyclone Reports issued by NHC and CPHC after hurricane season.  They are also included in the official hurricane database HURDAT2.

The National Hurricane Center has a great Glossary of Terms that are used in weather forecasts. Some important terms from that glossary are below.

Hurricane Watch – A Hurricane Watch is an announcement that hurricane force winds are possible within the specified area in association with a tropical cyclone. A hurricane watch is issued 48 hours in advance of the anticipated onset.

Hurricane Warning – Hurricane warnings are issued 36 hours in advance and are announced when hurricane force winds are expected somewhere within the specified area in association with a cyclone. This warning can remain in effect in the face of other hazards, such as flooding even if the winds drop to below hurricane force.

Advisory – An advisory contains all tropical cyclone watches and warnings in effect along with details concerning tropical cyclone locations, intensity and movement, and precautions to be taken. 

Maximum sustained wind – This is determined as winds that last for an average of at least one minute at the surface of a hurricane or about 33 feet (10 meters).

Gusts – are classified as a 3-5 second burst of wind higher than the maximum sustained wind.

Storm Surge Watch – A storm surge watch is the possibility of a life-threatening inundation from rising water moving inland from the shoreline, and it is usually issued 48 hours from the anticipated event in association with an ongoing tropical storm.

Storm Surge Warning – The danger of a life-threatening inundations from rising water moving inland, and usually issued 36 hours in advance of the event in association with an ongoing tropical storm.

Storm Track – A storm track is a representation of a tropical cyclone’s predicted path, location, and intensity over its lifetime. The best track contains the cyclone’s latitude, longitude, maximum sustained winds, and minimum sea level pressure at 6-hour intervals.

Storm Intensity – Hurricane intensity refers to the amount of energy a hurricane is carrying with it. Hurricane intensity and size are not closely related.

Reference: Powell, M.D., S.H. Houston, and T.A. Reinhold, 1996:”Hurricane Andrew’s Landfall in South Florida, Part I: Standardizing measurements for documentation of surface wind fields.” Wea. Forecast. v.11, p.329-349

The Atlantic Oceanographic and Meteorological Laboratory (AOML) supports these organizations by doing hurricane research with both observations and model experiments in order to provide guidance and integrate new technology into the forecast models. These experimental models are tested rigorously and submitted to the NCEP for verification before they are integrated into the operational models and sent to the NHC for use in the public forecast.

There are a number of different seasonal forecasts currently being issued for various basins. Some of these are fairly new, while the oldest and most well known (Prof. Bill Gray’s forecast from Colorado State[CSU]) has been issued for almost four decades. Over the last twenty years, seasonal forescasts have been issued by the National Oceanic and Atmospheric Administration (NOAA), and Tropical Storm Risk (TSR).

Click here for a comparison of the CSU, NOAA, and TSR seasonal outlook numbers.

North Atlantic Basin:

• Dr. Klotzbach, Department of Atmospheric Science, Colorado State University
• National Oceanic and Atmospheric Administration, CPC/AOML/NHC Team  – Forecast Archive
• Maritza Ballester, Cuban Institute of Meteorology
• Mark Saunders, Tropical Storm Risk, Department of Space and Climate Physics, University College London
• Florida State University COAPS
• International Research Institute for Climate and Society; Columbia University
• European Centre for Medium Range Weather Forecasts; Reading, England

NE Pacific Basin:

• National Oceanic and Atmospheric Administration, CPC/AOML/NHC Team
• European Centre for Medium Range Weather Forecasts; Reading, England
• National Meteorological Service; Mexico City, Mexico
• International Research Institute for Climate and Society; Columbia University

NW Pacific Basin:

• Mark Saunders, Tropical Storm Risk, Department of Space and Climate Physics, University College London
• Prof. Johnny C. L. Chan, Laboratory for Atmospheric Research, Dept. of Physics & Mat. Sci., City University of Hong Kong
• National Climate Centre, PRC
• European Centre for Medium Range Weather Forecasts; Reading, England
• International Research Institute for Climate and Society; Columbia University

Australian Basin:

• Mark Saunders, Tropical Storm Risk, Department of Space and Climate Physics, University College London
• International Research Institute for Climate and Society; Columbia University
• European Centre for Medium Range Weather Forecasts; Reading, England
• Macquarie University, Sydney, Australia

South China Sea:

• Prof. Johnny C. L. Chan, Laboratory for Atmospheric Research, Dept. of Physics & Mat. Sci., City University of Hong Kong

South Pacific Basin:

• International Research Institute for Climate and Society; Columbia University
• European Centre for Medium Range Weather Forecasts; Reading, England
• Macquarie University, Sydney, Australia

The major hurricane track forecast models run operationally for the Atlantic, Eastern Pacific, and Central Pacific hurricane basins are:

1. The basic model that is used as a “no-skill” forecast to compare other models against is CLIPER (CLImatology and PERsistence), which is a multiple regression statistical model that best utilizes the persistence of the current motion and also incorporates climatological track information (Aberson 1998). Surprisingly, CLIPER was difficult to beat with numerical model forecasts until the 1980s.
2. The NOAA Global Forecast System (GFS), formerly known as the Aviation and MRF models (Lord 1993) has been used for track forecasting since the 1992 hurricane season. An ensemble of lower-resolution runs is available four times daily. Current information on the GFS
3. A triply-nested movable mesh primitive equation model developed at the Geophysical Fluid Dynamics Laboratory (Bender et al 1993), known as the GFDL model, has provided forecasts since the 1992 hurricane season. One version (GFDL) uses GFS fields for boundary conditions; a second version (GFDN) uses NAVGEM fields for boundary conditions. Current information on the GFDL model
4. A doubly-nested movable mesh primitive equation non-hydrostatic model known as HWRF (for the Hurricane Weather Research and Forecast Model), has provided forecasts since 2006. It uses GFS fields for boundary conditions (Gopal et al 2012). Current information on HWRF
5. The United Kingdom Meteorological Office’s global Unified model is utilized for forecasting the tracks of tropical cyclones around the world (Radford 1994). NHC starting receiving these operationally in 1996. Current information on the Unified Model
6. The United States Navy Global Environmental Model (NAVGEM) is also a global numerical model that shows skill in forecasting tropical cyclone track (Fiorino et al. 1993). This model was also first received operationally at the National Hurricane Center during 1996. An ensemble of lower-resolution runs is available twice daily. Current information on NAVGEM
7. The Canadian Meteorological Center’s Global Environmental Multi-scale Model (GEM) provides forecasts twice per day. An ensemble of lower-resolution runs is available twice daily. Current information on GEM
8. The European Centre for Medium-Range Weather Forecast’s Integrated Forecast System (IFS) provides forecasts twice per day. It has proven to be the best model for track forecasting, and is the highest resolution global model available. An ensemble of lower-resolution runs is available twice daily.
9. The Japanese Meteorological Agency’s Global Spectral Model (GSM) provides forecasts, both in high-resolution deterministic runs and lower-resolution ensemble runs. Current information on the GSM

The full list of models used in the Atlantic and Eastern and Central Pacific is available to download here. Various types of consensus models (ensemble means) are available from these models.

Despite the variety of hurricane track forecast models, there are only a few models that provide operational intensity change forecasts for the Atlantic and Eastern and Central Pacific basins:

1. Similar to the CLIPER track model, the SHIFOR (Statistical Hurricane Intensity Forecast model) is used as a “no-skill” intensity change forecast. It is a multiple regression statistical model that best utilizes the persistence of the intensity trends and also incorporates climatological intensity change information (Jarvinen and Neumann 1979). SHIFOR has been difficult to exceed until recent years.
2. A statistical-synoptic model, SHIPS (Statistical Hurricane Intensity Prediction Scheme), has been available since the mid-1990s (DeMaria and Kaplan 1994). It takes current and forecasted information on the synoptic scale on the sea surface temperatures, vertical shear, moist stability, etc. with an optimal combination of the trends in the cyclone intensity.
3. The Logistic Growth Equation Model (LGEM) uses the same inputs as the SHIPS model but uses a dynamical scheme. The intensity is determined by a logistic growth equation constrained by the maximum potential intensity as derived from the sea surface temperature. LGEM differs from SHIPS in that it accounts for changes in environmental conditions rather than using values averaged over the forecast period.
4. The GFDL and HWRF models, described above in the track forecasting models, also issue forecasts of intensity change.
5. A statistical scheme for estimating the probability of rapid intensification has been developed (Kaplan et al 2010) and is now being used operationally. The RI scheme employs synoptic and persistence information from the SHIPS model to estimate the probability of rapid intensification (24 h increase in maximum wind of 35 mph or greater) every 6 hours.

Information on the performance of these models is available after each season here.

References: Aberson, Sim D. (1998): “Five-day tropical cyclone track forecasts in the North Atlantic basin” Weather and Forecasting, 13, pp.1005-1015

Marks, D.G. (1992): “The beta and advection model for hurricane track forecasting” NOAA Tech. Memo. NWS NMC 70, Natl. Meteorological Center; Camp Springs, Maryland, 89 pp.

Lord, S.J. (1993): “Recent developments in tropical cyclone track forecasting with the NMC global analysis and forecast system” Preprints of the 20th Conference on Hurricanes and Tropical Meteorology, San Antonio, Amer. Meteor. Soc., pp.290-291

Bender, M.A., R.J. Ross, R.E. Tuleya, and Y. Kurihara (1993): “Improvements in tropical cyclone track and intensity forecasts using the GFDL initialization system” Mon. Wea. Rev., 121, pp.2046-2061

Gopalakrishnan, S.G., S. Goldenberg, T. Quirino, X. Zhang, F. Marks, K-S Yeh, R. Atlas, V. Tallapragada (2012): “Toward Improving High-Resolution Numerical Hurricane Forecasting: Influence of Model Horizontal Grid Resolution, Initialization, and Physics” Wea. Forecasting, 27, pp.647-666.

Radford, A.M. (1994): “Forecasting the movement of tropical cyclones at the Met. Office” Met. Apps., 1, pp.355-363

Fiorino, M., J.S. Goerss, J.J. Jensen, E.J. Harrison, Jr.(1993): “An evaluation of the real-time tropical cyclone forecast skill of the Navy operations global atmospheric prediction system in the western North Pacific” Wea. Forecasting, 8, pp.3-24

Jarvinen, B.R., and C.J. Neumann (1979): “Statistical forecast of tropical cyclone intensity” NOAA Tech. Memo. NS NHC-10, 22pp.

DeMaria, M. and J. Kaplan (1994): “A statistical hurricane intensity prediction scheme (SHIPS) for the Atlantic basin” Wea. Forecasting, 9, pp.209-220

The Dvorak technique is a method used to estimate the intensity of a tropical cyclone based on satellite imagery. Developed by Vernon Dvorak in the 1970s, it analyzes cloud patterns associated with tropical storms. After a cloud pattern has been analyzed, meteorologists compare the imagery to a set of standard patterns, which have associated wind speeds and pressure values. By analyzing how closely the imagery matches the patterns, scientists can obtain a T-number (Tropical Number), which is a numerical value that corresponds to the intensity of the storm. Higher T-numbers indicate stronger storms. 

This T-number is converted to a Current Intensity (CI) number using a set of standardized tables. This CI number corresponds to a specific range of winds and pressures, giving meteorologists a method to interpolate the intensity of the cyclone from satellite imagery. 

An advanced version of the Dvorak technique is widely used by meteorologists and weather agencies because it allows for the continuous monitoring of tropical cyclones, even when direct measurements from aircraft reconnaissance are not available. It plays a crucial role in forecasting and assessing the potential impact of these storms.

The U.S. Government once supported research into methods of hurricane modification, known as Project STORMFURY.

It was an ambitious experimental program of research on hurricane modification carried out between 1962 and 1983. The proposed modification technique involved artificial stimulation of convection outside the eyewall through seeding with silver iodide. The invigorated convection, it was argued, would compete with the original eyewall, lead to the reformation of the eyewall at larger radius, and thus, through partial conservation of angular momentum, produce a decrease in the strongest winds.

Since a hurricane’s destructive potential increases rapidly as its strongest winds become stronger, a reduction as small as 10% would have been worthwhile. Modification was attempted in four hurricanes on eight different days. On four of these days, the winds decreased by between 10 and 30%, The lack of response on the other days was interpreted to be the result of faulty execution of the seeding or of poorly selected subjects.

These promising results came into question in the mid-1980s because observations in unmodified hurricanes indicated:

1. That cloud seeding had little prospect of success because hurricanes contained too much natural ice and too little supercooled water.
2. That the positive results inferred from the seeding experiments in the 1960s stemmed from inability to discriminate between the expected results of human intervention and the natural behavior of hurricanes.

For a couple decades NOAA and its predecessor tried to weaken hurricanes by dropping silver iodide – a substance that serves as an effective ice nuclei – into the rainbands of the storms. During the STORMFURY years, scientists seeded clouds in Hurricanes Esther (1961), Beulah (1963), Debbie (1969), and Ginger (1971). The experiments took place over the open Atlantic far from land. The STORMFURY seeding targeted convective clouds just outside the hurricane’s eyewall in an attempt to form a new ring of clouds that, hopefully, would compete with the natural circulation of the storm and weaken it. The idea was that the silver iodide would enhance the thunderstorms of a rainband by causing the supercooled water to freeze, thus liberating the latent heat of fusion and helping a rainband to grow at the expense of the eyewall. With a weakened convergence to the eyewall, the strong inner core winds would also weaken quite a bit. For cloud seeding to be successful, the clouds must contain sufficient supercooled water (water that has remained liquid at temperatures below the freezing point, 0°C/32°F). Neat idea, but in the end it had a fatal flaw. Observations made in the 1980s showed that most hurricanes don’t have enough supercooled water for STORMFURY seeding to work – the buoyancy in hurricane convection is fairly small and the updrafts correspondingly small compared to the type one would observe in mid-latitude continental super or multicells.

In addition, it was found that unseeded hurricanes form natural outer eyewalls just as the STORMFURY scientists expected seeded ones to do. This phenomenon makes it almost impossible to separate the effect (if any) of seeding from natural changes. The few times that they did seed and saw a reduction in intensity was undoubtedly due to what is now called “concentric eyewall cycles.” Thus nature accomplishes what NOAA had hoped to do artificially. No wonder the first few experiments were thought to be successes. Because the results of seeding experiments were so inconclusive, STORMFURY was discontinued. A special committee of the National Academy of Sciences concluded that a more complete understanding of the physical processes taking place in hurricanes was needed before any additional modification experiments. The primary focus of NOAA’s Hurricane Research Division today is better physical understanding of hurricanes and improvement of forecasts. To learn about the STORMFURY project as it was called, read Willoughby et al. (1985).

Reference: Willoughby, H.E., D.P. Jorgensen, R.A. Black, and S.L. Rosenthal (1985): “Project STORMFURY: A scientific chronicle 1962-1983” Bull. Amer. Meteor. Soc., 66, cover and pp.505-514

There have been numerous techniques that have been considered over the years to modify hurricanes: seeding clouds with dry ice or silver iodide, reducing evaporation from the ocean surface with thin-layers of polymers, cooling the ocean with cryogenic material or icebergs, changing the radiational balance in the hurricane environment by absorption of sunlight with carbon black, flying jets clockwise in the eyewall to reverse the flow, exploding the hurricane apart with hydrogen bombs, and blowing the storm away from land with giant fans, etc. As carefully reasoned as some of these suggestions are, they all share the same shortcoming: They fail to appreciate the size and power of tropical cyclones. For example, when Hurricane Andrew struck South Florida in 1992, the eye and eyewall devastated a swath 20 miles wide. The heat energy released around the eye was 5,000 times the combined heat and electrical power generation of the Turkey Point nuclear power plant over which the eye passed. The kinetic energy of the wind at any instant was equivalent to that released by a nuclear warhead.

Human beings are used to dealing with chemically complex biological systems or artificial mechanical systems that embody a small amount (by geophysical standards) of high-grade energy. Because hurricanes are chemically simple –air and water vapor – introduction of catalysts is unpromising. The energy involved in atmospheric dynamics is primarily low-grade heat energy, but the amount of it is immense in terms of human experience.

Attacking weak tropical waves or depressions before they have a chance to grow into hurricanes isn’t promising either. About 80 of these disturbances form every year in the Atlantic basin, but only about 5 become hurricanes in a typical year. There is no way to tell in advance which ones will develop. If the energy released in a tropical disturbance were only 10% of that released in a hurricane, it is still a lot of power. The hurricane police would need to dim the whole world’s lights many times a year.

Maybe the time will come when men and women can travel at nearly the speed of light to the stars, and we will then have enough energy for brute-force intervention in hurricane dynamics.

Until then, perhaps the best solution is not to try to alter or destroy the tropical cyclones, but just learn to co-exist with them. Since we know that coastal regions are vulnerable to the storms, building codes that can have houses stand up to the force of the tropical cyclones need to be enforced. The people that choose to live in these locations should be willing to shoulder a fair portion of the costs in terms of property insurance – not exorbitant rates, but ones which truly reflect the risk of living in a vulnerable region. In addition, efforts to educate the public on effective preparedness needs to continue. Helping other nations in their mitigation efforts can also result in saving countless lives. Finally, we need to continue in our efforts to better understand and observe hurricanes in order to more accurately predict their development, intensification, and track.

References: Simpson, R.H. and J. Simpson (1966): “Why experiment on tropical hurricanes ?” Trans. New York Acad. Sci., 28, pp.1045-1062 

Gray, W.M., W.M. Frank, M.L. Corrin, C.A. Stokes (1976): “Weather modification by carbon dust absorption of solar energy” J. Appl. Meteor., 15, pp.355-386

Gray, W.M., W.M. Frank, M.L. Corrin, C.A. Stokes, 1976: Weather Modification by Carbon Dust Absorption of Solar Energy, J. of Appl. Meteor., 15 4, pp. 355-386.

Woodcock, A.H., D.C. Blanchard, C.G.H. Rooth, 1963: Salt-Induced Convection and Clouds, J. of Atmos. Sci., 20, 2, pp. 159-169.

Blanchard, D.C., A.H. Woodcock, 1980: The Production, Concentration, and Vertical Distribution of the Sea-salt Aerosol, Ann. NY Acad. Sci., 338, 1, p. 330-347.

In the Atlantic basin (Atlantic Ocean, Gulf of Mexico, and Caribbean Sea) and in the eastern and central Pacific, as required, hurricane reconnaissance is carried out by two government agencies, the U.S. Air Force Reserves’ 53rd Weather Reconnaissance Squadron and NOAA’s Aircraft Operations Center (AOC). The U.S. Navy stopped flying hurricanes in 1974.

The 53rd WRS is based at Keesler AFB in Mississippi and maintains a fleet of ten WC-130 planes. These cargo airframes have been modified to carry weather instruments to measure wind, pressure, temperature and dew point as well as drop instrumented sondes and make other observations.

AOC is presently based at Linder Airfield in Lakeland, Florida and among its fleet of planes has two P-3 Orions, originally made as Navy sub hunters, but modified to include three radars as well as a suite of meteorological instruments and dropsonde capability. Starting in 1996 AOC added to its fleet a Gulfstream IV jet that is able to make observations from much higher altitudes (up to 45,000 feet).

The USAF planes are the workhorses of the hurricane hunting effort. They are often deployed to a forward base, such as Antigua, and carry out most of the reconnaissance of developing waves and depressions. Their mission in these situations is to look for signs of a closed circulation and any strengthening or organizing that the storm might be showing. This information is relayed by satellite to the hurricane specialists who evaluate this information along with data from other platforms.

The NOAA planes are more highly instrumented and are primarily used for scientific research on storms, but they may also be called upon for reconnaissance of mature hurricanes when they are threatening landfall, especially on U.S. territory.

The planes carry between six to fifteen people, both the flight crew and the weather crew. Flight crews consist of an aircraft commander, co-pilot, flight engineer, navigator, and electrical and data technicians. The weather crew might consist of a flight meteorologist, lead project scientist, cloud physicist, radar scientist, and dropsonde quality scientist.

The primary purpose of reconnaissance is to track the center of circulation, these are the co-ordinates that the National Hurricane Center issues, and to measure the maximum winds. But the crews are also evaluating the storm’s size, structure, and development and this information is also relayed to hurricane specialists via satellite link. Most of this data, which is critical in determining the hurricane’s threat, cannot be obtained from satellite.

The purposes of research are more varied. Onboard scientists direct the aircraft to those parts of the storm of interest, which might not be near the eye of the hurricane. Experiments might be planned to examine the outer rainbands or the hurricane’s interaction with the environment.

The NOAA G-IV jet usually does NOT penetrate the hurricane eye, but is assigned to fly synoptic scale patterns AROUND the storm, deploying dropsondes along the way, in order to profile the environmental flow that is moving the hurricane. In certain circumstances, a USAF WC-130 will also be assigned to fly a similar pattern in coordination with the G-IV to increase the coverage of this synoptic flow mission.

Whatever the mission’s purpose, information from all of these flights are shared via satellite with land-based forecasters to keep them current on the storm’s status. Radar and probe data are sent in real-time to be ingested into a variety of computer forecast models to ensure the best quality forecast.

Sorry, but only people who are part of the mission are allowed on military and public aircraft. This may include accredited members of the press, provided they are working on a current story involving the storm. If you are an accredited reporter and want to know how to arrange for your involvement in future flights with the:

Air Force Reserve Command’s Hurricane Hunters, please use the form for the Public Affairs Office of the 403rd Wing or call (228) 377-2056.

NOAA civilian hurricane aircraft, contact David Hall (301)713-7671 or Jonathan Shannon (863)267-1867.

Please note that seats are not always available on every flight, and that there is a limit of two seats per media outlet on a given flight. NOAA maintains a lengthy list of requests to fly aboard their aircraft during hurricane missions. If a hurricane is threatening landfall, local media will be given the first opportunity to fly. Due to the dynamics of hurricanes, flight plans can and do change right up until the last minute and flights are often cancelled. All of your contact information (cell numbers, pagers, home/office numbers) is extremely helpful in alerting you to changes.

The most incredible sight that I’ve ever seen is in the middle of a strong hurricane. One might not believe this, but most hurricane flights are fairly boring. They last 10 hours, there are clouds above you and clouds below – so all you see is gray, and you don’t feel the winds swirling around the hurricane.

But what does get interesting is flying through the hurricane’s rainbands and the eyewall, which can get a bit turbulent. The eyewall is a donut-like ring of thunderstorms that surround the calm eye. The winds within the eyewall can reach as much as 200 mph [325 km/hr] at the flight level, but you can’t feel these aboard the plane. But what makes flying through the eyewall exhilarating and at times somewhat scary, are the turbulent updrafts and downdrafts that one hits. Those flying in the plane definitely feel these wind currents (they sometimes makes us reach for the air-sickness bags). These vertical winds may reach up to 50 mph [80 km/hr] either up or down, but are actually much weaker in general than what one would encounter flying through a continental supercell thunderstorm. But once the plane gets into the calm eye of a hurricane like Andrew or Gilbert, it is a place of powerful beauty: sunshine streams into the windows of the plane from a perfect circle of blue sky directly above the plane, surrounding the plane on all sides is the blackness of the eyewall’s thunderstorms.

Directly below the plane peeking through the low clouds one can see the violent ocean with waves sometimes 60 feet high [20 m] crashing into one another. The partial vacuum of the hurricane’s eye (where one tenth of the atmosphere is gone) is like nothing else on earth. I would much rather experience a hurricane this way – from the safety of a plane – than being on the ground and having the hurricane’s full fury hit without protection.

The USAFR 53rd Hurricane Hunters have a ‘cyber flight’ through a hurricane. Visit the page here.

Contributed by Chris Landsea (NHC)
The Atlantic hurricane database (or HURDAT) extends back to 1851. However, because tropical storms and hurricanes spend much of their lifetime over the open ocean (some never hitting land) many systems were “missed” during the 19th and early 20th Centuries (Vecchi and Knutson 2008). Starting in 1944, systematic aircraft reconnaissance was commenced for monitoring both tropical cyclones and disturbances that had the potential to develop into tropical storms and hurricanes. This did provide much improved monitoring, but still about half of the Atlantic basin was not covered (Sheets 1990). Beginning in 1966, daily satellite imagery became available at the National Hurricane Center, and thus statistics from this time forward are most complete (McAdie et al. 2009). For hurricanes striking the USA Atlantic and Gulf coasts, one can go back further in time with relatively reliable counts of systems because enough people have lived along coastlines since 1900. Thus, the following records for the entire Atlantic Basin are divided into the pre-Satellite Era (1851-1965) and the Satellite Era (from 1966-present).

Named Storms = Tropical Storms, Hurricanes and Subtropical Storms
Hurricanes = Saffir-Simpson Hurricane Scale 1 to 5
Major Hurricanes = Saffir-Simpson Hurricane Scale 3, 4, or 5
“ACE” = Accumulated Cyclone Energy – An index that combines the numbers of systems, how long they existed and how intense they became. It is calculated by squaring the maximum sustained surface wind in the system every six hours that the cyclone is a Named Storm and summing it up for the season. It is expressed in 10⁴ kt².

References:

Landsea, C.W., G.A. Vecchi, L. Bengtsson, and T. R. Knutson, 2010: Impact of Duration Thresholds on Atlantic Tropical Cyclone Counts. Journal of Climate, 23(10), 2508-2519.

McAdie, C. J., C. W. Landsea, C. J. Neuman, J. E. David, E. Blake, and G. R. Hamner, 2009: Tropical Cyclones of the North Atlantic Ocean, 1851-2006. Historical Climatology Series 6-2,Prepared by the National Climatic Data Center, Asheville, NC in cooperation with the National Hurricane Center, Miami, FL, 238 pp.

Sheets, R.C., 1990: “The National Hurricane Center – Past, present, and future.”, Wea. Forecasting,5, 185-232.

Vecchi, G.A. and T. R. Knutson, 2008. “On estimates of historical North Atlantic tropical cyclone activity.”, J. Climate, 21, 3580.

Atlantic Basin: Individual years with the numbers in each category

Named Storms = Tropical Storms, Hurricanes and Subtropical Storms
Hurricanes = Saffir-Simpson Hurricane Scale 1 to 5
Major Hurricanes = Saffir-Simpson Hurricane Scale 3, 4, or 5
“ACE” = Accumulated Cyclone Energy – An index that combines the numbers of systems, how long they existed and how intense they became. It is calculated by squaring the maximum sustained surface wind in the system every six hours that the cyclone is a Named Storm and summing it up for the season. It is expressed in 10⁴ kt².

References:

Landsea, C.W., G.A. Vecchi, L. Bengtsson, and T. R. Knutson, 2010: Impact of Duration Thresholds on Atlantic Tropical Cyclone Counts. Journal of Climate, 23(10), 2508-2519.

McAdie, C. J., C. W. Landsea, C. J. Neuman, J. E. David, E. Blake, and G. R. Hamner, 2009: Tropical Cyclones of the North Atlantic Ocean, 1851-2006. Historical Climatology Series 6-2,Prepared by the National Climatic Data Center, Asheville, NC in cooperation with the National Hurricane Center, Miami, FL, 238 pp.

Sheets, R.C., 1990: “The National Hurricane Center – Past, present, and future.”, Wea. Forecasting,5, 185-232.

Vecchi, G.A. and T. R. Knutson, 2008. “On estimates of historical North Atlantic tropical cyclone activity.”, J. Climate, 21, 3580.

Click here for a complete list of hurricane landfalls in the continental United States.

The primary time of year for getting tropical cyclones is during the summer and autumn: July-October for the Northern Hemisphere and December-March for the Southern Hemisphere (though there are differences from basin to basin). The peak in summer/autumn is due to having all of the necessary ingredients become most favorable during this time of year: warm ocean waters (at least 26°C or 80°F), a tropical atmosphere that can quite easily kick off convection (i.e. thunderstorms), low vertical shear in the troposphere, and a substantial amount of large-scale spin available (either through the monsoon trough or easterly waves).

While one would intuitively expect tropical cyclones to peak right at the time of maximum solar radiation (late June for the tropical Northern Hemisphere and late December for the tropical Southern Hemisphere), it takes several more weeks for the oceans to reach their warmest temperatures. The atmospheric circulation in the tropics also reaches its most pronounced (and favorable for tropical cyclones) at the same time. This time lag of the tropical ocean and atmospheric circulation is analogous to the daily cycle of surface air temperatures – they are warmest in mid-afternoon, yet the sun’s incident radiation peaks at noon.

What never? Well, hardly ever.

In March, 2004 a hurricane DID form in the South Atlantic Ocean and made landfall in Brazil. But this still leaves the question of why hurricanes are so rare in the South Atlantic. Though many people might speculate that the sea surface temperatures are too cold, the primary reasons that the South Atlantic Ocean gets few tropical cyclones are that the tropospheric (near surface to 200mb) vertical wind shear is much too strong and there is typically no inter-tropical convergence zone (ITCZ) over the ocean (Gray 1968). Without an ITCZ to provide synoptic vorticity and convergence (i.e. large scale spin and thunderstorm activity) as well as having strong wind shear, it becomes very difficult to nearly impossible to have genesis of tropical cyclones.

In addition, McAdie and Rappaport (1991) documented the occurrence of a strong tropical depression/weak tropical storm that formed off the coast of Congo in mid-April of 1991. This storm lasted about five days and drifted toward the west-southwest into the central South Atlantic. So far, there has not been a systematic study as to the conditions that accompanied this rare event.

Hurricanes form both in the Atlantic basin (i.e. the Atlantic Ocean, Gulf of Mexico and Caribbean Sea) to the east of the continental U.S. and in the Northeast Pacific basin to the west of the U.S. However, the ones in the Northeast Pacific almost never hit the continental U.S., while the ones in the Atlantic basin strike the U.S. mainland just less than twice a year on average. There are two main reasons. The first is that hurricanes tend to move toward the west-northwest after they form in the tropical and subtropical latitudes. In the Atlantic, such a motion often brings the hurricane into the vicinity of the U.S. east coast. In the Northeast Pacific, a west-northwest track takes those hurricanes farther off-shore, well away from the U.S. west coast.

In addition to the general track, a second factor is the difference in water temperatures along the U.S. east and west coasts. Along the U.S. east coast, the Gulf Stream provides a source of warm (> 80°F or 26.5°C) waters to help maintain the hurricane. However, along the U.S. west coast, the ocean temperatures rarely get above the lower 70s, even in the midst of summer. Such relatively cool temperatures are not energetic enough to sustain a hurricane’s strength. So for the occasional Northeast Pacific hurricane that does track back toward the U.S. west coast, the cooler waters can quickly reduce the strength of the storm.  You may have remnants of such storms move over the Southwestern United States bringing heavy rainfall.

Recently Chenoweth and Landsea (2004), re-discovered that a hurricane struck San Diego, California on October 2, 1858. Unprecedented damage was done in the city and was described as the severest gale ever felt to that date nor has it been matched or exceeded in severity since. The hurricane force winds at San Diego are the first and only documented instance of winds of this strength from a tropical cyclone in the recorded history of the state. While climate records are incomplete, 1858 may have been an El Niño year, which would have allowed the hurricane to maintain intensity as it moved north along warmer than usual waters. Today if a Category 1 hurricane made a direct landfall in either San Diego or Los Angeles, damage from such a storm would likely be few to several hundred million dollars. The re-discovery of this storm is relevant to climate change issues and the insurance/emergency management communities risk assessment of rare and extreme events in the region.

Reference: Chenoweth, M., and C.W. Lansea (2004): “The San Diego hurricane of October 2, 1858” Bull. Amer. Meteor. Soc., 85, pp.1689-1697

The vast majority of Atlantic activity takes place during August-September-October, the climatological peak months of the hurricane season. The overall number of named storms (hurricanes) occurring in June and July (JJ) correlates at an insignificant r = +0.13 (+0.02) versus the whole season activity. In fact, there is a slight negative relationship between early season storms (hurricanes) versus late season – August through November – r = -0.28 (-0.35). Thus, the overall early season activity, be it very active or quite calm, has little bearing on the season as a whole. These correlations are based on the years 1944-1994.

A significant number of pre-season (April-May) and early season (JJ) storms are hybrid systems (neither fully tropical nor midlatitude lows).  So their formation mechanisms are very different from fully tropical systems that form in the Main Development Region (MDR).  So conditions favoring hybrid storm formation can be very different from those favoring tropical cyclone formation.

As shown in (Goldenberg 2000), if one looks only at the June-July Atlantic tropical storms and hurricanes occurring south of 22°N and east of 77°W (the eastern portion of the MDR for Atlantic hurricanes), there is a strong association with activity for the remainder of the year. According to the data from 1944-1999, total overall Atlantic activity for years that had a tropical storm or hurricane form in this region during JJ have been at least average and often above average. So it could be said that a JJ storm in this region is pretty much a “sufficient” (though not “necessary”) condition for a year to produce at least average activity. (I.e., Not all years with average to above-average total overall activity have had a JJ storm in that region, but almost all years with that type of JJ storm produce average to above-average activity.) The formation of a storm in this region during June-July is taken into account when the August updates for the Bill Gray and NOAA seasonal forecasts are issued.

The El Niño/Southern Oscillation (ENSO) resolves into a warm phase (El Niño), a cold phase (La Niña), and a neutral phase. During El Niño events (ENSO warm phase), tropospheric vertical shear is increased inhibiting tropical cyclone genesis and intensification, primarily by causing the 200 mb (12 km or 8 mi) westerly winds to be stronger (Gray 1984). La Niña events (ENSO cold phase) enhances activity. Recently, Tang and Neelin (2004) also identified that changes to the moist static stability can also contribute toward hurricane changes due to ENSO, with a drier, more stable environment present during El Niño events.

The Australian/Southwest Pacific shows a pronounced shift back and forth of tropical cyclone activity with fewer tropical cyclones between 145° and 165°E and more from 165°E eastward across the South Pacific during El Niño (warm ENSO) events. There is also a smaller tendency to have the tropical cyclones originate a bit closer to the equator. The opposite would be true in La Niña (cold ENSO) events. See papers by Nicholls (1979), Revell and Goulter (1986), Dong (1988), and Nicholls (1992). The western portion of the Northeast Pacific basin (140°W to the dateline) has been suggested to experience more tropical cyclone genesis during the El Niño year and more tropical cyclones tracking into the sub-region in the year following an El Niño (Schroeder and Yu 1995), but this has not been completely documented yet.

The Northwest Pacific basin, similar to the Australian/Southwest Pacific basin, experiences a change in location of tropical cyclones without a total change in frequency. Pan (1981), Chan (1985), and Lander (1994) detailed that west of 160°E there were reduced numbers of tropical cyclone genesis with increased formations from 160E to the dateline during El Niño events. The opposite occurred during La Niña events. Again there is also the tendency for the tropical cyclones to also form closer to the equator during El Niño events than average.

The eastern portion of the Northeast Pacific, the Southwest Indian, the Southeast Indian/Australian, and the North Indian basins have either shown little or a conflicting ENSO relationship and/or have not been looked at yet in sufficient detail.

Reference: Tang, B. H., and J. D. Neelin, 2004: “ENSO Influence on Atlantic hurricanes via tropospheric warming.” Geophys. Res. Lett.: Vol 31, L24204.

There is no debate that hurricane activity is strongly linked to short-term climate swings that last for approximately a year (ENSO) and for tens of years (known as “multi-decadal variability”), but there is an ongoing scientific debate about longer-term (30+ years) climate trends: how much is due to natural phenomena, how much is due to human activities, and how they affect tropical cyclone activity.

Atlantic hurricanes respond to the environment that they travel through. For example, when the tropical North Atlantic Ocean is warmer than usual, hurricanes tend to form more often and become stronger. However, when vertical wind shear is higher than normal over the basin, fewer storms form and are weaker.

Over the past 100 years and longer, the Atlantic hurricane environment has displayed climate swings known as “multi-decadal variability”, and hurricane activity has followed these swings. For example, in the 1940s through 1960s, ocean temperatures were warmer and hurricane seasons were more active than usual. This situation reversed during the 1970s and 1980s, which was a period of cooler ocean temperature and quieter than usual hurricane seasons. Since around the mid-1990s, we’ve been in another period of warmer than usual ocean temperatures and heightened hurricane activity.

Ocean temperatures in the region where most Atlantic hurricanes form and develop have been trending upwards as the Earth has gradually warmed since the mid-19th Century (top panel, Fig. 1). In addition to trending upwards, ocean temperatures show large multi-decadal climate swings from cooler to warmer than average. This becomes clearer when the warming trend is removed (middle panel). Atlantic hurricane activity has responded to these swings in a variety of ways. For example, the number of Atlantic major hurricanes (Saffir-Simpson categories 3–5) is greater during periods of warmer than usual temperatures (bottom panel).

Recent research describes two distinct types of Atlantic climate drivers: 1) Internal variability is caused by natural processes within the atmosphere and ocean climate system.  2) External variability is caused by forces outside of the atmosphere/ocean climate system.

Examples of natural internal forces are oceanic oscillations such as ENSO, meridional overturning circulation, and Saharan dust storms that blow mineral dust over the tropical Atlantic. The effects of the El Nino/Southern Oscillation are discussed in another section in detail.

Examples of external  climate forcing agents are solar variability, cosmic radiation changes, and air pollution such as industrial particulate and sulfur emissions.

The Atlantic meridional overturning circulation, which transports ocean heat from the tropics to higher latitudes and can cause substantial climate swings in the Atlantic region and beyond as this circulation increases or decreases.

Saharan dust storms have a similar effect on the Atlantic climate as the dust blows westward in the trade-winds off the African continent and blocks sunlight from reaching the ocean surface. Saharan dust storms are strongly seasonal, but can also exhibit multi-decadal swings that can cause similar swings in Atlantic ocean temperatures.

Our sun has 11-year and 22-year cycles in sunspot and magnetic activity, which affects the solar wind and Earth’s magnetic field.  It may also exhibit longer scale variability in its output. Along with changes in comic ray activity, this may alter Earth’s cloud cover in subtle ways and drive changes in ocean heat content.

Volcanic eruptions cause a transient cooling of ocean temperatures as they tend to block some of the incoming sunlight from reaching the surface. These natural eruptions tend to occur randomly and don’t exhibit any clear multi-decadal swings.

Finally, there is human-caused particulate and sulfate air pollution, which tends to block incoming sunlight similarly to volcanic eruptions and mineral dust. Human-caused sulfate pollution over the Atlantic exhibits a pronounced variability over time. Prior to the various Clean Air Acts and Amendments instituted by the United States and European countries in the 1970s, industrial sulfate emissions were much less regulated and air quality had become progressively worse. As the concentration of sulfate pollution over the Atlantic Ocean increased from the 1940s through 1970s, a cooling effect was noted as the pollution blocked incoming sunlight. According to some studies, as sulfate pollution concentrations decreased during and after the 1970s, the offsetting cooling effect is believed to have been reduced.

In November 2006 the global community of tropical cyclone researchers and forecasters as met at the 6^th International Workshop on Tropical Cyclones of the World Meteorological Organization in San Jose, Costa Rica. They released a statement on the links between anthropogenic (human-induced) climate change and tropical cyclones, including hurricanes and typhoons. The following is a summary of their report.

1. There have been a number of recent high-impact tropical cyclone events around the globe. These include 10 landfalling tropical cyclones in Japan in 2004, five tropical cyclones affecting the Cook Islands in a five-week period in 2005, Cyclone Gafilo in Madagascar in 2004, Cyclone Larry in Australia in 2006, Typhoon Saomai in China in 2006, and the extremely active 2004 and 2005 Atlantic tropical cyclone seasons – including the catastrophic socio-economic impact of Hurricane Katrina.
2. Some recent scientific articles have reported a large increase in tropical cyclone energy, numbers, and wind-speeds in some regions during the last few decades in association with warmer sea surface temperatures. Other studies report that changes in observational techniques and instrumentation are responsible for these increases.

Consensus Statements by International Workshop on Tropical Cyclones-VI (IWTC-VI) Participants :

1. Though there is evidence both for and against the existence of a detectable anthropogenic signal in the tropical cyclone climate record to date, no firm conclusion can be made on this point.
2. No individual tropical cyclone can be directly attributed to climate change.
3. The recent increase in societal impact from tropical cyclones has largely been caused by rising concentrations of population and infrastructure in coastal regions.
4. Tropical cyclone wind-speed monitoring has changed dramatically over the last few decades, leading to difficulties in determining accurate trends.
5. There is an observed multi-decadal variability of tropical cyclones in some regions whose causes, whether natural, anthropogenic or a combination, are currently being debated. This variability makes detecting any long-term trends in tropical cyclone activity difficult.
6. It is likely that some increase in tropical cyclone peak wind-speed and rainfall will occur if the climate continues to warm. Model studies and theory project a 3-5% increase in wind-speed per degree Celsius increase of tropical sea surface temperatures.
7. There is an inconsistency between the small changes in wind-speed projected by theory and modeling versus large changes reported by some observational studies.
8. Although recent climate model simulations project a decrease or no change in global tropical cyclone numbers in a warmer climate, there is low confidence in this projection. In addition, it is unknown how tropical cyclone tracks or areas of impact will change in the future.
9. Large regional variations exist in methods used to monitor tropical cyclones. Also, most regions have no measurements by instrumented aircraft. These significant limitations will continue to make detection of trends difficult.
10. If the projected rise in sea level due to global warming occurs, then the vulnerability to tropical cyclone storm surge flooding would increase.

A PDF version of the official report is available here.

This figure shows, at any particular location, what the chance is that a tropical storm or hurricane will affect an area during an individual month. We utilized the years 1944 to 1999 in the analysis and counted hits when a storm or hurricane was within about 100 miles (165 km).

Typically, for someone visiting the tropics during June through November, the chance to experience (or even be threatened by) a hurricane is very small.

To put this into perspective, if you made 50 one week trips to Puerto Rico in September, you would only experience a direct hit in ONE of those 50 visits. So the chances to get impacted by a hurricane are quite small for relatively short trips. And the case chosen here is the WORST possible, as all other locations in all other months have smaller chances of being hit by a hurricane.

Despite the chance being small, one should know in advance what your hotel’s, cruise company’s, etc. policy is for guests when a hurricane is coming, what actions they plan and what refund policies they have (if any). As is described above, a direct hit by a hurricane is a very rare event for a short visit and if I had a chance – for example – to go on a cruise in the Caribbean Sea during hurricane season, I would go without hesitation.

The forward speed of hurricanes is very latitude dependent. Typically, Atlantic hurricanes track along the western side of the subtropical ridge in the western Atlantic. As they recurve (turn more northerly) from their westward track they usually slow down. If they reach the midlatitudes, they can interact with upper-level troughs and pick up speed.

In the table below, the forward speed of hurricanes in the HURDAT database have been averaged in 5 degree latitude bins :

While there are many cases where the forward speed over the 6 hour interval in the hurricane database is zero, such as Mitch in 1998, the highest speed in the database is for unnamed Tropical Storm #6 in 1961. As it got caught up by a midlatitude trough over the mid Atlantic states, it went speeding off northeastward over Maine and New Brunswick at a maximum speed of 112.25 km/hr (60.57 kt or 69.75 mph). The fastest hurricane in the record was Emily in 1987, whose maximum speed reached 110.48 km/hr (59.61 kt or 68.65 mph) as it raced over the North Atlantic, before it turned extratropical.

The American Meteorological Society (AMS) publishes the Monthly Weather Review which has annual summaries of Atlantic basin tropical cyclones, Atlantic basin tropical disturbances, and Northeast Pacific (east of 140W) basin tropical cyclones. These summaries have a substantial amount of data and analysis of the storms.

Weatherwise prints annual summaries of both the Atlantic and Northeast Pacific basins which are less technical than the Monthly Weather Review articles, but come out months earlier.

Mariner’s Weather Log has articles from all of the global basins in annual summaries. These are descriptive and non-technical.

For the tropical cyclones of the Southeast Indian/Australia and the Australia/Southwest Pacific basins, Australia’s Bureau of Meteorology publishes Australia Meteorological and Oceanographic Journal has a very thorough annual summary.

The Indian journal Mausam carries an annual summary of tropical cyclone activity over the North Indian Ocean.

In addition to these summaries, many other AMS journals publish scholarly articles about tropical cyclones, especially the Bulletin of the AMS, Journal of Climate, Journal of Atmospheric Sciences, and Weather and Forecasting. International journals that often carry similar type articles are Geophysical Research Letters, Journal of the Meteorological Society of Japan, Nature, Quarterly Journal of the Royal Meteorological Society,Science, and Weather.

Hurricanes: Their Nature and Impacts on Society
An excellent introductory text into hurricanes (and tropical cyclones in general), this book by R.A. Pielke, Jr. and R.A. Pielke, Sr. provides the basics on the physical mechanisms of hurricanes without getting into any mathematical rigor. The book also discusses hurricane policy, vulnerability and societal responses and ends with an in-depth look at Hurricane Andrew’s forecast, impact and response. Roger A. Pielke, Jr. is a Sociologist at the Environmental and Societal Impacts Group at the National Center for Atmospheric Research in Boulder, Colorado, USA. Roger A. Pielke, Sr. is a Professor of Atmospheric Science at Colorado State University (USA).
John Wiley & Sons, Chichester, UK, 1997, 279 pp.

Meteorology Today for Scientists and Engineers
This paperback book is designed to accompany C. Donald Ahrens’ introductory book “Meteorology Today.” For a concise mathematical description of hurricanes that has NO calculus and NO differential equations, then I would suggest obtaining a copy of this book by Rolland B. Stull
West Publ. Co., Minneapolis/St. Paul, MN, 2000, 385 pp.
Chapter 16 Hurricanes p.289-304.

Global Perspectives on Tropical Cyclones: From Science to Mitigation
edited by Johnny C. L. Chan and Jeffrey D. Kepert
This book is a completely rewritten, updated and expanded new edition of the original Global Perspectives on Tropical Cyclones published in 1995. It presents a comprehensive review of the state of science and forecasting of tropical cyclones together with the application of this science to disaster mitigation, hence the tag: From Science to Mitigation.Since the previous volume, enormous progress in understanding tropical cyclones has been achieved. These advances range from the theoretical through to ever more sophisticated computer modeling, all underpinned by a vast and growing range of observations from airborne, space and ocean observation platforms. The growth in observational capability is reflected by the inclusion of three new chapters on this topic. The chapter on the effects of climate change on tropical cyclone activity is also new, and appropriate given the recent intense debate on this issue. The advances in the understanding of tropical cyclones which have led to significant improvements in forecasting track, intensity, rainfall and storm surge, are reviewed in detail over three chapters. For the first time, a chapter on seasonal prediction is included. The book concludes with an important chapter on disaster mitigation, which is timely given the enormous loss of life in recent tropical cyclone disasters.
World Scientific, 2010, 448 pp.ISBN: 978-981-4293-47-1 or 978-981-4293-48-8 (ebook).

Global Guide to Tropical Cyclone Forecasting
For the tropical cyclone forecaster and also of general interest for anyone in the field and those with a non-technical interest in the field, the loose-leaf book Global Guide to Tropical Cyclone Forecasting(1993) by G.J. Holland (ed.), World Meteorological Organization, WMO/TD-No. 560, Report No. TCP-31 is a must get.

North Carolina’s Hurricane History, Florida’s Hurricane History
These two books are an amazing documentaries of the hurricanes which have struck the states of North Carolina and Florida from 1526 until 1996 and 1546-1995, respectively. The author Jay Barnes – Director of the North Carolina Aquarium – tells the stories of the hurricanes and their effects upon the people of the state in an easily readable style with numerous photographs.
University of North Carolina Press, Chapel Hill, NC, 1998, 330pp. 

Atlantic Hurricanes
A classic book describing tropical cyclones primarily of the Atlantic basin, but also covering the physical understanding of tropical cyclone genesis, motion, and intensity change at the time. Written in 1960, by Gordon E. Dunn and Banner I. Miller, this book provides insight into the knowledge of tropical cyclones as of the late 1950s. It is interesting to observe that much of what we know was well understood at this pre-satellite era. Gordon E. Dunn was the Director of the National Hurricane Center and Banner I. Miller was a research meteorologist with the National Hurricane Research Project.
Louisiana State Press, 1960, 326pp (revision 1964)

Hurricanes, Their Nature and History
Before Dunn and Miller’s book, Ivan Ray Tannehill came out with an authoritative reference on the history, structure, climatology, historical tracks, and forecasting techniques of Atlantic hurricanes as was known by the mid-1930s. This is one of the first compilations of yearly tracks of Atlantic storms – he provides tracks of memorable tropical cyclones all the way back to the 1700s and shows all the storm tracks yearly from 1901 onward. The first edition came out in 1938 and the book went through at least nine editions (my book was published in 1956). Mr. Tannehill was engaged in hurricane forecasting for over 20 years and also lead the Division of Synoptic Reports and Forecasts of the U.S. Weather Bureau.
Princeton University Press, 1956, 308 pp.

Into the Hurricane 
(Published in Britain as “The Devil’s Music”)
Author Pete Davies spent the summer of 1999 looking at Atlantic hurricanes, traveling to Honduras to see the aftermath of Hurricane Mitch, and flying on research missions with NOAA’s Hurricane Research Division. He explores the science of why the storms occur and how to predict them, and recounts the impacts of Hurricane Floyd.
Henry Holt and Company. 2000, 264 pp., ISBN: 0-8050-6574-1.

The Divine Wind 
(translated into Chinese) Hurricanes are presented in verse, art, history, and science in this all-encompassing book of the science and culture of hurricanes. Author Kerry Emanuel discusses hurricane forecasting, historical events and human impacts. The book includes many artworks, figures, and photographs, plus a description of flying into hurricanes.
Oxford University Press, 2005, 296 pp.,ISBN-10: 0195149416.

A Global View of Tropical Cyclones
(A revised version of this book is Global Perspectives on Tropical Cyclones listed above.)
A very thorough book dealing with the technical issues of tropical cyclones for the state of the science in the mid-1980s by Elsberry, Holland, Frank, Jarrell, and Southern.
University of Chicago Press, 1987,195 pp.

The Hurricane
(1997 revision titled “Hurricanes: Their Nature and Impacts on Society” by Pielke and Pielke is listed above.)
A very good introductory text into hurricanes (and tropical cyclones in general), this book by R.A. Pielke provides the basics on the physical mechanisms of hurricanes without getting into any mathematical rigor. This first version is just 100 pages of text with another 120 pages devoted toward all of the tracks of Atlantic hurricanes from 1871-1989. Roger A. Pielke is a professor of Atmospheric Science at Colorado State University.
Routledge Publishing, New York, 1990, 279 pp. (revision 1997)

Hurricanes
An introductory text book for young readers on hurricanes by Sally Lee.
Franklin Watts Publishing, New York, 1993, 63 pp.

Cyclone Tracy, Picking up the Pieces
Twenty years after Cyclone Tracy, this book recreates, by interviews with survivors, the events during and after the cyclone that nearly destroyed Darwin, Australia by B. Bunbury
Fremantle Arts Centre Press, South Fremantle, Australia, 1994, 148 pp.

Beware the Hurricane!
This book tells “the story of the cyclonic tropical storms that have struck Bermuda and the Islanders’ folk-lore regarding them” by Terry Tucker.
The Island Press Limited, Bermuda, 1995, 180 pp.

Florida Hurricanes and Tropical Storms, Revised Edition
This recent book provides a historical perspective of Florida Hurricanes extending from 1871 to 1996 by J.M. Williams and I. W. Duedall
Florida Sea Grant College Program, University of Florida Press, Gainesville, FL, 1997, 146 pp.

Hurricanes of the North Atlantic
This book by J. B. Elsner and A. B. Kara focuses on the statistics and variability of Atlantic hurricanes as well as detailed discussions on how hurricanes impact the insurance industry and how integrated assessments can be made regarding these storms. The book provides very valuable information on hurricane frequencies, intensities and return periods that are not easily available elsewhere. Also sections are devoted on the development of seasonal (and longer) hurricane forecast models and their performance.
Oxford University Press, New York/Oxford, 1999, 488 pp.

Natural Disasters – Hurricanes
This reference book by P. J. Fitzpatrick provides a very useful compilation of a wide range of topics on Atlantic hurricanes. Of particular interest is the chronology of advances in the science and forecasting of hurricanes along with biographical sketches of researchers and forecasters prominent in the field. This book is an excellent resource in answering questions on many issues in the field.
ABC-CLIO, Santa Barbara, CA, 1999, 286 pp.

Tropical Cyclones of the North Atlantic Ocean, 1851-2006
Researchers and those who follow Atlantic hurricanes should all have a copy of the atlas. Previous versions:
Tropical Cyclones of the North Atlantic Ocean, 1871-1998
Tropical Cyclones of the North Atlantic Ocean, 1871-1992
Tropical Cyclones of the North Atlantic Ocean, 1871-1986
Tropical Cyclones of the North Atlantic Ocean, 1871-1980
Tropical Cyclones of the North Atlantic Ocean, 1871-1977
Tropical Cyclones of the North Atlantic Ocean, 1871-1963
North Atlantic Tropical Cyclones, 1886-1958
National Climatic Data Center, Asheville, NC, in cooperation with the Tropical Prediction Center/National Hurricane Center, Miami, FL, 2006, 238 pp.

Hurricanes and Florida Agriculture
Dr. John A. Attaway, former Scientific Research Director of the Florida Department of Citrus, wrote this well-researched history and litany of the impacts that hurricanes have had upon agriculture in Florida.
Florida Science Source, Inc., Lake Alfred, FL, 1999, 444 pp.

There is an undeniable drama to hurricanes; their massive scale affecting the lives of thousands, the foreshadowing of impending doom, and their ponderous pace as they approach the shore. This has made them ideal plot elements in many fictional works. Below is an admittedly partial list of some novels, plays, poems, and movies which have used hurricanes as a major dramatic element.

Based on data from 19810-2022
(1980/82 to 2019/2022 for the Southern Hemisphere):

*Note that the data includes subtropical storms in the Atlantic basin numbers. (Neumann 1993)

** Note that the data includes storms and hurricanes that formed in the Central Pacific.

These values are based on data supplied by the WMO Regional Meteorological Center responsible for tropical cyclone forecasting for that particular basin.

Reference: Neumann, C.J. (1993): “Global Overview” – Chapter 1″ Global Guide to Tropical Cyclone Forecasting, WMO/TC-No. 560, Report No. TCP-31, World Meteorological Organization; Geneva, Switzerland

Here is a list of tropical cyclones that have crossed from the Atlantic basin to the Northeast Pacific and vice versa. To be considered the same tropical cyclone an identifiable center of circulation must be tracked continuously and the cyclone must have been of at least tropical storm strength in both basins (i.e. sustained winds of at least 34 kt, or 18 m/s). This record only goes back to 1923. Before the advent of geostationary satellite pictures in the mid-1960s, the number of Northeast Pacific tropical cyclones was undercounted by a factor of 2 or 3. Thus the lack of many of these events during the 1960s and earlier is mainly due to simply missing the Northeast Pacific TCs.

There has not been a recorded case where the same tropical cyclone crossed from the Atlantic into the Northeast Pacific then crossed back into the Atlantic, but Hattie/Simone/Inga in 1961 came close. There is no evidence that a single center of circulation persisted through several crossings of land, but the envelope of moisture and instability from one system helped spawn the next.

• Atlantic Tropical Storm Bonnie (July 2022) struck the Costa Rica/Nicaragua border then crossed over to the eastern Pacific.  It would later strengthen into a Category 3 hurricane before dissipating in the central Pacific.
• Atlantic Hurricane Otto (November 2016) made landfall in Nicaragua.  Its circulation remained intact and it reformed into a tropical storm in the Northeast Pacific, but quickly dissipated after encountering hostile wind shear.
• Northeast Pacific Tropical Storm Trudy (October 2014) made landfall on southern Mexico on October 18th and the circulation dissipated over the rugged terrain of Mexico. The moisture associated with the remnants moved into the southern Gulf of Mexico where a new circulation developed and intensified into a tropical depression on the 22nd. The depression weakened into a low pressure and crossed the Yucatan peninsula reaching the Caribbean Sea where it intensified into Tropical Storm Hanna on the 27th before making landfall near the Nicaraguan/Honduran border.
• Northeast Pacific Hurricane Barbara (May 2013) made landfall on the Tehuantepec peninsula on May 29th and its center of circulation dissipated before it reached the Gulf of Mexico. However, its envelope of moisture continued northward and from this Atlantic Tropical Storm Andrea formed on June 5th in the northeast Gulf.
• Northeast Pacific Tropical Storm Alma (May 2008) became a remnant low in the Atlantic where it merged with another tropical wave which generated Atlantic Tropical Storm Arthur.
• Atlantic Hurricane Iris (October 2001) become a remnant low over Central America and regenerated in the Northeast Pacific as Tropical Storm Manuel.
• Atlantic Hurricane Cesar (July 1996) became Northeast Pacific Hurricane Douglas.
• Atlantic Tropical Storm Bret (August 1993) became Hurricane Greg in the Northeast Pacific.
• Northeast Pacific Hurricane Cosme became Atlantic Tropical Storm Allison (June 1989).
• Atlantic Hurricane Joan (October 1988) became Northeast Pacific Hurricane Miriam.
• Atlantic Hurricane Greta (September 1978) became Northeast Pacific Hurricane Olivia.
• Atlantic Hurricane Fifi (September 1974) became Northeast Pacific Tropical Storm Orlene.
• Atlantic Hurricane Irene (September 1971) became Northeast Pacific Tropical Storm Olivia.
• Atlantic Hurricane Francelia (September 1969) made landfall in Belize, dissipating over Guatemala and eastern Mexico. The remnants redeveloped into Tropical Storm Glenda over the Northeastern Pacific on September 8th, moving parallel to the Mexican coast until dissipating on the 12th.
• Atlantic Hurricane Hattie (October-November 1961) after dissipating over Guatemala contributed to the formation of Northeast Pacific Tropical Storm Simone which crossed the isthmus of Teuhantepec and merged with other disturbed weather which later formed Atlantic Tropical Storm Inga.
• A Northeast Pacific Tropical Storm (September-October 1949) became an Atlantic Hurricane (Storm #10) and made landfall in TX.
• A Northeast Pacific Tropical Storm (October 1923) became an Atlantic Hurricane (Storm #6) and made landfall in LA.

Naming Conventions for Basin-Jumping Hurricanes

If the system remains a tropical cyclone as it moves across Central America, then it will keep the original name. Only if the tropical cyclone dissipates with just a tropical disturbance remaining, will the hurricane warning center give the system a new name assuming it becomes a tropical cyclone once again in its new basin.

Four hurricanes occurred simultaneously on two occasions. The first occasion was August 22, 1893, and one of these eventually killed 1,000- 2,000 people in Georgia and South Carolina. The second occurrence was September 25, 1998, when Georges, Ivan, Jeanne and Karl persisted into September 27, 1998 as hurricanes. Georges ended up taking the lives of thousands in Haiti. In 1971 from September 10 to 12, there were five tropical cyclones at the same time; however, while most of these ultimately achieved hurricane intensity, there were never more than two hurricanes at any one time.

Reference: Blake, E.S., E.N. Rappaport, J.D. Jarell, and C.W. Landsea, 2005: “The Deadliest, Costliest, and Most Intense United States Hurricanes from 1851 to 2004 (and Other Frequently Requested Hurricane Facts.) NOAA Technical Memorandum NWS-TPC-4, 48 pp.

This table, updated from Jarrell et al. (2001), shows the number of hurricanes affecting the United States and individual states, i.e., direct hits. The table shows that, on the average, close to seven hurricanes every four years (~1.75 per year) strike the United States, while about three major hurricanes cross the U.S. coast every five years (0.60 per year). Other noteworthy facts, updated from Jarrell et al. (2001), are:

• Forty percent of all U.S. hurricanes hit Florida
• Sixty percent of Major hurricanes strikes have hit either Florida or Texas
• Pennsylvania’s only hurricane strike between 1851-2022 was in 1898 (from Blake et al. 2005).

Notes:

State totals will not equal U.S. totals and Texas and Florida totals will not necessarily equal sum of sectional totals since storms may be counted for more than one state or region.

Regional definitions are found in Appendix A of Jarrell et al. (2001).

References: Blake, E.S., E.N. Rappaport, J.D. Jarell, and C.W. Landsea, 2005: “The Deadliest, Costliest, and Most Intense United States Hurricanes from 1851 to 2004 (and Other Frequently Requested Hurricane Facts.) NOAA Technical Memorandum NWS-TPC-4, 48 pp.

Jarell, J.D., B.M. Mayfield, E.N. Rappaport, and C.W. Landsea, 2001: “The Deadliest, Costliest, and Most Intense United States Hurricanes from 1900 to 2000 (and Other Frequently Requested Hurricane Facts.) NOAA Technical Memorandum NWS-TPC-3, 30 pp.

This table shows the incidence of major hurricanes by months for the U.S. mainland and individual states. September has as many major hurricane landfalls as October and August combined. Texas and Louisiana are the prime targets for pre-August major hurricanes. The threat of major hurricanes increases from west to east during August with major hurricanes favoring the U.S. East Coast by late September. Most major October hurricanes occur in southern Florida (from Blake et al. 2005).

Note: State totals do not equal U.S. totals.
Texas and Florida totals do not necessarily equal the sum of sectional entries.
Florida and Texas regional definitions are found in Appendix A.

Reference: Blake, E.S., E.N. Rappaport, J.D. Jarell, and C.W. Landsea, 2005: “The Deadliest, Costliest, and Most Intense United States Hurricanes from 1851 to 2004 (and Other Frequently Requested Hurricane Facts.) NOAA Technical Memorandum NWS-TPC-4, 48 pp.

This table summarizes the occurrence of the last hurricane and major hurricane to directly hit the most populated coastal communities from Brownsville, Texas to Eastport, Maine.  There are many illustrative examples of the uncertainty of when a hurricane might strike a given locality. After nearly 70 years without a direct hit, Pensacola, Florida was hit directly by Hurricane Erin in 1995 and major Hurricane Ivan in 2004 within 10 years. Miami, which expects a major hurricane every nine years, on average, has been struck only once since 1950 (in 1992). Tampa has not experienced a major hurricane since 1921.

Last direct hit by any hurricane or a major hurricane at certain populated coastal communities through 2022.

Reference: Blake, E.S., E.N. Rappaport, J.D. Jarell, and C.W. Landsea, 2005: “The Deadliest, Costliest, and Most Intense United States Hurricanes from 1851 to 2004 (and Other Frequently Requested Hurricane Facts.) NOAA Technical Memorandum NWS-TPC-4, 48 pp.

This table shows the total and average number of tropical storms, and those which became hurricanes, by month, for the period 1851-2020. It also shows the monthly total and average number of hurricanes to strike the U.S. since 1851.

Excludes subtropical storms

This table ranks the top ten countries by most tropical cyclone strikes. These numbers are approximated from the IBTrACS database and include only those storm tracks that intersected the coastline at hurricane intensity (≥ 65 kt) and does NOT include storms that remained just offshore but may have affected the country.

However, it should be noted that some basins have longer histories of such activity and this might bias these counts. So the following is the ranking if we only look at storms since 1970, when world-wide satellite coverage became available.

The most intense mainland United States hurricanes by central pressure (1851-2018)

Notes:
Includes only major hurricanes at their most intense landfall.
^&Highest category justified by winds.
^#Classified 4 because of estimated winds.
⁺Cape Fear, NC area only; was a category 2 at final landfall.
^%Storm post-tropical at landfall

** SANDY 2012 was not classified a tropical cyclone when it came ashore but is placed in this table for reference relative to other storms.

Reference: The Deadliest, Costliest, and Most Intense United States Tropical Cyclones from 1851 to 2006 (and other Frequently Requested Hurricane Facts) NOAA Technical Memorandum NWS TPC-5 April 15, 2007, Eric S. Blake, Edward N. Rappaport, Christopher W. Landsea.

This table ranks the top 30 years by deaths. In most years the death totals are the result of a single, major hurricane.

Notes:
¹ Could have been as high as 12,000.
² Considered too high in 1915 reference.