A tropical cyclone rainfall climatology is developed to determine rainfall characteristics of past tropical cyclones. A tropical cyclone rainfall climatology can be used to help forecast current or upcoming tropical cyclone impacts. The degree of a tropical cyclone rainfall impact depends upon speed of movement, storm size, and degree of vertical wind shear. One of the most significant threats from

tropical cyclone A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Dep ...

s is heavy rainfall. Large, slow moving, and non-sheared tropical cyclones produce the heaviest rains. The intensity of a tropical cyclone appears to have little bearing on its potential for rainfall over land, but satellite measurements over the last several years show that more intense tropical cyclones produce noticeably more rainfall over water. Flooding from tropical cyclones remains a significant cause of fatalities, particularly in low-lying areas.

Anticipating a flood event

While inland

flood A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrol ...

ing is common to tropical cyclones, there are factors which lead to excessive rainfall from tropical cyclones. Slow motion, as was seen during

Hurricane Danny (1997) Hurricane Danny was the only hurricane to make landfall in the United States during the 1997 Atlantic hurricane season, and the second hurricane and fourth tropical storm of the season. The system became the earliest-formed fifth tropical or subtr ...

and

Hurricane Wilma Hurricane Wilma was an extremely intense and destructive Atlantic hurricane which was the most intense storm of its kind and the second-most intense tropical cyclone recorded in the Western Hemisphere, after Hurricane Patricia in 2015. Part o ...

, can lead to high amounts of rainfall. The presence of mountains/hills near the coast, like across much of

Mexico Mexico (Spanish language, Spanish: México), officially the United Mexican States, is a List of sovereign states, country in the southern portion of North America. It is borders of Mexico, bordered to the north by the United States; to the so ...

Haiti Haiti (; ht, Ayiti ; French: ), officially the Republic of Haiti (); ) and formerly known as Hayti, is a country located on the island of Hispaniola in the Greater Antilles archipelago of the Caribbean Sea, east of Cuba and Jamaica, and s ...

, the

Dominican Republic The Dominican Republic ( ; es, República Dominicana, ) is a country located on the island of Hispaniola in the Greater Antilles archipelago of the Caribbean region. It occupies the eastern five-eighths of the island, which it shares with ...

Central America Central America ( es, América Central or ) is a subregion of the Americas. Its boundaries are defined as bordering the United States to the north, Colombia to the south, the Caribbean Sea to the east, and the Pacific Ocean to the west. ...

Madagascar Madagascar (; mg, Madagasikara, ), officially the Republic of Madagascar ( mg, Repoblikan'i Madagasikara, links=no, ; french: République de Madagascar), is an island country in the Indian Ocean, approximately off the coast of East Afric ...

Réunion Réunion (; french: La Réunion, ; previously ''Île Bourbon''; rcf, label= Reunionese Creole, La Rényon) is an island in the Indian Ocean that is an overseas department and region of France. It is located approximately east of the island o ...

China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...

, and

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the n ...

acts to magnify rainfall potential due to forced upslope flow into the mountains. Strong upper level forcing from a trough moving through the Westerlies and its associated

cold front A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough of low pressure. It often forms behind an extratropical cyclone (to the west in the Northern ...

, as was the case during

Hurricane Floyd Hurricane Floyd was a very powerful Cape Verde hurricane which struck the Bahamas and the East Coast of the United States. It was the sixth named storm, fourth hurricane, and third major hurricane in the 1999 Atlantic hurricane season. Floyd tr ...

, can lead to high amounts even from systems moving at an average forward motion. Larger tropical cyclones drop more rainfall as they precipitate upon one spot for a longer time frame than average or small tropical cyclones. A combination of two of these factors could be especially crippling, as was seen during

Hurricane Mitch Hurricane Mitch is the second-deadliest Atlantic hurricane on record, causing over 11,000 fatalities in Central America in 1998, including approximately 7,000 in Honduras and 3,800 in Nicaragua due to cataclysmic flooding from the slow motion ...

. During the 2005 season, flooding related to slow-moving

Hurricane Stan Hurricane Stan was a relatively weak but deadly tropical cyclone that affected areas of Central America and Mexico in early October 2005. The eighteenth named storm and eleventh hurricane of the 2005 Atlantic hurricane season, Stan formed from ...

's broad circulation led to 1,662–2,000 deaths.Dennis, Katrina, Rita, Stan, and Wilma "Retired" from List of Storm Names
." ''

NOAA The National Oceanic and Atmospheric Administration (abbreviated as NOAA ) is an United States scientific and regulatory agency within the United States Department of Commerce that forecasts weather, monitors oceanic and atmospheric conditio ...

.'' Retrieved on June 14, 2008.

General distribution within a tropical cyclone

Isaac Cline was the first to investigate

rain Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...

fall distribution around tropical cyclones in the early 1900s. He found that a larger proportion of rainfall falls in advance of the center (or eye) than after the center's passage, with the highest percentage falling in the right front quadrant. Father Viñes of

Cuba Cuba ( , ), officially the Republic of Cuba ( es, República de Cuba, links=no ), is an island country comprising the island of Cuba, as well as Isla de la Juventud and several minor archipelagos. Cuba is located where the northern Caribb ...

found that some tropical cyclones have their highest rainfall rates in the rear quadrant within a training (non-moving) inflow band. Normally, as a tropical cyclone intensifies, its heavier rainfall rates become more concentrated around its center. Rainfall is found to be heaviest in tropical cyclone's inner core, whether it be the

eyewall The eye is a region of mostly calm weather at the center of tropical cyclones. The eye of a storm is a roughly circular area, typically in diameter. It is surrounded by the ''eyewall'', a ring of towering thunderstorms where the most severe weat ...

central dense overcast The central dense overcast, or CDO, of a tropical cyclone or strong subtropical cyclone is the large central area of thunderstorms surrounding its circulation center, caused by the formation of its eyewall. It can be round, angular, oval, or irr ...

, within a degree latitude of the center, with lesser amounts farther away from the center. Most of the rainfall in tropical cyclones is concentrated within its radius of gale-force (34 knots/39 mph/63 km/h) winds.Corene J. Matyas
Relating Tropical Cyclone Rainfall Patterns to Storm Size.
Retrieved on 2007-02-14. Rainfall is more common near the center of tropical cyclones overnight. Over land, outer bands are more active during the heating of the day, which can act to restrict inflow into the center of the cyclone. Recent studies have shown that half of the rainfall within a tropical cyclone is stratiform in nature.David M. Roth
Tropical Cyclone Rainfall Presentation (July 2007).
Retrieved on 2007-07-19. The chart to the right was developed by Riehl in 1954 using meteorological equations that assume a gale radius of about , a fairly symmetric cyclone, and does not consider topographic effects or vertical wind shear. Local amounts can exceed this chart by a factor of two due to topography. Wind shear tends to lessen the amounts below what is shown on the table.

Relation to storm size

Larger tropical cyclones have larger rain shields, which can lead to higher rainfall amounts farther from the cyclone's center. This is generally due to the longer time frame rainfall falls at any one spot in a larger system, when compared to a smaller system. Some of the difference seen concerning rainfall between larger and small storms could be the increased sampling of rainfall within a larger tropical cyclone when compared to that of a compact cyclone; in other words, the difference could be the result of a statistical problem.

Slow/looping motion on rainfall magnitude

Storms which have moved slowly, or loop, over a succession of days lead to the highest rainfall amounts for several countries. Riehl calculated that 33.97 inches (863 mm) of rainfall per day can be expected within one-half degree, or 35 miles (56 km), of the center of a mature tropical cyclone. Many tropical cyclones progress at a forward motion of 10 knots, which would limit the duration of this excessive rainfall to around one-quarter of a day, which would yield about 8.50 inches (216 mm) of rainfall. This would be true over water, within 100 miles (160 km) of the coastline, and outside topographic features. As a cyclone moves farther inland and is cut off from its supply of warmth and moisture (the ocean), rainfall amounts from tropical cyclones and their remains decrease quickly.

Vertical wind shear impact on rainfall shield

Vertical

wind shear Wind shear (or windshear), sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizont ...

forces the rainfall pattern around a tropical cyclone to become highly asymmetric, with most of the precipitation falling to the left and downwind of the shear vector, or downshear left. In other words, southwesterly shear forces the bulk of the rainfall north-northeast of the center. If the wind shear is strong enough, the bulk of the rainfall will move away from the center leading to what is known as an exposed circulation center. When this occurs, the potential magnitude of rainfall with the tropical cyclone will be significantly reduced.

Effect of interaction with frontal boundaries/upper level troughs

As a

interacts with an upper-level trough and the related surface front, a distinct northern area of precipitation is seen along the front ahead of the axis of the upper level trough. This type of interaction can lead to the appearance of the heaviest rainfall falling along and to the left of the tropical cyclone track, with the precipitation streaking hundreds of miles or kilometers downwind from the tropical cyclone.Norman. W. Junker
Hurricanes and extreme rainfall.
Retrieved on 2006-02-13. The stronger the upper trough picking up the tropical cyclone, the more significant the left of track shift in the rainfall distribution tends to be.

Mountains

Moist air forced up the slopes of coastal hills and mountain chains can lead to much heavier rainfall than in the coastal plain. This heavy rainfall can lead to landslides, which still cause significant loss of life such as seen during

Global distribution

Globally, tropical cyclone rainfall is more common across the northern hemisphere than across the southern hemisphere. This is mainly due to the normal annual tropical cyclone distribution, as between half and two-thirds of all tropical cyclones form north of the equator. Rainfall is concentrated near the 15th parallel in both hemispheres, with a less steep dropoff seen with latitude across the northern hemisphere, due to the stronger warm water currents seen in that hemisphere which allow tropical cyclones to remain tropical in nature at higher latitudes than south of the equator. In the southern hemisphere, rainfall impacts will be most common between January and March, while north of the equator, tropical cyclone rainfall impacts are more common between June and November.

receives over half of its rainfall from typhoons.

United States tropical cyclone rainfall statistics

Between 1970–2004, inland flooding caused a majority of the tropical cyclone-related fatalities in the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

. This statistic changed in 2005, when

Hurricane Katrina Hurricane Katrina was a destructive Category 5 Atlantic hurricane that caused over 1,800 fatalities and $125 billion in damage in late August 2005, especially in the city of New Orleans and the surrounding areas. It was at the time the cost ...

's impact alone shifted the most deadly aspect of tropical cyclones back to

storm surge A storm surge, storm flood, tidal surge, or storm tide is a coastal flood or tsunami-like phenomenon of rising water commonly associated with low-pressure weather systems, such as cyclones. It is measured as the rise in water level above the ...

, which has historically been the most deadly aspect of strong tropical cyclones. On average, five tropical cyclones of at least tropical depression strength lead to rainfall across the contiguous United States annually, contributing around a quarter of the annual rainfall to the southeast United States. While many of these storms form in the Atlantic basin, some systems or their remnants move through Mexico from the Eastern Pacific basin. The average storm total rainfall for a tropical cyclone impacting the lower 48 from the Atlantic basin is about 16 inches (406 mm), with 70–75 percent of the storm total falling within a 24-hour period. The highest point total was seen during

Hurricane Harvey Hurricane Harvey was a devastating Category 4 hurricane that made landfall on Texas and Louisiana in August 2017, causing catastrophic flooding and more than 100 deaths. It is tied with 2005's Hurricane Katrina as the costliest ...

2017 File:2017 Events Collage V2.png, From top left, clockwise: The War Against ISIS at the Battle of Mosul (2016-2017); aftermath of the Manchester Arena bombing; The Solar eclipse of August 21, 2017 ("Great American Eclipse"); North Korea tests a s ...

, when 60.58 inches (1,538.7 mm) fell in southeast

Texas Texas (, ; Spanish: ''Texas'', ''Tejas'') is a state in the South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2020, it is the second-largest U.S. state by ...

Printed media

#Ivan Ray Tannehill. Hurricanes. Princeton University Press: Princeton, 1942. #Herbert Riehl. Tropical Meteorology. McGraw-Hill Book Company, Inc.: New York, 1954. #Terry Tucker. Beware the Hurricane! Hamilton Press: Bermuda, 1966.