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Tropical cyclones
Formation and naming
Development - Structure Naming - Seasonal lists - Full list
Effects
Effects Watches and warnings - Storm surge Notable storms - Retired hurricanes
Climatology and tracking
Basins - RSMCs - TCWCs - Scales Observation - Rainfall forecasting - Rainfall climatology

While flooding is common to tropical cyclones near a landmass, there are a few factors which lead to excessive rainfall from tropical cyclones. Slow motion, as was seen during Hurricane Danny (1997) and Hurricane Wilma, can lead to high amounts. The presence of topography near the coast, as is the case across much of Mexico, Haiti, the Dominican Republic, much of Central America, Madagascar, Réunion, China, and Japan acts to magnify amounts due to upslope flow into the mountains. Strong upper level forcing from a trough moving through the Westerlies, as was the case during Hurricane Floyd, can lead to high amounts even from systems moving at an average forward motion. A combination of two of these factors could be especially crippling, as was seen during Hurricane Mitch in Central America.^[1]

One of the most significant threats from tropical cyclones is heavy rainfall. Between 1970-2004, inland flooding from tropical cyclones caused a majority of the fatalities from tropical cyclones in the United States.^[2] This statistic changed in 2005, when Hurricane Katrina's impact alone shifted the most deadly aspect of tropical cyclones back to storm surge, which has historically been the most deadly aspect of strong tropical cyclones.^[3] During the 2005 season, flooding related to Hurricane Stan's broad circulation lead to 1662-2000 deaths.^[4]

Contents 1 Rainfall distribution around a tropical cyclone 1.1 General distribution 1.2 Storm size 1.3 Slow/looping motion on rainfall magnitude 1.4 Vertical wind shear 1.5 Interaction with frontal boundaries/upper level troughs 2 Tools used to forecast tropical cyclone rainfall 2.1 Climatology and persistence 2.2 Numerical weather prediction 2.2.1 Kraft rule of thumb 2.2.2 The 8 inch/203 mm rule of thumb 2.2.3 The 16 inch/406 mm rule of thumb 3 See also 4 References 4.1 Printed media 4.2 World Wide Web

See also: Tropical cyclone rainfall climatology

Isaac Cline underwent the first investigations into rainfall distribution around tropical cyclones in the early 1900s. He found that a larger proportion of rainfall typically 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 found that some tropical cyclones can have their highest rainfall rates in the right rear quadrant within a training (non-moving) inflow band (Tannehill 1942). Rainfall is found to be strongest in their inner core, within a degree of latitude of the center, with lesser amounts farther away from the center (Riehl 1954). Most of the rainfall in hurricanes is concentrated within its radius of gale-force winds.^[5]

Larger tropical cyclones have larger rain shields, which can lead to higher rainfall amounts farther from the cyclone's center.^[5] This is generally due to the longer time frame rainfall falls at any one spot in a larger system, as long as forward motion is similar to that of 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.

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,^[6] 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.^[7]

Vertical wind shear 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.^[8] 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.

As a tropical cyclone 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.^[9]

The Hurricane Research Division of the Atlantic Oceanographic and Meteorological Laboratory created the r-CLIPER (rainfall climatology and persistence) model to act as a baseline for all verification regarding tropical cyclone rainfall. The theory is, if the global forecast models cannot beat predictions based on climatology, then there is no skill in their use. There is a definite advantage to using the forecast track with r-CLIPER because it could be run out 120 hours/five days with no problems.^[10] The short range variation which uses persistence is the Tropical Rainfall Potential technique (TRaP) technique, which uses satellite-derived rainfall amounts from microwave imaging satellites and extrapolates the current rainfall configuration forward for 24 hours along the current forecast track.^[11] This technique's main flaw is that it assumes a steady state tropical cyclone which undergoes little structural change with time, which is why it is only run forward for 24 hours into the future.^[12]

Further information: Tropical cyclone prediction model

Computer models can be used to diagnose the magnitude of tropical cyclone rainfall. Since forecast models output their information on a grid, they only give a general idea as to the areal coverage of moderate to heavy rainfall. No current forecast models run at a small enough grid scale (1 km or smaller) to able to detect the absolute maxima measured within tropical cyclones. Of the United States forecasting models, the best performing model for tropical cyclone rainfall forecasting is known as the GFS, or Global Forecasting System.^[13] The GFDL model has been shown to have a high bias concerning the magnitude of heavier core rains within tropical cyclones.^[14]

During the 1950's and 1960's, this rule of thumb came into being, developed by R. H. Kraft. It was noted from rainfall amounts (in imperial units) reported by the first order rainfall network in the United States that the storm total rainfall fit a simple equation: 100 divided by the speed of motion in knots.^[9] This rule works as long as a tropical cyclone is moving and only the first order or synoptic station network (with observations spaced about 60 miles/100 km apart) are used to derive storm totals. The main problem with this rule is that the rainfall observing network is denser than either the synoptic reporting network or the first order station networks, which means the absolute maximum is likely to be underestimated. Another problem is that it does not take the size of the tropical cyclone or topography into account.

Rusty Pfost, now the head of the Miami National Weather Service Forecast Office, did a study in 1999 reviewing rainfall totals from tropical systems affecting Florida between 1960 and 1998. He found that for tropical cyclones moving at greater than 6 knots, the average storm total was normally in the 5–10 inch (127–254 mm) range. Slower moving storms usually forced greater than 15 inches (381 mm) of rain to fall.^[9]

David Roth, a forecaster at the Hydrometeorological Prediction Center, determined that the average amount for all tropical cyclones impacting the United States was 13.34 inches (339 mm) between 1991 and 2005.^[15] When removing the storms that grazed the domain, an average of near 16 inches/406 mm was obtained. Using this latter amount appears to work best for systems that experience little vertical wind shear and are of at least average size. Amounts measured in small/midget tropical cyclones showed storm total amounts closer to 6 inches/152 mm. Operationally, variations to these amounts are introduced if the cyclone encounters topography or a nearby frontal zone, sea surface temperatures underneath the tropical cyclone are anticipated to drop below 26 Celsius prior to landfall, or the storm is significantly sheared.

• Rule of thumb
• Tropical cyclones
• Tropical cyclone rainfall climatology
• Tropical cyclone observation

1. Ivan Ray Tannehill. Hurricanes. Princeton University Press: Princeton, 1942.
2. Herbert Riehl. Tropical Meteorology. McGraw-Hill Book Company, Inc.: New York, 1954.