United States tropical cyclone rainfall climatology

Long term averages

On average, five tropical cyclones or their remnants lead to rainfall across the contiguous United States each year, contributing around a quarter of the annual rainfall to the southern 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.

Highest known tropical cyclones amounts for the lower 48 since 1976

Below is a list of the top ten highest known storm total rainfall amounts from individual tropical cyclones across the lower 48 since 1976. Much of the rainfall information was provided by the National Climatic Data Center in Asheville, North Carolina.

# 1219 mm/48.00 inches Amelia 1978
# 1143 mm/45.00 inches Claudette 1979
# 1033 mm/40.68 inches Allison 2001
# 977 mm / 38.46 inches Georges 1998
# 932 mm / 36.71 inches Danny 1997
# 707 mm / 27.85 inches Alberto 1994
# 652 mm / 25.67 inches Allison 1989
# 649 mm / 25.56 inches Dennis 1981
# 635 mm / 25.00 inches T.D. #1 1992
# 611 mm / 24.06 inches Floyd 1999David Roth. [http://www.hpc.ncep.noaa.gov/tropical/rain/tcmaxima.html Tropical Cyclone Rainfall Maxima.] Retrieved on 2007-03-15.]

Maximum tropical cyclone rainfall per state for the Lower 48

State maxima relating to tropical cyclones and their remnants are shown on the left, color coded by amount. Tropical cyclones from the Atlantic Basin have the most sway along the Gulf coast and Eastern Seaboard. The impact of tropical cyclones and their remnants originally from the eastern Pacific stretches as far east as Michigan and Indiana. It should be stated that rainfall related to the low pressure area once associated with a tropical cyclone, or its remnants aloft, are included in this sample. No additional rainfall from pre-existing upper lows as seen before cyclones such as Hurricane Fran of 1996 or from upper cyclones that closed off behind former tropical cyclones such as Hurricane Juan of 1985 was included.

The state of Texas has the highest amounts, followed by Florida, Alabama, and Mississippi. This is because over the 31 year sample, the storms that have become quasi-stationary like 2001's Tropical Storm Allison, 1978's Tropical Storm Amelia (1978), and Hurricane Danny (1997) are dominating the statistics. Out West, the same can be said for Hurricane Kathleen (1976) and the remains of Hurricane Olivia of the 1982 Pacific hurricane season. As the United States tropical cyclone rainfall climatology works back earlier into the 1970s, the amounts across the West and the Upper Ohio Valley are likely to increase due to the influence of Hurricane Agnes of 1972 and Hurricane Eloise in 1975. Data from the 1950s will increase amounts in New England.

Average and record statistics per time frame for the lower 48

To the right is a graphic showing averages and extremes for a 15 year sample of tropical cyclones and their remnants affecting the contiguous United States. The units of the rainfall amounts are in inches, while the time units are in hours. The bars in the graph express the percent of the storm total rainfall, which is defined to be 100 percent in the final column. Note that, on average, as much as one-fourth of the total occurs in 2-3 hours, while half falls within 12 hours, and almost three-quarters of the storm total falls within a 24 hour period. Cases where a cyclone scraped the coast were not separated out from those that made a more direct landfall. Also, Pacific and Atlantic cases were not separated. This all explains the average storm total of the sample being depressed to 338.8 mm/13.34". On the bottom of the graphic are listed the averages per time frame and the records. The records were mainly set during Tropical Storm Allison (2001) and Hurricane Danny (1997). This graphic will be updated as the climatology pushes farther back in time.cite web | url = http://www.hpc.ncep.noaa.gov/tropical/rain/tcrainfall.html | title = Tropical Cyclone Rainfall Data | accessdate = 2007-01-13 | last = Roth | first = David | date = 2007-01-01 | work = Hydrometeorological Prediction Center Home Page | publisher = National Weather Service ]

United States rules of thumb for tropical cyclone rainfall forecasting

Kraft rule

During the late 1950s, this rule of thumb came into being, developed by R. H. Kraft. [Frank Marks. [http://ams.confex.com/ams/last2000/techprogram/paper_12681.htm WSR-88D Derived Rainfall Distributions in Hurricane Danny (1997).] Retrieved on 2007-04-13.] 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.Norman W. Junker. [http://www.hpc.ncep.noaa.gov/research/mcs_web_test_test_files/page0024.htm Hurricanes and Extreme Rainfall.] Retrieved on 2007-03-15.] 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. Canada uses a modified version of the Kraft rule which divides the results by a factor of two, which takes into account the lower sea surface temperatures seen around Atlantic Canada and the prevalence of systems undergoing vertical wind shear at their northerly latitudes. [David M. Roth [http://www.hpc.ncep.noaa.gov/research/roth/TC_QPF_talk072007.ppt Tropical Cyclone Rainfall (July 2007 presentation).] Retrieved on 2007-07-19.] 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.

Eight inch or 203 mm rule

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.

ixteen inch or 406 mm rule

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. [David Roth. [http://www.ofcm.gov/ihc06/Presentations/06%20session6%20Decision-making%20Products/s6-04roth.ppt Tropical Cyclone QPF.] Retrieved on 2007-03-15.] When removing the storms that grazed the domain, an average of near 16 inches/406 mm was obtained. [David Roth. Developing a Recent Tropical Cyclone Rainfall Climatology For the United States. 25th Conference on Hurricanes and Tropical Meteorology. American Meteorological Society, 2002.] 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 mountain zones, interacts with a nearby front, or the storm is significantly sheared.

ee also

*List of wettest tropical cyclones in the United States
*Tropical cyclone
*Tropical cyclone rainfall climatology
*Tropical cyclone rainfall forecasting

References

External links

* [http://www.hpc.ncep.noaa.gov/tropical/rain/tcrainfall.html Individual Tropical Cyclone Rainfall Pages for United States]