Extreme Rainfall Associated with Landfalling Tropical Cyclones along the Northern Gulf Coast

 

satellite imagery

 

 

Rainfall associated with landfalling tropical cyclones has long been a personal interest of mine, especially extreme rainfall amounts and where they occur in relation to the passage of the eye. One of the old, long established starting point rules has been to estimate the maximum rainfall associated with landfalling tropical cyclones by taking 100 and dividing it by the storm speed in knots. For example, a landfalling tropical cyclone moving at 25 knots would produce an estimated rainfall maximum of 4 inches, whereas a storm moving at 10 knots would produce an estimated maximum of 10 inches.

This study, which will be presented in poster form at the upcoming National Weather Association meeting in Oklahoma City, used rainfall data from cooperative observers in 30 tropical cyclones that made landfall between Lake Charles, LA, and Apalachicola, FL, between 1960 and 1997. I wanted to test the old 'rule', and perhaps come up with a better relationship to use operationally.

Here's the raw data that I used. The speeds are general estimates of storm motion derived from the historical plots available from the TPC home page.

name dates month year landfall spd (kts) max rptd rain (in)
ethel 15,16 sep 1960 8.5 8
florence 26,27 sep 1960 12 9.96
hilda 3,4 oct 1964 9.6 17.71
unnamed 14,15,16 jun 1965 22 8.71
betsy 9,10 sep 1965 25 5.8
camille 17,18 aug 1969 20 10.6
becky 21,22,23 july 1970 21 7.64
fern 5,6,7 sep 1971 6.8 6.61
edith 15,16 sep 1971 20.5 8.29
agnes 19,20 jun 1972 13.8 7.17
carmen 7,8 sep 1974 14.6 4.65
eloise 23,24 sep 1975 27.5 9.27
babe 4,5,6 sep 1977 9.2 10.65
debra 28,29 aug 1978 15.8 8.29
bob 11,12 july 1979 24.6 7
frederic 12,13 sep 1979 19.8 10.11
chris 10,11 sep 1982 12.5 9.65
danny 15,16 aug 1985 16.3 8.57
elena 1,2,3 sep 1985 20.8 8.75
juan II 31,1 oct,nov 1985 23 4.18
kate 21,22 nov 1985 16.7 8.32
beryl 8,9,10 aug 1988 6.7 9.16
florence 9,10 sep 1988 19.8 4.89
andrew 25,26,27 aug 1992 16.7 11.2
alberto 3,4,5 july 1994 16.7 14
beryl 14,15,16 aug 1994 6.3 10.69
erin 3,4 aug 1995 12.9 7.83
opal 4,5 oct 1995 25.8 19.06
allison 4,5 jun 1997 16.7 7.46
danny I 17,18 jul 1997 8.3 9.2

 

These three storms were included in the study but were also excluded when it came time to find a best fit straight line because they were very slow moving and produced torrential rainfall amounts.

 

juan I 28,29,30,31 oct 1985 3 14.18
alberto II 3,4,5,6,7,8,9 july 1994 2.5 27.61
danny II 19,20,21 jul 1997 2.5 35.31

 

Using this data, I plotted the speed on the y axis and the maximum reported rainfall on the x axis to come up with a scatter diagram:

 

 

 

satellite imagery

 

 

As you can see, a best fit line will be parabolic, but is unexpectedly straight between 6 and 30 knots! If the three points mentioned previously are excluded, a best fit straight line can be drawn with a y intercept of 9.75 inches and a slope of -0.039. Thus, one can estimate the maximum rainfall from a landfalling tropical cyclone moving greater than 6 knots by using the formula

 

Rmax = -0.039 x ( speed (kts)) + 9.75 inches

 

with a standard deviation of 3.3 inches (pretty big!). For example, if a landfalling tropical cyclone is moving at 15 knots, then 9.17 inches of rain +/- 3.3 inches can be expected. Of course, with such a huge standard deviation, perhaps the best thing to do is always state that from 6 to 12 inches of rain can be expected with any storm moving faster than 6 knots.

For storms moving less than 6 knots, all bets are off. Only three data points are available for these very slow speeds, and the maximum rainfall amounts range from 14 to over 35 inches! It's very clear from Alberto and Danny in this decade that stalling or very slow moving tropical systems are a nightmare and should be taken very seriously.

Next week we'll look at the geographical distribution of the heaviest rainfall and it's relation to speed and angle of approach to the coast.

RLP

 


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