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)|
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.
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:
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.