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John Wheeler, WDAY, Published May 02 2014

Weather Talk: Destructive weather can be a matter of geometry

Sometimes when the weather is highly destructive, it is because of an extremely powerful storm. Other times, it is a matter of geometry.

Last Tuesday and Tuesday night, a portion of the Gulf Coast region from Mobile, Ala., to Pensacola, Fla., received from 12 to 22 inches of rain. That rain, all of it, fell within 24 hours. The destruction was devastating, especially in and around Pensacola as roads and yards were washed out to a depth of many feet by the rushing flood water.

The thunderstorms were big storms, to be sure. But the ridiculous rain totals were a product of training thunderstorms. This is a process in which thunderstorms form and move over the same area over and over again. This happens not because the storms are necessarily all that strong but because there are so many repeating storms.

It requires a stationary trigger point, usually an area of low pressure, and a stationary front along which the storms move. A similar thing happened over the Fargo area in July 2000. The result was 7 to 10 inches of rain in one night and more than a billion dollars in property damage.

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