April 5, 2005
By Michael Schirber
LiveScience Staff Writer

The time to be most wary of a tornado is a spring afternoon in Texas or Oklahoma with thunderstorms brewing. But twisters do not limit themselves to these conditions or locations, a new study shows.

"If you're driving in a midnight rain in October near Lake Michigan, remember that a tornado is not outside the realm of possibility," says Robert Trapp of Purdue University.

Trapp and his colleagues studied more than 3,800 tornadoes in the United States from 1998 to 2000. Many of these were not of the typical variety that form in Tornado Alley – the flat, twister-prone region through the central plain states.

"In the heart of Tornado Alley, twisters most often develop from relatively small 'cell' storms that look like blotches on a Doppler radar weather map," Trapp said.

The conventional wisdom is that the tornado threat goes down when the cells merge into 100-mile-long line storms. But Trapp’s team found this to be wrong, especially beyond the Alley. For example, about half of Indiana’s 20 tornadoes a year come from line storms.

Nationwide, 79 percent of tornadoes arise out of cells, whereas 18 percent form from line storms, according to the study, which was supported by the National Science Foundation and reported in the February issue of the journal Weather and Forecasting.

"This implies that we may be overlooking many tornado-breeding storms in the Midwest and elsewhere," Trapp said.

While Texas leads the nation with an average of 125 tornadoes every year, and Florida has more per square mile than anywhere else, a handful of twisters strike several out-of-the-Alley states, including Maine (2 per year), Arizona (3) and California (5). [Map]

Most of what meteorologists know about tornadoes is based on studying cell-type storms in the central plains, however. This prejudice may be because these tornadoes tend to be stronger and more frequent than those born outside the Alley.

"We're not trying to be alarmist with these findings," Trapp said. "But we hope that people will stay alert to tornado risk even outside the traditional severe storm season."

http://www.livescience.com/forcesofnature/050405_tornado_midwest.html