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Where is Tornado Alley, and Why?

UPDATED May 10, 2010

By WeatherBug Meteorologists

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Most Americans are familiar with the term Tornado Alley. But do you really know where it`s located? Have you ever thought about why tornadoes occur most often in the spring and why they tend to be concentrated where they do?

The American Meteorological Society defines Tornado Alley as the corridor stretching from central Texas, northward into Oklahoma, Kansas and Nebraska, and eastward into central Illinois and Indiana. This region contains the maximum tornado frequency in the continental United States.

Recent research at the NOAA National Severe Storm Laboratory, however, has focused on the likelihood that tornadoes will cause significant damage in a much larger region. They designate a swath from central Texas, northward to South Dakota as having the greatest risk of "significant tornadoes." Such twisters are rated EF2 or higher on the Enhanced Fujita Tornado Scale and have winds over 113 mph.

The Plains are most vulnerable to tornado outbreaks because the region is the place where cold, dry air flowing south from Canada most frequently clashes with warm, moist air moving northward from the Gulf of Mexico.

A feature called the dry line can also provide a trigger. The dry line is a boundary between hot, dry desert air advancing east from the Desert Southwest versus the warm, moist airmass from the Gulf of Mexico. Tornado-producing thunderstorms frequently form just ahead of the dryline.

Tornadoes also form and survive best in areas with few obstructions, i.e. flat terrain and few trees, so the Plains are an ideal breeding ground. Hilly, forested areas in the East, and mountainous terrain in the West tend to suppress tornado formation.

Terrain, unstable air, and a triggering mechanism, such as a cold front or dry line, are not enough to cause tornadoes, however. Wind shear, or changes in wind direction and speed with height, is necessary to get a thunderstorm to rotate and produce a tornado.

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