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                     Deadly US Twister Hit 'Tornado Alley'

   Monday`s deadly tornado in the central U.S. state of Oklahoma struck in
   an area commonly known as "Tornado Alley" -home to most of the nation`s
   tornadoes.
   Tornado Alley is generally seen to span north to south from South
   Dakota to Texas between the Rocky Mountains and the Appalachian
   Mountains, but there is no official consensus on the boundaries.
   The region is susceptible to tornadoes when cold, dry air moving south
   from Canada meets warm, moist air from the Gulf of Mexico and warm, dry
   air moving east across the Rockies. The collision of warm and cold air
   breeds severe thunderstorms, which spawn tornadoes.
   The National Weather Service`s Storm Prediction Center says Tornado
   Alley is a nickname used by the media and stresses the it only
   indicates the area where most tornadoes occur. It says violent or
   killer tornadoes happen outside Tornado Alley every year and can strike
   almost anywhere in the United States, as well as in Canada and
   overseas.
   Studies have shown the deadliest twisters in the U.S. happen to the
   east of Tornado Alley, in an area known as "Dixie Alley." Florida also
   experiences a large number of tornadoes due to its frequent
   thunderstorms, but they are typically weak.
   Tornadoes can strike at any time, but they form most often in May and
   June in the late afternoon to early evening.
   On May 3, 1999, Moore, Oklahoma, the town devastated by Monday`s
   tornado, was hit by another deadly twister. It was one of more than 70
   tornadoes to touch down across Oklahoma and Kansas in just 21 hours.
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References

   1. http://www.voanews.com/content/deadly-us-twister-hit-tornado-alley/1665431.html