Moore: The Heart of Tornado Alley

The 1999 twister that struck Moore, Oklahoma, part of a 74-tornado outbreak pulverized entire neighborhoods with winds reaching 318 miles per hour, the strongest ever recorded on Earth. In 2013, a 2-Mile-Wide tornado tore down almost the exact path as its 1999 predecessor, and resulted in catastrophic damage to the town.
Meteorologists say it's extremely rare for any town to get hit twice by killer tornadoes. Why does Moore have such terrible luck? A brief guide:
Why is this part of Oklahoma so vulnerable to killer tornadoes?
The short explanation is that Moore sits at the heart of Tornado Alley. Tornado Alley extends from the Rocky Mountains in the West to the Appalachians in the East. Seventy-five percent of the world's tornadoes hit in the U.S. That's more than 1,000 per year. (Canada, at No. 2, has just 100 per year.) And nowhere are they more common than in the swath of the nation's midsection centered around Texas, Oklahoma, Kansas, and the states to the north.
What makes tornadoes so common there?
The Rockies block moist air from flowing eastward. This clears the way for frigid Arctic air to stream south from Canada over the Great Plains. These cold blasts then run into warm, humid air coming up from the Gulf of Mexico. When these cold and warm air masses collide, they cause powerful rotating updrafts and downdrafts that can create dangerous thunderstorms known as "supercells," which in turn spawn powerful tornadoes.

 http://theweek.com/articles/464148/why-tornado-alley-prone-disaster