Map Showing Tornado Risk Areas in the Conterminous United States, also known as 'Tornado Alley'.
The categories shown reflect the tornado recurrence interval at a single point. In the highest risk areas, a tornado of significant strength would be expected to occur once every 2000 years, at each point in the area shown. For the high-risk areas, the interval would be once every 5000 years. The data base is 800 cases of significant tornadoes for the period 1954-1992.
Source: U.S. Geological Survey