Most tornadoes occur east of the Rocky Mountains in a region known as "Tornado Alley." The vast majorities of tornadoes occur in the deep south as well as the broad, relatively flat basin between the Rockies and the Appalachians, however no state is immune. Often, the most dangerous tornadoes occur in the deep South and Southeast, where low visibility because of trees and hills lead to a false sense of security during severe weather outbreaks. Furthermore, the South and Southeast tends to be more heavily populated than the Plains states. This region has been given the nickname "Dixie Alley."
Peak months of tornado activity in the U.S. are April, May, and June, but tornadoes have occurred in every month of the year. Although they are possible any time of the day or night, tornadoes tend to occur in the late afternoon and early evening hours, when the atmospheric conditions are most ripe for supercell thunderstorms. They are most common from 4pm to 9pm.
Tornado Alley is so active with severe storms because of its unique location.
Tornado Alley
Tornado Alley is a nickname given to the plains region of the U.S. that experiences a high frequency of tornadoes, many of which are violent tornadoes (classified as EF-3 or greater). With the Rocky Mountains to the west, and the warm waters of the Gulf of Mexico to the south, this region is in a ripe location to produce supercell thunderstorms.
Midlatitude cyclones, like the one illustrated above with the red "L" and fronts, are the large-scale phenomena that spawn storms that create tornadoes. As midlatitude cyclones move eastward off the Rockies, they can tap into the warm, humid air from the Gulf of Mexico. This air surges northward at the low levels as cold, dry air at upper levels is pulled south by the storm. These are the basic ingredients for instability, which every thunderstorm needs to grow. Warm, moist air is less dense than cold, dry air. In this density game, the surface air will begin to rise, and in an unstable atmosphere, what goes up will keep going up. Air moving upward is what creates our clouds, rain, and our thunderstorms. If the upper-level jet stream is strong in this storm, the turning of the winds with height, or "wind shear," will aid in creating supercell thunderstorms and tornadoes are possible. All tornadoes are spawned from a parent supercell, but not all supercells produce tornadoes.
Damage from the 1999 F5 tornado that struck Moore, Oklahoma. Photo courtesy NOAA.