Tornado Alley is a nickname for an area that consistently experiences a high frequency of tornadoes each year.

The area that has the most strong and violent tornadoes includes eastern SD, NE, KS, OK. Northern TX, and eastern Colorado.

The relatively flat land in the Great Plains allows cold dry polar air from Canada to meet warm moist tropical air from the Gulf of Mexico. A large number of tornadoes form when these two air masses meet, along a phenomenon known as a "dryline."

The highest frequency of tornadoes shifts geographically depending on the season. During the cooler months the South East U.S. receives the highest number of tornadoes.

During the months of March - May the threat shifts into the central United States. Then during the summer months the highest concentration of tornadoes tends to shift farther north.

Of course the central plains may be considered part of the tornado alley almost all-year-round. Tornado alley produces supercell thunderstorms in part because of a steady supply of warm, humid air. Supercells that spawn the biggest tornadoes need low-level winds that shift direction and grow stronger just above the ground.

The Gulf of Mexico provides an abundance of tropical moisture blowing into the Plains on south and southeast winds. Meanwhile the higher and drier elevations of the rocky mountain region allow a hot, dry layer of air to blow over the region from the southwest at about 3,000 feet above the ground.