The central plains of the United States are in a unique geographical position when it comes to tornado formations.
Seventy-five percent of the world's tornadoes occur in the United States and most of those occur in "Tornado Alley," which is located from Iowa/Nebraska to Texas.
Texas has the most tornadoes per year, 126, but this is also the biggest state as well.
The reason why we have tornado alley in the Plains has much to do the with geography.
The Gulf of Mexico, which is located to the south, supplies a rich and necessary amount of low level moisture to fuel thunderstorms.
Then, you have the Rocky Mountains to the west, and with the higher elevation, it is hard for the moisture from the Gulf of Mexico to reach back that far west, so you have a dry air mass in the western Plains.
This sets up a frequent clash of air masses between humid and dry creating a boundary where low level winds converge, aiding in making the air rise, which helps to create a few severe thunderstorms.
When you add in a storm system with strong winds aloft moving from west to east over this boundary, as south winds occur at the surface in the humid air and severe thunderstorms can greatly increase in coverage.
This is due to increased shear in the atmosphere, or winds blowing in different directions with height allowing building thunderstorms to rotate and grow, which is essential for a tornado to form.
Colder air can come in from the northwest, adding a third air mass to the equation. This only enhances the chance of tornado formation in Tornado Alley.