The United States experiences more tornadoes than any place else in the world. They are most prevalent in a line from West Central Texas northward to South Dakota, perhaps 500 miles wide. This is where the ingredients in the mixing bowl come together most often.
Cool dry air from the Rocky Mountains mixes with warm dry air from the desert Southwest. When these two air masses collide with warm, moist air being pulled up from the Gulf of Mexico, coupled with wind shear, the ingredients are all in place to produce supercell thunderstorms; the mothers of most tornadoes.
However, Tornado Alley does not mean that the weather conditions are favorable only in that part of the United States. Tornadoes have occurred in all fifty states, but are most common east of the Rocky Mountains. In fact, the worst tornado outbreak in modern times didn't even occur in what is commonly referred to as Tornado Alley. The "Super Outbreak", as it is called, occurred east of the Mississippi River and affected thirteen states on April 3rd, 1974. The final toll was 148 tornadoes is less than 24 hours. At one time, there were 15 tornadoes on the ground simultaneously in Indiana alone!
Most often, Tornado Alley is the target for storm chasers as the flat lands and good road systems (sometimes) allow the chasers to better observe the development of supercell thunderstorms and study the formation of tornadoes. This serves a dual purpose. Some chasers have tours, where regular citizens interested in severe weather pay to travel with the chasers. Some chase for science, some for the thrill, but all play an integral role in keeping the local National Weather Service offices informed of the location and intensity of tornadoes. As good a tool as Doppler Radar is for detecting rotation, it is still limited in its capacity to tell if a tornado is on the ground.
Oklahoma City is definitely ground zero for tornadoes. In its history, Oklahoma City has been struck over 100 times. Other major metropolitan areas in Tornado Alley have seen their share, but nothing like Oklahoma City.
In fact, some of the most commonly hit areas in the country are not even in what is commonly referred to as Tornado Alley. Tom Grazulis is the foremost authority on tornado history in the United States. He is the author of two books that list every significant tornado is the States he could find during his exhaustive study. His research has helped in a study that identifies at least four Tornado Alleys. The research was done by Casey Crosbie and Chris Boyles of the Storm Prediction Center in Norman, Oklahoma. Here is their list of the top 10 cities most frequented by violent tornadoes:
1: Oklahoma City, Oklahoma
2: Tulsa, Oklahoma
3: Shreveport, Louisiana
4: South Bend, Indiana
5: Indianapolis, Indiana
6: Huntsville, Alabama
7: Topeka, Kansas
8: Springfield, Illinois
9: Jackson, Mississippi
10: Nashville, Tennessee
To conclude, Tornado Alley is just a name given to a region in the United States. It got its name in the 40's and 50's, when scientists thought that a mere 200 tornadoes hit the United States in any given year. They estimated wind speeds as high as 500 miles per hour during this era. We now know that a figure of 1,200 tornadoes is more feasible and wind speeds rarely exceed 300 mph. As our understanding improves, we now know that Tornado Alley is much different than is commonly perceived. For further reading, here are a few very interesting sites:
These sites are full of information and links to other great sites concerning all types of severe weather.