Why can a tornado happen in St. Louis Missouri?

Answer 1

Missouri is partly within a region commonly called Tornado Alley, a region of the United States that has the highest amount of tornado activity in the world. The region is the site of frequent collisions of warm, moist air from the Gulf of Mexico, cool air from Canada, and dry air from the Rockies. These collisions result in very strong thunderstorms, with the moist air acting as their "fuel." Additionally, the region often has a layer of stable air called a cap that suppresses storms. This behaves like the lid on a pressure cooker, allowing the atmosphere below it to become increasingly unstable until storms develop explosively. Finally, the region often sees strong vertical wind shear, or a variation of wind speed and direction with altitude. This tilts them in such a way that the updraft and downdraft portions becomes separated and don't interfere with one another, letting the already strong storm further strengthen and last longer. The wind shear also sets the storms rotating, turning them into supercells, the most powerful thunderstorms on earth. It is the rotation within these supercells that can develop into a tornado.

Other parts of the world do see such conditions and thunderstorms at least occasionally, but not with the same frequency as in Tornado Alley.

Answer 2

Tornadoes can hit almost anywhere. St Louis is just a little bit east of Tornado Alley, the most tornado prone region in the world, and contrary to popular belief, there is nothing that protects major cities from tornadoes.