Why do the most powerful Tornadoes occur in the US?

Answer 1

In short, the Central United States has just about the perfect climate for producing tornadoes.

The long version: Most strong tornadoes in the U.S. occur between the Rockies and the Appalachians, particularly on the Great Plains in a region called Tornado Alley. In the region between the two mountain ranges cold air from Canada collides with warm, moist air from the Gulf of Mexico, forming a cold front. The cold front lifts the warm, moist air, sparking the development of thunderstorms. In Tornado Alley a mass of dry air from the Rockies may also move in, forming a dry line. This can also generate thunderstorms and can be even more conducive for the formation of strong tornadoes than a cold front. Activity is especially intense at the triple point, where a dry line intersects a cold front

Other key factors come into play as well. There is a layer of stable air about 6,000 feet above the ground called a cap, which initially prevents thunderstorms from developing. This can lead to great instability as the lower atmosphere heats up throughout the day. When the storms break through the cap the atmosphere will contain an enormous amount of latent energy to power them. Finally there is wind shear, a change in wind speed and direction with altitude. This strengthens the storms by separating the updraft from the downdraft so they don't interfere with each other and effectively increases the vertical pressure gradient, strengthening the updraft even more. It also give the storms the rotation they need to produce tornadoes.

When all these ingredients come together, conditions are ripe for an outbreak with strong tornadoes. Such a setup, however, is rare outside the U.S.