Why do so many tornadoes and thunderstorms happen in the United States?

Answer 1

The United States of America has regions with diverse climates. In addition, the US has 3 major areas of advancing fronts that can influence the kinds of weather patterns that can spawn thunderstorms and tornadoes. (This reply rightly omits any fronts originating off the Pacific or Atlantic coasts.)

The 3 major areas that generate our weather are:

1. A front over the Great Plains that comes nearly straight across, west to east. It may start off lower, from Kansas and Oklahoma, with a northeasterly track before turning to the east.
2. A front from Canada that comes over the Great Lakes and and stay high in upper States, or take a deep southerly tract.
3. Weather fronts arising from the Gulf of Mexico can be mediated or intensified by the 2 named fronts, above, or can mediate and intensify them. - It can intensify one of the other fronts by bringing up warm, moist atmosphere from the Gulf. If the Canadian front does not drop low enough into the US, or is not strong enough, the Gulf moisture adds to the west-east front coming across the Plains. Depending how far west the Gulf front extends over Texas, it can pack a wallop of air heated over southern states and packed with moisture from the Gulf. If heading up from a hurricane, this kind of front can lower barometers even inland in northern states like OH and PA. On the other hand, a strong Canadian front with its usually cooler air can push toward a Gulf front, preventing it from coming as far north. Wicked storms can still take place in Southern States, but the west to east path of advancing fronts from the plains won't get as much moist fuel from the Gulf.

Because of the unique patterns of fronts and climates, many areas are always at risk for a "one-off" tornado. For example, in 1985, Niles Ohio was hit with a strong twister. Unusual places can be hit, such as in Uniontown PA in Feb 2018 that got hit with an F1.

Other places are always at high risk for frequent thunderstorms and tornadoes, specifically The Great Plains which includes Tornado Alley. There, storms can be intense, with hail and lightning. Tornadoes can be a mile wide in the air, and cover 1 mile distances on the ground. A twister can touch down, weaken, disappear into the clouds, move on and strengthen, before then touching down again miles away (hit-skip), thus extending its life and potential for damages.