When tornadoes form in north America where does the air come from?

Often, but not always, tornadoes form when three different airs masses collide. The first, and probably most important, is a warm, moist air mass from the Gulf of Mexico. Next comes a cool air mass from northern Canada, and finally a dry air mass from the Rockies. When the collision occurs the warm, moist air mass essentially becomes fuel for very strong thunderstorms. Given a few other factors these storms can produce tornadoes.