It is a thing that exists as far as the experiences of the residents east of the Rockies and toward Appalachia, running from the Gulf up into the Canadian hinterlands up the middle of the continent. If you've ever looked at the strong lines of thunderstorms as they sweep across the prairies, you know that the storms form because of the cold air coming down from Canada hitting the warm air that funnels up from the Gulf of Mexico. The convergence effect isn't hindered by updrafts off of mountains.

There may not be a built structure, there is a corridor of weather, and calling it Tornado Alley makes perfect sense if you've ever lived here. Tornado Alley is at least as well defined as "Bible Belt," and there is no such thing as a Bible Belt, either, but we all understand what it means (and alas, I have dual citizenship in Tornado Alley and the Bible Belt.)

SRS