In the recent Straight Dope Classic #3, Do tornadoes only occur in North America? (Jan 10 2014, originally Aug 6 1993, http://www.straightdope.com/columns/read/1083/do-tornadoes-only-occur-in-north-america)
Cecil reports that tornadoes common form in the US because “you get warm, moist surface winds blowing up from the Gulf of Mexico, while cool high-altitude winds blow over the tops of the Rockies.”
This is not right. Tornadoes form so commonly in the US and Canada because there is one long plain that extends from the Arctic Ocean to the Gulf of Mexico without any geographical features to block the air masses from the Arctic from meeting those from the Gulf.
The Rockies actually block the winds from either direction, preventing the air from extending westward, they do not generate the cold air. This can be seen in a map of tornado locations, which sweep at a NW-SE diagonal that tracks the front of the mountain range. This is also one of the reasons that tornadoes do not form west of the Rockies - where there is warm air from the Pacific currents that give the west coast such a pleasant climate compared to the east coast at identical latitudes.
So the cold Arctic air sweeps across the lake-littered glacial tundra of Canada’s Northwest Territories into Canada’s Great Plains of Saskatchewan, Manitoba. northern Ontario and eastern Alberta, and onward to the US Great Plains where they meet the Gulf air coming north.
The many lakes of the tundra and plains of Canada and the northern US are also remnants of the glaciers. You will see a similar pattern between lake location and tornado formation because the Rockies blocked the glaciers as well. And the existence of this vast low-lying plain explains the childhood mystery of why there are fossils of fish and other marine life in Kansas.
An additional consequence was the formation of one of the world’s few high latitude desert regions in the Great Basin between the Rockies and the various mountain ranges that cross the region to the Pacific coastal chains (the other being the Taklamakan-Gobi Desert system where various mountain ranges and plateaus enclose the region in an east-west manner).