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Thank you for all of your support! I had asked you questions in the past and you were so timely and helpful. Why do tornadoes happen more in the Midwest of the United States? Students came up with it is because the land is flatter. Thought I should check it with the experts.


Tornadoes occur mostly in the plains states east of the Rocky Mountains and in the early Spring they occur primarily in association with cold fronts moving through the southeastern United States. Most people do not realize that the center of activity for early Spring thunderstorms and tornadoes is the southeast. Later, the center of activity moves to the southern plains states and in late Spring and early summer it moves to the central, northern, and eastern plains states (Midwest - Iowa, Illinois, Minnesota, Michigan, Ohio). The reason for this is, primarily, the position of the jet stream and associated warm and cold fronts. As Spring progresses, the jet stream moves westward and northward in response to surface warming that starts in the south and progresses westward and northward.

The convection that feeds severe thunderstorms is most easily started in flatter terrain. More hilly or mountainous terrain disrupts the convective process, but it is not the most important factor in the development of severe thunderstorms and tornadoes. Vertical wind shear, which the jet stream provides in abundance, is needed to spawn a tornado.

David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory

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