Acording to Atlas of Michigan (East Lansing: Michigan State University Press, 1977), which places this paragraph above a path map of an apparently really nasty tornado that hit southeastern Michigan on 12 April 1965:

Tornadoes are usually spawned by an advance of a strong cold front into a mass of warm, moist air from the Gulf of Mexico, the presence of dry air over the frontal zone of middle levels of the atmosphere, and high-speed winds aloft fostered by the jet stream. Such conditions, although they combine relatively infrequently in Michigan, have resulted in some devastating tornadoes in the state.

The atlas is useful for lots of things, though it’s obviously somewhat out of date (its color photograph of a grinning and youthful-looking William Milliken being the most obvious indication). Among its interesting factoids: the Washtenaw County area usually gets its first snowfall of an inch or over between November 22-29 or shortly thereafter, which pretty much jibes with our experience this past autumn.