Dear Tom,
I’ve noticed that the heavy, drenching rains in recent months have stayed to the north and northwest of Chicago in Lake and McHenry Counties. Why this pattern?
Erik Rasmussen, River Forest, IL
Dear Erik,
Illinois counties bordering Wisconsin have indeed borne the brunt of recent heavy rains. The area is but the southernmost extension of a 250-mile wide band which has been saturated–a zone which extends north across central and northern Wisconsin. Recent 30 day rains have fallen at rates two or more times “normal”. Records of observer Phil King in McHenry County’s Hebron tell the story. Rainfall there since mid-April has totaled 20.25 inches, 8 inches more than Chicago during the same period.
The tendency of fronts to drop south over Lake Michigan’s chilly spring/early summer waters, then stall over its south end, may be a factor. When fronts at that location are overrun by moisture, the heavy rains fall some distance to the north, favoring locations like Lake and McHenry counties, among others. But a “feedback,” not unlike the one that contributed to 1993’s massive flooding in Iowa and on the Mississippi, may also be occurring. Wet ground, with the help of plants, feed moisture back to the atmosphere, enhancing later storms. Once these feedbacks get going, they can persist for a while.
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Tom Skilling is chief meteorologist at WGN-TV. His weather forecasts can be seen Monday through Friday on WGN News at noon and 9 p.m.
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