During the summer months, the primary jet stream is juxtapositioned along the U.S. and Canadian border. Winds aloft over northern Illinois are weaker, and weather systems often stall out across the area or are slow-moving. Low-level ENE flow associated with high pressure to the north or a lake-breeze off lake Michigan collides with a southwesterly flow from an established warm moisture-laden air mass, creating vertical motion–a lifting of air, and the convective thunderstorm-producing process begins. Embedded pockets of cooler air (“short waves”) drifting in the weak flow aloft, enhance storm development–leading to prolonged rains over smaller areas and localized flash flooding. Prime times for these convective storms are late at night or during the afternoon. In 2001, an unusually active summer, 8 heavy rainfall events occurred in the city–the most intense on August 2 when 4.8 inches fell in 3 hours, filling the deep tunnel and forcing the release of polluted storm water into Lake Michigan.
WGN-TV/Paul Dailey, Thomas Valle
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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN-TV News at noon and 9 p.m.
WGN-TV meteorologists Steve Kahn, Richard Koeneman and Paul Dailey plus weather producer Bill Snyder contribute to this page.
