Dear Tom,
On a summer night when the forecast was for clear weather, I was awakened at 4 a.m. by a torrential downpour. What happened?
Jim O’Shea, Chicago
Dear Jim,
Without specifics, it is impossible to know what happened on the night in question. One possibility is that you experienced a “derecho”, a fast-moving thunderstorm cluster that often hits the Midwest during the early morning hours in the summer.
Strong thunderstorms form during the evening in the northern Plains, then move southeast under a strong northwestly jet stream at speeds in excess of 50 or 60 m.p.h., producing damaging winds and heavy rainfall. The high rate of speed means that storms that don’t even exist in the afternoon when forecasts are made, can form in the Plains in the evening and reach Chicago before morning, occasionally catching forecasters by surprise.
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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN News at noon and 9 p.m.
Write to: ASK TOM WHY, 2501 Bradley Place, Chicago, IL 60618 or asktomwhy@wgntv.com (Mail volume precludes personal response.)
