Dear Tom,
What determines when there will be lake-effect snow in the Chicago area?
I have noticed that many times northeast winds off the lake do not produce snow. At other times, the northeast winds will wallop us with white stuff. What is the difference?
Alan Brunettin, Evanston
Dear Alan,
Wind direction is obviously important because the wind must blow from lake to land in order for lake-effect snow to occur, but air temperatures are absolutely critical as well. Snow will not develop if air moving across Lake Michigan is not sufficiently cold.
Forecasters monitor air temperatures 4,000-5,000 feet above the lake to predict snow (air below that height is usually warmed by the lake), and the onset temperature is about 15 degrees — but the colder, the better.
Air that cold (4,000-5,000 feet aloft) provides sufficient instability to mix moisture evaporated from the lake upward high enough to generate clouds and snow. Once snow occurs, favorable winds can carry it to Chicago.
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