Huge fluctuations in the level of Lake Michigan

Ask Tom Why
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Dear Tom,
On May 6, I saw huge fluctuations in the level of Lake Michigan at Montrose Beach for about 20 minutes. What caused this?
— Albert Legzdins
Dear Albert,
You were probably seeing a seiche. “Probably” because we have received no other reports of the phenomenon and no thunderstorms had occurred. A seiche (pronounced “say-sh”) in southern Lake Michigan is usually the result of thunderstorms moving from the northwest and, upon hitting the lakefront, producing a gust front and air-pressure jump at the lake. The air pressure depresses the lake surface and establishes a wave that moves east across the lake, reflects off the Lower Michigan and Indiana shore and returns to the Chicago shore. It can result in a sudden lake rise of several feet in a period of a minute or two, followed by a sudden drop.

The largest seiche to occur on the Chicago lakefront struck on June 26, 1954. Thunderstorms moving 50-60 mph approached the southeast Wisconsin and Chicago area lakefront about 7:30 am, setting up an air-pressure jump that depressed the Lake Michigan surface and established a surge that pushed east/southeast across the lake. The surge reflected off the Lower Michigan and northern Indiana shore and, traveling at about 30 mph, returned to Chicago at about 9:35 am.

In less than one minute, the lake rose two to four feet on the Illinois shore of the lake, but reached a maximum height of ten feet from about the North Avenue Beach to Wilmette. Eight people were drowned in the Montrose Harbor area.


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