The water temperatures along coastal regions of Lakes Michigan and Superior took an astounding 25 to 30 degree plunge overnight on Monday. Parts of Lake Superior bottomed-out at 38°F, while parts of eastern Lake Michigan hit 41°F. These temperatures are more common in April than the last week in July.
The water managed to get this cold through upwelling, which occurs when cold waters towards the bottom of a body of water rises towards the surface. WOOD-TV meteorologist Bill Steffen described what caused the upwelling event:
We had a strong north wind behind a cold front and that pushed the surface water toward the middle of the lake, allowing much colder water from 100 feet below to rise to the surface.
The cold water's effect on the weather will remain localized, with cooler temperatures along the coast as well as persistent fog over coastal waters.
As seen by the satellite imagery at the top of this post, the rapidly-chilled water created a thick layer of fog on Lake Michigan where temperatures dropped the most. The cold water cooled the air above it through conduction, dropping the air temperature to its dew point and condensing its water vapor.
The average temperature for Lake Michigan at the end of July is around 68°F, and the average temperature for Lake Superior sits around 60°F.
[Images via MODIS and GLERL]