By Marc Kavinsky and Ashley Sears, NWS Milwaukee

The prolonged period of unseasonably warm temperatures during July and August across southern Wisconsin and surrounding regions pushed the Lake Michigan sea surface temperature to 80 degrees several times during the month of August at the south Lake Michigan buoy located 43 miles east-southeast of Milwaukee.  The buoy reached 80F on August 1st, August 10th, and August 12th. 

Checking the historical database for the south Lake Michigan buoy which dates back to 1981, the warmest sea surface temperature ever recorded at the south Lake Michigan buoy was 81 degrees, set on August 18th, 1995.  Overall, the buoy has recorded temperatures at 80 or above just six times between the time period of 1981 and 2009, with all occurrences being recorded in the month of August.

The two graphics below compare the average daily sea surface temperature from April through September 13 across Lake Michigan (graphic 1) and Lake Superior (graphic 2) to the 30 year sea surface temperature average for the individual lakes. The daily average temperature for each lake is depicted by the red line. The blue line provides the 30 year average. Comparing the daily average to the 30 year average, it can be noted that both Lake Michigan and Lake Superior have observed above normal temperatures through the summer months. The sharp decrease, especially recorded in the Lake Superior observations, is contributed to two major wind events that occurred in early September that allowed mixing of the lake, bringing the cooler temperatures to the surface.

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Graphics Courtesy of Jay Austin, Large Lakes Observatory - University of Minnesota, Duluth. Data Courtesy of the National Data Buoy Center

The sea surface temperature image below taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) equipment on a polar orbiting satellite in mid-July show the sea surface temperature had warmed into the middle to upper 70s across the southern mid-lake waters. 

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The below sea surface temperature image taken by the Advanced Very High Resolution Radiometer (AVHRR) equipment on a polar orbiting satellite on September 12th shows that the sea surface temperature had cooled into the middle 60s in the southern mid-lake waters.  Sea surface temperatures just east of Southeast Wisconsin have cooled into the 50s.

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The coolest temperatures were located east of Port Washington and Sheboygan, where a steeper dropoff in the bathymetry of Lake Michigan and deeper, colder waters were located. 

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Check out the NOAA National Geophysical Data Center Great Lakes Bathymetry viewer.

See this post here.