The prolonged period of unseasonably warm temperatures during July and August across northern Illinois, northwest Indiana, 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 80 °F 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.
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.
The following graphics are a monthly breakdown of the average daily surface temperature observed this year versus the average daily surface temperature for the period between 1981 and 2009.
The last graphic depicts the daily average surface temperature from this year against the monthly average temperature from the time period between 1981 and 2009. The Lake Michigan sea surface temperatures remained very warm through August, until a strong cold front swept across the area early in September. Since then, cooler air temperatures and bouts of gusty west winds have cooled Lake Michigan surface temperatures by 10 to 20 degrees.
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.
Here is a bathymetry image of southern Lake Michigan from the National Geophysical Data Center.
Check out the NOAA National Geophysical Data Center Great Lakes Bathymetry viewer.
Marc Kavinsky - NWS Milwaukee/Sullivan
Gino Izzi - NWS Chicago/Romeoville