(Updated to include closeup of central and southern Lake Michigan surface temperatures from early Saturday afternoon.)
Lake Michigan water temperatures have been warming accordingly in response to the unseasonably warm temperatures we have been experiencing this spring. As of Friday morning, the average Lake Michigan water temperature was 57oF (14oC). This is about 2-4oC above the 19 year average, from 1992 to 2011.
Here is a graphical illustration of the actual sea surface temperature taken early Saturday afternoon, as measured by the MODIS instrument on the passing polar orbiting satellite. Most of southern Lake Michigan had warmed into the upper 50s to middle 60s. Notice that the far southern shallower part of Lake Michigan, north of Indiana has warmed into the middle to upper 60s. Most inland lakes and even those larger river expanses have warmed into the 70s.
Below is the MODIS image from May 10th showing lake temperatures mostly in the middle 40s to lower 50s.
And on April 6th, lake temperatures were mostly in the lower to middle 40s.
The below image shows how the 2012 Lake Michigan average temperature compares to the past 5 years. The mid-March record warmth can be clearly seen by the temperature trace being warmer by about 5 degrees over the past 5 years.
Since then, however, the average Lake Michigan water temperature has cooled but has remained about 2-4 degrees C above the average from April into early June.
National Weather Service Milwaukee/Sullivan