The unseasonably warm weather of the past week along with the warm spring temperatures have contributed to Lake Michigan surface temperatures warming into the upper 60s to upper 70s. The below image was taken Friday morning, July 6th, by the MODIS instrument package on polar-orbiting satellite Terra or Aqua. Most of southern Lake Michigan had warmed into the 70s as indicated by the orange color.
In contrast, below is the MODIS image taken one year ago, on July 6th, 2011. Surface temperatures were about 5 to 10 degrees cooler, mostly in the low to mid 60s.
The graph below shows this year's average lake temperature compared to the previous five years. As you can see, this year is about four degrees warmer than the average lake temperature of the past five years.
In fact, the south mid lake buoy reached 80 degrees on Friday, July 6th. This is rather rare, as in the 31 years of data, a temperature of 80 degrees has only been recorded during six prior episodes. This is also the earliest 80 degrees has ever been recorded at the south mid lake buoy.*
It is also worth noting that during the past warm period between, July 1st and July 6th, the water temperature rose 10 degrees!*
Furthermore, the average water temperature at that particular buoy on July 6th is 63 degrees, while the average temperature record on July 6, 2012 was a whopping 78 degrees.*
*Information from a NWS Chicago Public Information Statement
National Weather Service - Milwaukee/Sullivan