Sunday afternoon, July 15th, a very warm pool of water briefly passed by the northern Lake Michigan buoy. This pushed the lake water temperature up to 79° at 5:00 pm. This didn't last long, as the warm pool moved on, and the water temperature dropped back to 76° by 7:00 pm (see the graph below, which depicts water temperatures over the last several days). Temperatures had been running in the lower 70s.
This buoy was first splashed in 1979. This is only the second time that water temperatures, briefly, popped up to levels this warm. In fact, this is the second warmest water temperature ever observed at this buoy. The warmest temperature was 80°, which occurred for an hour on August 10th, 2010. We can't be entirely sure that this hasn't occurred other times as well, since 1979. The water temperature record is not continuous; there have been some gaps in the data. The only other time the data showed water temperatures, as high as 77°, was July 16th, 2005.
Is there any significance to this? Not really. All this really says is that the weather pattern has resulted in an extended period of light winds. Periods of stronger winds mix the water. Usually, occasional low pressure systems keep the water mixed enough so that, just as temperatures reach a peak, brisk winds mix the water and drop them back down. The last day there was a lengthy period of appreciable winds, this year (20-30 kts), was April 26th.
The warmest water temperature averages 68° each year, but they have been into the lower to middle 70s in June, July, August, and September.
The image below shows the mean Lake Michigan water temperatures (over the entire lake) for the last several years. The black line is 2012. Temperatures lake-wide are running around 3°C above the average of the last few years. Note that you can see when the mean temperature is knocked down by a wind event.
This next image compares this year to the 20-year long-term average. While this seems impressive, all it would take is a decent wind event, and the water would "overturn", mixing the temperatures lower.
The current lake temperatures are below:
Prepared by JH 7.16.12