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Spike in Great Lakes temperature has winter implications

Dr. Doug Gillham
Meteorologist, PhD

Tuesday, September 26, 2017, 4:59 PM - A late September heat wave has water temperatures in the Great Lakes running at their warmest levels of 2017, and warmer than what is typical for July and August.

During early September each of the Great Lakes were cooler than normal, but as of September 26 each of the lakes had caught up to or even surpassed where they were this time last year when lakes were exceptionally warm after a hot summer.

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The recent temperature trend is especially remarkable on Lake Huron, where the average surface temperature has risen by over 3.5oC during the past 2 weeks. As of September 26, the average surface water temperature on Lake Huron was 21.2oC, which is a degree warmer than both the peak temperature during the summer and the typical lake temperature for July and August.

Lake Superior is also at its warmest levels of the year with a temperature of 16.7oC. This is a degree above the summer peak for this year and the long-term average peak summer temperature.

Lake Erie has also gained nearly four degrees during the past two weeks, but at 23.7oC it is just below its peak summer temperature of 24.0oC.

Likewise, Lake Ontario gained just under four degrees during the heat wave and with a temperature of 22.8oC it is just below its summer peak of 23.1oC.

How will the Warm Lakes Impact Late Fall and Winter?

While it is remarkable to see the lakes trending warmer rather cooler during late September, much of the warming that has occurred during the past two weeks has been within the top couple of metres of water. Two weeks of warm weather during late September does not have the same impact on the heat content of the lakes as a hot summer like 2016.

For each of the Great Lakes, the average water temperature for the entire lake volume (not just the surface) is still about a degree cooler than this time last year. So, a period of strong winds that mix up the lake water would offset some of the recent warming. However, with a mild October expected, the Great Lakes will likely still be warmer than normal as we head into late fall and winter.

One of the key drivers of lake effect snow is the difference in temperature between the Great Lakes and the cold air crossing over them. A larger temperature difference tends to increase the potential for heavy lake effect snow.

Therefore, the potential for heavy lake effect snow could be higher once Arctic air does track into the region. However, there are several others factors that impact lake effect snow totals which would have to be evaluated on a case by case basis.

The warmer water could also delay the build-up of ice on the Great Lakes. This would extend the opportunity for lake effect snow later into the winter, especially downwind of Lake Erie and Georgian Bay which are the first to freeze.

However, the warmer Great Lakes can also reduce the potential for accumulating snow near lake shore areas when temperatures are marginal for snow versus rain, especially during the late fall and early winter.

Last November and December there were a few systems that brought significant snow to parts of southern Ontario, but an east wind off a relatively warm Lake Ontario was the key to areas downwind of the lake receiving rain (or wet snow that did not accumulate) rather than significant snowfall.

Sources: NOAA Coast Watch |

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