Dry Winds into the Head of Lake Superior

Sometimes east to east-northeast winds funnel dry air into the head of Lake Superior so vigorously that the onset of snow is suppressed for several hours. In extreme cases snow can hold off for more than six hours. The northern edge of the snow that is actually reaching the ground will approach Duluth, MN and Superior, WI from the south and then just stop. Snow needs to spread well out over the western third of the lake. Once that happends the air upstream can be saturated enough so that it is not too dry by the time it reaches Duluth. A strong high pressure system over Ontario or the Hudson Bay is frequently the culprit for supplying the dry air.

Stable air over the lake also helps keep the air dry. If the temperature of the air is near that of the water, little mixing will occur to transport moisture higher into the air above the lake. Late March through early May is a favorable time to have air temperatures similar to the water. Also note that the moderated air has an increased dew point depression, making the air that much harder to saturate.

The following surface map shows a typical Duluth Spring snowstorm scenario with a strong wound up surface low to the south, and a strong Hudson Bay high pressure system to the north. The storm occurred from April 10th through April 11th. Off and on light snow started to fall around 7:30 PM CST April 10th, but steadier snow did not start until after 9:30 PM and even that was fairly light. Heavier snow, accompanied by thunder, started falling after 11 PM. Take THAT you dry air!

For more on lapse rates over a warm lake see the “Lake Influenced Lapse Rate” section of “Topics in Lake Effect Snow Forecasting” from the MetEd website.