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Dry Winds into the Head of Lake Superior

Sometimes east to northeast winds in the first few thousand feet from the surface will 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 six hours or longer. 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 far enough northeast over the lake so that the air is near saturated by the time it reaches the western end of the lake. 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 cold air tends to have an increased dew point depression making the air harder to saturate.

The following surface maps show 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 10 through April 11. Off and on light snow started to fall around 7:30 PM CST (8:30 PM CDT) April 10, but steadier snow did not start until after 9:30 PM CST and even that was fairly light. Heavier snow, accompanied by thunder, started falling after 11 PM CST. So much for dry air!

Surface analysis - 6 PM CST Friday, April 10, 2008
NWS Hydrometeorological Prediction Center (now called the Weather Prediction Center)

High resolution surface analysis - 6 PM CST Friday, April 10, 2008
NWS Weather Prediction Center