Convergence at the Head of Lake Superior

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Low level convergence at the head of Lake Superior is a combination of frictional convergence and directional convergence resulting from high terrain funneling air into the head of the lake. When air crosses the shore line, the increased friction from the land slows the air down. Faster moving air upstream converges on the air over the land. The best wind direction for convergence over Duluth is a direction from slightly north of east which contains an onshore component.

The blockage of the hill increases the convergence beyond what would occur if the land were just flat. The hill also deflects some of the air so that the air flows parallel to the hill. The difference between the direction of the deflected air and the direction of air approaching from farther over the lake results in directional convergence of the two air flows. Directional convergence also occurs when east-northeast winds are funneled between higher elevations on both sides of the narrowing western part of Lake Superior.

The added lift produced by the convergence is an very effective snow enhancer. Any storm that approaches from a generally west or southwest direction will likely produce an east to east-northeast wind as it passes to the south of Duluth.

Definition of Frictional Convergence in AMS Glossary of Meteorology