The mesoscale vortices that occasionally form over the Great Lakes during winter are interesting local features that may develop during a northerly flow of cold, dry Arctic air over the relatively warm and ice-free lake surfaces. These phenomena result from the unique combinations of local mesoscale meteorological processes including diabatic heating from the lake surfaces causing the formation of low-level clouds; organized convergence over the lakes of thermally driven land breezes created by the strong temperature contrast between the cold, snow covered land surfaces and the warm, ice free lake surfaces; low-level cyclonic rotation imparted to the clouds by the concave shape of the lakeshore; mid-level subsidence with a low-level subsidence inversion high enough to allow a sufficient depth for the unstable mixing layer above the lake surface; and synoptic-scale winds light enough to avoid disrupting formation of the developing circulation above the lake surface.

This case study examines the features that contributed to the striking image on visible satellite imagery of the rare appearance of two mesocyclones simultaneously on Lakes Huron and Michigan during February 20, 2008.