PICTURE OF THE MONTH Observation of Coexisting Mesoscale Lake-Effect Vortices over the Western Great Lakes

Mesoscale vortices are one of several types of wintertime lake-effect structures that are observed over the Great Lakes region as cold arctic air is rapidly modified from below by warm lake waters. These vortices are thought to occur less frequently than other lake-effect structures, such as wind-parallel snowbands (e.g., Kristovich 1993), which are responsible for greater than half of the yearly snowfall downwind of the western Great Lakes (Kelly 1986), and more intense shore-parallel snowbands, which can produce localized heavy snowfalls (e.g., Braham 1983). Through the examination of visible satellite imagery, Forbes and Merritt (1984) found 14 cases of mesoscale vortex formation over the Great Lakes in the years 1978–82. This suggests that, on average, a mesoscale vortex will occur within the Great Lakes region less than four times a winter. During an early winter lakeeffect event on 13 November 1995, intense land-breeze circulations and four mesoscale vortices (.50-km diameter) developed over Lakes Superior, Huron, and Michigan simultaneously (Fig. 1). This unique observation of coexisting lake-effect vortices over the western Great Lakes has not previously been reported in the literature and suggests that atmospheric conditions favorable for vortex development were widespread throughout the region. Previous cases have typically observed a single vortex, suggesting that favorable meteorological conditions were localized. The author is aware of only two other reported observations of simultaneous mesoscale vortices within the Great Lakes region (Schoenberger 1986; Pitts et al. 1977). However, the lake-effect vortices that Schoenberger (1986) and