A multiscale analysis of a severe weather-producing Great Lakes cutoff cyclone: Part II: Mesoscale structure and evolution

At 1800 UTC 25 May 2006, a cutoff cyclone was slowly progressing eastward across the northern United States toward the western shore of Lake Michigan. A potent mesoscale short-wave trough, the first in a series of such troughs which likely developed in association with barotropic instability, was rotating cyclonically around the southern periphery of the cutoff cyclone. Ahead of this mesoscale short-wave trough, two robust convective lines formed. The first line initiated along a surface front that was partially coincident with the southeastern shore of Lake Michigan, where ambient baroclinity from the horizontal land–lake temperature gradient locally enhanced the magnitude of the temperature gradient associated with the surface front. A second line formed approximately 100 km farther southeast along a surface trough that had formed in situ along the western edge of a dry slot. These two convective lines formed as individual cells, coalesced into solid lines, grew upscale, and combined into a single large mesoscale convective system (MCS). Both convective lines initiated in an environment of moderate convective available potential energy (~1500 J kg-1) and moderate 0-6 km shear (~15 m s-1). In total, ~500 reports of severe weather occurred in association with these two convective lines and resulting MCS.

This presentation will provide both an observational and a WRF-model simulated investigation of the processes and ingredients that contributed to this widespread severe weather outbreak. A detailed observational analysis of the evolution of the mesoscale environment within the Great Lakes region will be presented. The WRF model simulations will be utilized to ascertain the importance of Lake Michigan to MCS development. Within the simulated environment, Lake Michigan and its associated natural baroclinity will be removed in an attempt to elucidate the role that the land–lake temperature gradient, and resulting enhancement of the baroclinic zone associated with the surface front, had on convective initiation and evolution.