A Case Study of the 30 June-1 July 2011 Lake Michigan-Crossing MCS

Mesoscale convective systems (MCSs) are ubiquitous features across the central and eastern United States during the warm season. Many of these MCSs traverse the Great Lakes and pose an important forecasting challenge. While conventional wisdom suggests that mature MCSs might dissipate upon crossing lake waters that are typically cooler than the surrounding land, observational evidence reveals that MCSs can persist or even intensify upon crossing these relatively cool lake waters. This presentation will examine multiple MCSs that formed near and over Lake Michigan during the 30 June to 1 July 2011 period in order to ascertain why these MCSs were poorly forecast by numerical models.

During this two-day period, three separate MCSs formed and regenerated along the western shore and directly over Lake Michigan. Forecasts predicted ~0.25 to 0.5 inches of rain, however, 6–8 inches accumulated in some localized areas. These systems resulted in over 17.5 million dollars in damage. A north–south-oriented warm front located directly over Lake Michigan served as a focusing mechanism for repeated convective development over Lake Michigan. Additionally, a robust low-level jet stream in the presence of significant low-level moisture and CAPE contributed to the heavy-precipitation event. Given the cool temperatures of Lake Michigan, the surface boundary was not well resolved by surface observations or numerical models, which likely contributed to the large forecast errors observed.