The effects of Lake Michigan on mature mesoscale convective systems

Warm-season mesoscale convective systems (MCSs) that traverse the Great Lakes pose an important forecasting issue. Conventional wisdom suggests that mature MCSs might dissipate upon crossing lake waters that are typically cooler than the surrounding land. However, observational evidence reveals that MCSs can persist or even intensify. As these MCSs interact with the lake-modified air, they undergo structural and evolutionary changes that vary from case-to-case, based on the environmental conditions present. This presentation will document environmental conditions associated with warm-season MCSs that cross Lake Michigan and examine MCS–lake interactions in an attempt to ascertain the spectrum of conditions under which past MCSs persisted and dissipated.

Climatological results from 2002 to 2007 indicate that 43% of warm-season MCSs persisted upon crossing Lake Michigan. Persistence is favored for MCSs that cross during the evening/overnight and the mid-summer. However, MCSs can persist at other times as well, such as during the early warm season (April–June), when lake water temperatures are only 3–10°C. Consequently, the lake water temperature is not a good predictor of MCS persistence. Rather, the very shallow near-surface lake inversion (buoy air temperature at 5 m minus lake water temperature) is typically much stronger when MCSs persist, particularly in the early warm season. Additionally, MCSs often persist in association with an intense low-level jet (LLJ) stream, while the amount of CAPE and shear immediately upstream of Lake Michigan do not appear to discriminate between MCSs that persist and dissipate. Synoptic-scale composites generally agree with climatological results and show that MCSs usually persist in the equatorward-entrance region of a stronger upper-level jet stream than MCSs that dissipate.

Selected observational and WRF-simulated case studies will accentuate climatological and compositing results, and highlight the differing combinations of environmental and lake conditions that distinguish between MCSs that persist and those that dissipate. For instance, MCSs that persist usually are associated with convective cold pools that are much deeper than the very shallow, stable dome of cold air over Lake Michigan. Persisting MCSs typically feed on “surface” flow characteristic of adjoining landmasses that is advected by a strong LLJ above this lake cold dome.