Maintenance of Mesoscale Convective Systems over the Great Lakes

Mesoscale convective systems (MCSs) are ubiquitous features across the upper Midwest during the warm season. MCSs that traverse the Great Lakes and vicinity during the warm season are of interest because the cooler lake waters relative to the surrounding land may decrease (increase) surface-based CAPE (CIN) and result in the weakening of the MCSs. However, some MCSs are observed to maintain their strength, or even intensify, as they traverse the Great Lakes, suggesting that these MCSs may be feeding on an accelerated “surface” flow that lies just above a shallow surface-based stable dome of cold air over the Great Lakes. The purpose of this presentation will be to document warm-season MCSs that maintain their strength or intensify over the Great Lakes and examine the dynamical and physical mechanisms controlling the convective maintenance.

One such example was the 7 June 2008 MCS that developed over northwestern Iowa. The cold pool from the MCS was intense and acted to extend a surface boundary into southern Minnesota and Wisconsin as the MCS progressed eastward. Enhanced 0–1-km shear along, and isentropic ascent over, the cold-pool-induced surface boundary led to additional convective development and numerous associated tornado, severe wind, and severe hail reports in the wake of the MCS. The MCS then crossed Lake Michigan and did not weaken based on composite radar reflectivity. Severe wind reports continued to occur as the eastern extent of the MCS reached western Michigan.

A detailed 10-year climatology of MCSs that were maintained while crossing the Great Lakes will be presented along with a careful examination of individual MCSs from the climatology, such as the 7 June 2008 case. Preliminary results show that midlevel dry air often allows for the formation of a strong and deep surface cold pool in many of the cases, and the balance between strong 0–3-km shear and the cold pool promotes vigorous ascent. The depth of the MCS cold pool as the systems approach the Great Lakes is often much larger than the depth of the surface-based cold dome over the lake, allowing for continued ascent over the lake. Additionally, an intense low-level jet is often present, advecting warm unstable air into the MCS region, just above the shallow surface-based cold dome of the lake.