Maintenance of mesoscale convective systems over Lake Michigan

Mesoscale convective systems (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 Lake Michigan.

Two such examples are the 4 July 2003 and 7 June 2008 MCSs that developed over the upper Midwest. Surface boundaries were created in the wake of each MCS, associated with the southern edge of intense cold pools. Enhanced 0–1-km shear along, and isentropic ascent over, the cold-pool-induced surface boundaries resulted in additional convective development and numerous severe weather reports. Both MCSs then crossed Lake Michigan and did not weaken based on composite radar reflectivity observations. Severe wind reports continued to occur as the eastern extent of the both MCSs reached western Michigan.

A detailed examination of the mesoscale features and environments associated with MCSs that were maintained while crossing Lake Michigan will be presented. The MCSs considered are from a 7-year (2002–2008) climatology, which includes the two aforementioned MCSs. 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 Lake Michigan is often much larger than the depth of the 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 a shallow, intense surface-based inversion associated with the cold dome of the lake.