6.1 Maintenance and Dissipation of mesoscale convective systems that encounter the Great Lakes

Tuesday, 28 October 2008: 8:30 AM
North & Center Ballroom (Hilton DeSoto)
Nicholas D. Metz, Univ. at Albany/SUNY, Albany, NY; and L. F. Bosart

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 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 cool air over the Great Lakes. The purpose of this presentation will be to explore the dynamical and physical mechanisms that control whether a warm-season MCS will strengthen or weaken when it crosses the Great Lakes.

The varying effects of the Great Lakes on MCSs were highlighted on 7–8 June 2008 when an MCS that crossed central Lake Michigan maintained strength, while a large supercell that crossed southern Lake Michigan quickly dissipated. Around 1700 UTC 7 June, an MCS developed over southwestern Wisconsin, along a surface boundary created by previous convection. Warm advection overspread the surface boundary, and the MCS encompassed the southern half of Wisconsin by 0000 UTC 8 June, causing numerous tornado, severe wind, and severe hail reports. As the MCS crossed Lake Michigan, composite reflectivity did not show any weakening and severe wind reports occurred over western-Lower Michigan as the eastern extent of the MCS emerged from the eastern shore of Lake Michigan. Finally, by 1000 UTC 8 June, portions of the MCS that remained over Lake Michigan weakened quickly. During the same period over central Illinois at 2100 UTC 7 June, a supercell that produced tornado reports moved towards the northeast. As the isolated cell encountered Lake Michigan by 0100 UTC 8 June, to the immediate south of the aforementioned MCS, the supercell weakened and dissipated.

A detailed examination and analysis of the MCS over Wisconsin and the separate supercell over northern Illinois that both subsequently crossed Lake Michigan on 7–8 June 2008 will be presented. The MCS was unaffected by its passage over Lake Michigan while the supercell rapidly dissipated once it reached Lake Michigan. Our analysis will attempt to document the physical processes responsible for the observed difference in behavior of the two convective systems once they interacted with Lake Michigan. Additionally, other examples of MCSs that maintained their strength or dissipated while crossing the Great Lakes will also be presented to augment our findings from the 7–8 June 2008 convective event.

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