A Multiscale Analysis of Upstream Precursors associated with Serial Severe Weather Events across the Upper Midwest

Between 30 June and 1 July 2011, a heavy-rain-producing mesoscale convective system (MCS) occurred over Lake Michigan. A second MCS subsequently progressed across Minnesota, Iowa and Wisconsin on 1 July 2011 resulting in more than 200 severe weather reports. These two MCSs contained limited predictability, even at relatively short time scales, partially the result of antecedent variability in the upstream large-scale flow evolution over the North Pacific. The objective of this presentation is to demonstrate that the antecedent large-scale flow evolution and multiscale flow interactions during 20–30 June 2011 resulted in a favorable mesoscale environment for serial MCS development over the Upper Midwest of the United States on 30 June and 1 July 2011.

The antecedent large-scale flow evolution was strongly influenced by early-season tropical cyclones (TCs) Haima and Meari in the western North Pacific (WNP). The recurvature and subsequent interaction of these TCs with the extratropical large-scale flow was associated with Rossby wave train amplification on 22–24 June 2011 over the WNP and Rossby wave train dispersion across North America on 28–30 June 2011. The Rossby wave train dispersion was associated with trough (ridge) development over western (central) North America at the time of MCS development over the Upper Midwest U.S. The resulting mesoscale environment featured conditions conducive to heavy rainfall and severe weather including reduced static stability associated with the eastward advection of an elevated mixed layer, increased lower-tropospheric moisture, increased shear associated with strong warm-air advection, and upward vertical motion coincident with strong surface boundaries.