Far Upstream Precursors to Severe and Extreme Weather over the Midwestern United States

Between 30 June and 1 July 2011, a heavy-rain-producing mesoscale convective system (MCS) occurred over Lake Michigan. A second MCS subsequently occurred over Minnesota, Iowa and Wisconsin on 1 July 2011 resulting in more than 200 severe weather reports. The antecedent large-scale flow evolution was strongly influenced by early-season tropical cyclones (TCs) Haima and Meari in the western North Pacific. The recurvature and subsequent interaction of these TCs with the extratropical large-scale flow was associated with Rossby wave train (RWT) amplification on 22–26 June 2011 over the western North Pacific and dispersion across North America on 28–30 June 2011. The RWT dispersion was associated with trough (ridge) development over western (central) North America at the time of MCS development over the Midwestern United States. The evolution of the large-scale flow was particularly conducive to heavy rainfall and severe weather as a surface-based mixed layer over the Intermountain Western United States was advected eastward, transitioning to an elevated mixed layer (EML) over the Midwestern United States.

These two MCSs serve as motivation for a climatology of EML days and their relationship to severe weather over the Midwestern United States. The climatology illustrates that severe weather reports near Minneapolis, MN during the summer are twice as numerous on EML days as compared to normal. The increase in severe weather reports are primarily driven by more large hail and severe wind, which account for 95% of all severe weather reports on EML days. A time-lagged composite analysis indicates that RWT amplification over the central North Pacific and RWT dispersion across the eastern North Pacific and North American, as occurred prior to the 30 June–1 July period, is a common upstream precursor to EML days over the Midwestern United States. These results suggest that investigations of far upstream precursors to RWT amplification and dispersion over the North Pacific may be particularly useful in better understanding warm-season severe weather outbreaks over North America.