A Multiscale Analysis of Upstream Precursors associated with High-Impact Weather across the Upper Midwest

Lance Bosart's research philosophy emphasizes the importance of incorporating all available observational data to examine high-impact weather across temporal and spatial scales.  This multiscale paradigm can be utilized when examining the 30 June to 2 July 2011 period when two severe MCSs impacted the upper Midwest region.  The first resulted in >100 mm of rain in a ~24-h period, while the second produced >200 severe reports.  Furthermore, this period featured 100 (104) daily maximum high (low) temperature records across this same region.  This severe weather occurred in the presence of an elevated mixed layer (EML) that not only provided the necessary ingredients for the severe MCSs, but also contributed to the numerous record high temperatures. 

The antecedent large-scale flow evolution for this EML 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 over the WNP and dispersion across North America over the 22 June–2 July 2011 period. The Rossby wave train dispersion contributed to trough (ridge) development over western (central) North America at the time of MCS development.  The EML that contributed to the severe weather events was collocated with this central North American ridge.  A composite analysis of 99 warm-season EML days at Minneapolis, MN reveals a similar period of Rossby wave train amplification and dispersion across the WNP basin in the week leading up to EMLs across the upper Midwest.  These results suggest that EMLs and associated severe weather may often occur after an upstream event triggers a large-scale flow impact over the U.S. almost a week prior to convective activity.