Abstract
Abstract Two severe MCSs over the upper Midwest United States resulted in >100 mm of rain in a ~24-h period and >200 severe weather reports, respectively, during 30 June–2 July 2011. This period also featured 100 (104) daily maximum high (low) temperature records across the same region. These high-impact weather events occurred in the presence of an elevated mixed layer (EML) that influenced the development of the severe MCSs and the numerous record high temperatures. The antecedent large-scale flow evolution was influenced by early season Tropical Cyclone Meari over the western North Pacific. The recurvature and subsequent interaction of Meari with the extratropical large-scale flow occurred in conjunction with Rossby wave train amplification over the North Pacific and dispersion across North America during 22 June–2 July 2011. The Rossby wave train dispersion contributed to trough (ridge) development over western (central) North America and the development of an EML and the two MCSs over the upper Midwest United States. A composite analysis of 99 warm-season days with an EML at Minneapolis, Minnesota, suggests that Rossby wave train amplification and dispersion across the North Pacific may frequently occur in the 7 days leading up to EMLs across the upper Midwest. The composite analysis also demonstrates an increased frequency of severe weather and elevated temperatures relative to climatology on days with an EML. These results suggest that EMLs over the upper Midwest may often be preceded by Rossby wave train amplification over the North Pacific and be followed by a period of severe weather and elevated temperatures.
Highlights
The upper Midwest United States is frequently affected by 1) warm-season convection (e.g., Metz and Bosart 2010) that may lead to severe weather (e.g., Johns and Hirt 1987) and heavy precipitation (e.g., Ahijevych et al 2004) and 2) periods of elevated surface air temperatures and dewpoint temperatures that may lead to heat waves (Bentley and Stallins 2008; Duffy and Tebaldi 2012)
An Rossby wave train (RWT) that amplified over the western North Pacific on 22–26 June 2011 and dispersed across North America on 28 June–2 July 2011 led to the development of an elevated mixed layer (EML) over the upper Midwest
The presented case study identifies that the recurvature and extratropical transition of an early season tropical cyclones (TCs) (Meari) over the western North Pacific, and its interaction with the North Pacific waveguide, may impact preexisting RWT amplification and dispersion that can influence the variability in the occurrence of high-impact weather over North America on synopticto-intraseasonal time scales (e.g., 5–15 days) in the form of severe weather or extreme temperatures
Summary
The upper Midwest United States is frequently affected by 1) warm-season convection (e.g., Metz and Bosart 2010) that may lead to severe weather (e.g., Johns and Hirt 1987) and heavy precipitation (e.g., Ahijevych et al 2004) and 2) periods of elevated surface air temperatures and dewpoint temperatures that may lead to heat waves (Bentley and Stallins 2008; Duffy and Tebaldi 2012). 100 mm in areas surrounding Lake Michigan that exceeded the average rainfall for the month of June by 1.5 times in less than 48 h and left 400 000 people without power, killed 1 person, and caused 17 million dollars in damage, whereas the second MCS produced .200 severe weather reports, including seven tornadoes; resulted in 39 injuries; and caused over 13 million dollars in damage (NCDC 2011a,b; NOAA 2017). For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses)
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