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.

1. Introduction

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). These two high-impact weather phenomena occurred during 30 June–2 July 2011 in the presence of an elevated mixed layer (EML; e.g., Carlson et al. 1983) that influenced the development of a heavy-rain-producing mesoscale convective system (MCS) over Lake Michigan on 30 June–1 July 2011; a severe-weather-producing MCS over Minnesota, Iowa, and Wisconsin on 1–2 July 2011; and record warmth over the upper Midwest between 30 June and 2 July 2011 (Figs. 1 and 2). The first MCS produced rainfall totals > 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). These two MCS events bookended a period with 100 daily maximum high temperature records with an average Tmax = 35.2°C and 104 daily maximum low temperature records with an average Tmin = 22.7°C over North Dakota, South Dakota, Minnesota, Wisconsin, Iowa, and Michigan (Fig. 1a; NOAA 2016). The purpose of this study is to 1) provide an overview of the antecedent upstream conditions that led to the development of an EML, 2) illustrate the in situ meso–synoptic-scale conditions that led to the subsequent development of these two MCSs and record warmth during 30 June–2 July 2011, and 3) provide historical context for the evolution of the antecedent conditions prior to the development of the EML and high-impact weather events over the upper Midwest.

Previous studies have shown that the sensible weather over the United States can be related to upstream conditions over the Pacific Ocean (e.g., Namias 1955). Recently, large-scale flow reconfigurations over the North Pacific have been shown to influence high-impact weather events over North America (e.g., Archambault et al. 2013). The cause of these large-scale flow reconfigurations is often attributed to synoptic-scale transient disturbances that contribute to the onset of blocking (e.g., Sanders and Gyakum 1980; Shutts 1983, 1986; Colucci 1985, 1987; Mullen 1986; Pelly and Hoskins 2003; Berrisford et al. 2007), the development of persistent large-scale positive and negative geopotential height anomalies (e.g., Namias 1955; Dole and Gordon 1983; Dole 1986a,b; Dole and Black 1990; Black and Dole 1993; Higgins and Schubert 1994; Dole 2008), or changes in the storm track over the North Pacific, which are closely related to variability in the structure of the North Pacific jet stream (Nakamura 1992; Matthews and Kiladis 1999; Higgins et al. 2000; Harnik and Chang 2004; Nakamura and Shimpo 2004; Otkin and Martin 2004; Archambault et al. 2008, 2010; Martius et al. 2008; Cordeira and Bosart 2010; Moore et al. 2010; Archambault et al. 2013).

Archambault et al. (2013) illustrate that interactions among recurving and extratropical transitioning western North Pacific tropical cyclones (TCs) and the North Pacific jet stream are a critical process in large-scale flow reconfigurations that produce Rossby wave train (RWT) amplification and dispersion via downstream development (e.g., Harr and Elsberry 2000; Klein et al. 2002; Atallah and Bosart 2003; McTaggart-Cowan et al. 2007; Riemer et al. 2008; Harr and Dea 2009; Riemer and Jones 2010; Archambault et al. 2015; Torn and Hakim 2015; Quinting and Jones 2016; Grams and Archambault 2016). Both TC-influenced and non-TC-influenced RWTs can disperse downstream along the North Pacific waveguide and can influence the evolution of the flow over the North Pacific and North America (e.g., Chang 1993; Orlanski and Chang 1993; Orlanski and Sheldon 1995; Danielson et al. 2004, 2006; Cordeira and Bosart 2010; Archambault et al. 2015; Torn and Hakim 2015; Bentley and Metz 2016) and over Europe (Martius et al. 2008; Wirth and Eichhorn 2014), where they may contribute to the development of high-impact weather.

Forecasting the development of tropospheric deep moist convection that may result in severe MCSs (i.e., severe convection) and high-impact weather without the use of high-resolution numerical weather prediction models has traditionally been performed using an ingredients-based approach (McNulty 1978; Doswell 1987; Johns and Doswell 1992; Doswell et al. 1996; Moller 2001; Brooks et al. 2003). The three ingredients considered necessary for the development of severe and nonsevere convection are generally considered to be lift (e.g., forcing for ascent), moisture, and instability. The latter ingredient is often observed as enhanced convective available potential energy (CAPE) in the presence of steep (i.e., quasi–dry adiabatic) temperature lapse rates (LRs; e.g., ≥8.0 K km−1) in the lower and middle troposphere that can lead to large air parcel accelerations along updrafts and downdrafts. This region of steep LRs often develops in association with an EML (e.g., Carlson et al. 1983).

The EML is an example of an atmospheric structure that may develop as a result of the evolution of the large-scale flow over the eastern North Pacific and western North America and contribute to the development of both severe and nonsevere convection or contribute to an absence of convection over the United States (Carlson et al. 1983; Lanicci and Warner 1991a,b,c; Banacos and Ekster 2010). The EML develops initially as a surface-based, deep, neutrally stratified mixed layer over the elevated terrain of the Intermountain West beneath the climatological warm-season subtropical upper-tropospheric anticyclone that expands northward during April–July (Lanicci and Warner 1991a). Synoptic scaling of the stability (i.e., LR) tendency equation (Bluestein 1992, Eq. 5.9.8) indicates that geostrophic LR advection is the largest source (approximately one to two orders of magnitude larger than the other terms) for local changes in the LR. The surface-based mixed layer can therefore be advected to the east or northeast downstream of an approaching trough in the upper troposphere (e.g., Banacos and Ekster 2010). This mixed layer may then overrun a cooler, moister air mass whose source region may have originated over the Gulf of Mexico (Lanicci and Warner 1991b) or developed in situ via evapotranspiration (e.g., Sun and Barros 2015). The thermodynamic profile of the EML and its inversion is commonly referred to as a type-1 tornado sounding (Fawbush and Miller 1954) and is characterized by lower-tropospheric moist, potentially unstable air that is capped by potentially warm, nearly dry-adiabatic air in the middle troposphere. The strength of this capping inversion may simultaneously inhibit the development of convection, or in the presence of sufficient mesoscale or synoptic-scale forcing for ascent, lead to the development of severe convection. The presence of an EML therefore indicates regions of potential for buoyant instability aloft (Carlson and Ludlam 1968; Doswell et al. 1985) that may contribute to or inhibit the development of severe convection.

The remainder of this paper documents the evolution of a prominent EML that 1) developed over the Midwest United States following a large-scale flow reconfiguration over the North Pacific and western North America, 2) influenced the development of the two aforementioned high-impact MCSs in the presence of meso–synoptic-scale lifting mechanisms and lower-tropospheric moisture, and 3) influenced record warmth across the Midwest. The mobility of this EML in the presence of geostrophic advection and its influence on the development of severe convection and record warmth over the Midwest is also the basis for investigating a local climatology of steep LRs in order to provide historical context for the evolution of the antecedent conditions prior to high-impact weather events over the Midwest. Section 2 provides an overview of the data and methods employed in this study. Section 3 provides an overview of the antecedent upstream conditions that helped produce an EML, while section 4 highlights the mesoscale conditions associated with the two MCSs of interest. Section 5 places the results in context using a historical analysis that focuses on a climatology of steep LRs. Finally, section 6 provides a discussion of the results in relation to previous literature, while a section 7 offers a concluding summary.

2. Data and methods

The synoptic-scale analysis prior to and during the 30 June–2 July 2011 severe weather events is performed using 0.5° latitude × 0.5° longitude gridded data from the National Oceanic and Atmospheric Administration/National Centers for Environment Prediction (NOAA/NCEP) Global Forecast System (GFS) 0-h analyses (NOAA 2012a) and the 2.5° latitude × 2.5° longitude gridded data from the NCEP–National Center for Atmospheric Research (NCAR) reanalysis (Kalnay et al. 1996). The mesoscale analysis is performed using the 13-km gridded data from the Rapid Update Cycle 0-h analyses (NOAA 2012b). These three gridded analyses were acquired from the NOAA/National Centers for Environmental Information (NCEI) National Operational Model Archive and Distribution System (http://nomads.ncdc.noaa.gov/data.php). Radar data were also obtained from NCEI (NOAA 2014). Figure 1 was constructed from severe weather reports acquired from the NOAA/Storm Prediction Center (SPC) Severe Weather Database Browser (NOAA 2017) that curates official National Weather Service severe weather information from the Storm Data publications at NCEI, daily weather records acquired from NCEI (NOAA 2016), and gridded quantitative precipitation estimate (QPE) data acquired from the UCAR Earth Observing Laboratory (EOL; Lin 2011). Infrared satellite imagery (see Fig. 5b) was downloaded and adapted from Weather Home, Kochi University in Kochi, Japan (http://weather.is.kochi-u.ac.jp).

An algorithm for the detection of an EML was developed by Farrell (1988) and Farrell and Carlson (1989) and was used in climatological studies by Lanicci and Warner (1991a). Their algorithm examines the temperature and relative humidity profiles of sounding data to determine the presence of the EML and its inversion (i.e., lid) based on a number of criteria. Herein, the likely presence of an EML is indicated based on convective instability (e.g., Hales 1982) through steep, quasi–dry adiabatic (e.g., ≥8.0 K km−1) midtropospheric temperature LRs (i.e., “steep LRs”). Note that whether or not severe convection, nonsevere convection, or no convection is likely in an environment that contains an EML is dependent upon the characteristics of the moist air contained beneath the EML and its inversion and the presence of mesoscale or synoptic-scale forcing for ascent (e.g., Doswell et al. 1995, 1996). A 34-yr climatology of steep LRs is generated from temperature and geopotential height observations collected from the NCEI integrated global radiosonde archive (Durre et al. 2006). The climatology is produced for Chanhassen, Minnesota (MPX), using observations from 1000 to 1400 UTC during June–August (JJA) from 1974 through 2007 (Cordeira et al. 2008) by identifying vertically continuous isobaric layers between 900 and 400 hPa with a temperature LR ≥ 8.0 K km−1 and a layer-mean potential temperature ≥ 30°C over a depth ≥ 150 hPa. Various thresholds for the layer-mean potential temperature and LR criterion were tested with very little difference in the resulting statistics or composite analyses. The 99 dates that satisfied the aforementioned criterion are considered warm-season EML days. These dates are used to generate composite gridded analyses from NCEP–NCAR reanalysis data, composite time series of the number of days with at least one severe weather report (e.g., hail, wind, and tornado) within a 4° latitude × 4° longitude box surrounding MPX (i.e., southeast Minnesota and western Wisconsin), and composite time series of daily temperature-derived quantities at Minneapolis, Minnesota (MSP). Temperature data for MSP are acquired from the NCEI global surface summary of the day (https://data.noaa.gov/dataset/global-surface-summary-of-the-day-gsod).

3. Antecedent upstream conditions

The focus of this section is on the spatial and temporal evolution of the large-scale flow over the North Pacific and North America during 22–30 June 2011 that influenced the high-impact weather events during 30 June–2 July 2011. The evolution of the large-scale flow was likely influenced by the life cycles of two early season TCs, Haima and Meari, that developed east of the Philippines on 17 and 21 June 2011, respectively (Fig. 3). TC Haima traveled from east to west and made landfall in southeast China on 22 June 2011, whereas TC Meari traveled poleward into the Yellow Sea, underwent extratropical transition, and made landfall in North Korea on 27 June 2011 (Fig. 3).

Both TCs were located within environments that contained integrated water vapor values near 70 mm beneath a subtropical upper-tropospheric ridge at 250 hPa at 0000 UTC 24 June 2011 (Fig. 4a). The 250-hPa midlatitude flow at this time was characterized by quasi-zonal wind from East Asia to 160°E that contained an embedded 70 m s−1 jet streak located north of the Sea of Japan (Fig. 4a). This jet streak had intensified and elongated compared to 2 days earlier (not shown), and this intensification and elongation occurred in the presence of poleward water vapor transport from TC Haima over East China along and east of a mei-yu front (Chen 1988). The poleward elongation of this region of enhanced integrated water vapor is illustrated in Fig. 4a. The poleward transport of water vapor from TC Haima and poleward elongation of enhanced integrated water vapor toward the equatorward entrance region of an intensifying upper-tropospheric jet streak is consistent with the dynamics outlined for a possible predecessor rain event (PRE; e.g., Cote 2007; Wang et al. 2009; Galarneau et al. 2010; Byun and Lee 2012) over eastern China, South Korea, and Japan. Further analysis of the possible PRE is beyond the scope of the current investigation.

The subsequent poleward movement and extratropical transition of TC Meari occurred in association with upper-tropospheric ridge development near 40°N , 130°E, trough development along 155°E, and ridge amplification along 170°W that suggests RWT amplification at 0000 UTC 26 June 2011 (Fig. 4b). In the present case, ridge amplification near 40°N, 130°E appears to amplify a preexisting RWT that is illustrated by an upstream trough located along 110°E and the aforementioned ridge located over eastern Asia near 130°E at 0000 UTC 26 June 2011 (Fig. 4b). Recent studies indicate that the negative upper-tropospheric potential vorticity (PV) advection by the diabatically driven divergent outflow associated with a TC in the presence of an upstream trough may contribute to amplification of preexisting RWTs (e.g., Archambault et al. 2013, 2015). The orientation of the 300–200-hPa layer-mean irrotational wind in the vicinity of TC Meari relative to the upstream trough and the southeast-to-northwest upper-tropospheric PV gradient located over eastern China at 0000 UTC 26 June 2011 (Fig. 5a) illustrate the contribution of diabatically driven divergent TC outflow (Fig. 5a) and negative PV advection by the irrotational wind values less than −10 PVU day−1 (1 PVU = 10−6 K kg−1 m2 s−1; Fig. 5b) as a catalyst for the amplification of the preexisting RWT over the North Pacific. The corresponding infrared satellite image at 0000 UTC 26 June 2011 corroborates the location of the diabatically driven divergent TC outflow relative to the locations of the upstream trough and downstream jet streak (Fig. 5b).

The dispersion of the RWT across the eastern North Pacific and western North America is observed in the progression of the 250-hPa geopotential heights and integrated water vapor imagery on 26–30 June 2011 (Figs. 4b–d). The eastern extent of the RWT was collocated with an upper-tropospheric trough and short-wave ridge immediately west of the U.S. West Coast at 0000 UTC 26 June 2011 (Fig. 4b) that developed into a prominent ridge over the Gulf of Alaska and trough approaching the U.S. West Coast by 0000 UTC 28 June 2011 (Fig. 4c). The RWT progressed eastward and amplified over North America by 30 June 2011 such that the trough located west of the U.S. West Coast became located firmly over the western United States and a downstream ridge amplified over the Intermountain West (Fig. 4d).

The RWT amplification over the North Pacific and propagation over North America resulted in a large-scale flow reconfiguration over the United States that can be illustrated by the evolution of the 700–500-hPa layer-mean geopotential heights, wind, and temperature LRs (Fig. 6). The aforementioned trough–ridge dipole at 250 hPa at the leading edge of the RWT propagation approaching the U.S. West Coast at 0000 UTC 28 June 2011 (Fig. 6a) is associated with midtropospheric geostrophic southwesterly winds that promote the displacement of steep surface-based LRs > 8.5 K km−1 from the elevated terrain over the Intermountain West toward the western portions of the upper Midwest United States during 29–30 June 2011 (Figs. 6b,c) and eastern portions of the upper Midwest United States during 30 June–1 July 2011 (Figs. 6c,d).

A summary of the RWT amplification and propagation is illustrated by a time–longitude analysis of the 250-hPa meridional wind anomaly, 250-hPa total wind speed, and 700–500-hPa temperature LRs for 20 June–10 July 2011 averaged over 40°–50°N for longitudes located over the North Pacific and North America (Fig. 7). The analysis illustrates the intensification of the North Pacific jet stream along 135°–165°E on 24–25 June 2011 prior to the amplification of the RWT on 26 June 2011. The RWT disperses downstream with a group velocity of ~23 m s−1 and a phase speed of ~10 m s−1. This Rossby wave group velocity and phase speed are graphically approximated for a reference latitude at 45°N from the time and longitude information in Fig. 7 and are slightly (noticeably) higher than composite values for RWT propagation influenced by midlatitude winter cyclones (TCs undergoing extratropical transition) provided by Torn and Hakim [(2015); see their Fig. 10 for a comparison of midlatitude winter cyclones and TCs]. Upon dispersing across North America, the leading edge of the RWT and collocated southerly 250-hPa meridional wind anomaly is associated with the eastward displacement of steep LRs > 8.5 K km−1 at 700–500 hPa from along the 105°W meridian toward 90°W and locations over the upper Midwest United States.

4. Mesoscale analysis and observations

The focus of this section is on the two separate MCSs that developed between 1800 UTC 30 June and 0600 UTC 2 July 2011 (Fig. 2). The first MCS (MCS1) developed over Lake Michigan on 1800 UTC 30 June (Fig. 2b). MCS1 remained quasi-stationary with back-building convection over Lake Michigan over the subsequent 12 h before moving southward into northern Indiana at 0600 UTC 1 July (Figs. 2c,d). MCS1 subsequently redeveloped along the eastern shore of Lake Michigan at 1200 UTC 1 July (Fig. 2e) before finally dissipating along the Lake Michigan–Indiana shoreline after 1800 UTC 1 July (Fig. 2f). The quasi-stationary and back-building convection produced by MCS1 led to large amounts of precipitation > 100 mm over the Lake Michigan region (Fig. 1b). The second MCS (MCS2) developed over north-central Nebraska and south-central South Dakota at ~1500 UTC 1 July and is observed 3 h later over east-central South Dakota (Fig. 2f). MCS2 moved eastward across southwest Minnesota (Fig. 2g) before weakening over Wisconsin and Iowa by 0600 UTC 2 July (Fig. 2h). MCS2 was responsible for a majority of the severe reports shown in Fig. 1a.

Prior to and during the formation of MCS1 (1800 UTC 30 June), steep (>8.0–9.0 K km−1) surface-based LRs from locations over the Rocky Mountains progressed eastward as an EML to near Lake Michigan (Figs. 6c–d). MCS1 formed on the eastern periphery of an amplified upper-tropospheric ridge at 250 hPa (Fig. 8a) and on the east periphery of a moist and unstable environment characterized by integrated water vapor values > 45 mm and CAPE > 3000 J kg−1, respectively (Figs. 2d and 8b,c). The formation of MCS1 coincided with meso–synoptic-scale forcing for ascent in conjunction with robust lower-tropospheric frontogenesis > 2 K (100 km)−1 (3h)−1 (Fig. 8b) and warm-air advection (>30 K day−1) proximate to a warm-frontal boundary (Fig. 9). Note that MCS1 did not form in the region of steepest LRs located upstream (Fig. 6d), where CIN values over Minnesota and Wisconsin approached −500 J kg−1 (Fig. 8a). MCS1 formed in a location where the meso–synoptic-scale forcing for ascent along a warm front likely provided enough lift for air parcels to reach their level of free convection along the eastern periphery of the EML (Figs. 8d and 9). MCS1 was responsible for the few severe weather reports and prolific rainfall on 1–2 July near Lake Michigan (Fig. 1).

MCS2 formed over north-central Nebraska and south-central South Dakota on 1 July (Figs. 2e,f) along the western periphery of the amplified upper-tropospheric ridge at 250 hPa and the region of steepest LRs (Figs. 6d and 10a). MCS2 also formed in a moist and unstable environment characterized by integrated water vapor values near 50 mm and CAPE values between 2000 and 5000 J kg−1, respectively (Figs. 10b,c). The formation of MCS2 also coincided with meso–synoptic-scale forcing for ascent in conjunction with robust lower-tropospheric frontogenesis > 2 K (100 km)−1 (3h)−1 (Fig. 10b) and warm-air advection (>40 K day−1) ahead of an approaching cold-frontal boundary and sea level pressure trough (Figs. 10b,d and 11). Note that MCS2 did not form in the region of steepest LRs located downstream (Fig. 6d), where CIN values over Minnesota and Wisconsin were from −100 to −300 J kg−1 (Fig. 10a). MCS2 formed in a location where the meso–synoptic-scale forcing for ascent along the upstream cold front likely provided enough lift for air parcels to reach their level of free convection along the western periphery of the EML (Figs. 10d and 11). MCS2 was largely responsible for a majority of the severe weather reports on 2 July over central and southeast Minnesota (Fig. 1a).

The previous mesoscale discussion demonstrates that the EML likely contributed to an environment favorable for severe convection, but also demonstrates that the severe convection occurred along the periphery of the steepest LRs in regions of meso–synoptic-scale forcing for ascent. The region of the steepest LRs over southeast Minnesota and Wisconsin (Fig. 6) coincided with the regions of highest CAPE (Figs. 8c and 10c) and the highest CIN directly beneath the EML-related capping inversion. A comparison of 12-hourly soundings at MPX illustrates this point (Fig. 12). The eastward advancement of the EML between 1200 UTC 30 June and 1200 UTC 1 July (Figs. 6c,d) is associated with lower-tropospheric (e.g., surface–700 hPa) warm-air advection, an increase in surface-based CAPE values at MPX from 2064 to 4732 J kg−1, and an increase in lower-tropospheric dewpoint temperatures (Fig. 12). The EML resulted in a substantial capping inversion at ~800–850 hPa that initially served to limit convective initiation proximate to MPX on 1 July in the absence of notable meso–synoptic-scale forcing for ascent, and later served to promote convection and the formation of MCS2 on 2 July in the presence of meso–synoptic-scale forcing for ascent related to the upstream cold front (Figs. 10d and 11). Consequently, 100 daily record high maximum and 104 daily record high minimum temperatures were recorded in the absence of severe weather while the MPX region was under the apex of this amplified upper-tropospheric ridge, EML, and strong capping inversion.

5. Composite analysis of steep LRs and sensible weather impacts

The 30 June–1 July 2011 case study analysis illustrated how the development of an EML influenced severe convection and record warmth over the upper Midwest United States. The development of this EML was linked to an RWT that was amplified by TCs over the western North Pacific. This upstream influence on the evolution of an EML and the occurrence of high-impact weather over the United States motivates an examination of a climatology of steep LRs over MPX during 1974–2007 in order to determine whether EMLs in this region are frequently linked to characteristic upstream synoptic-scale precursors over the North Pacific.

a. Composite analysis

A composite analysis of 850–600-hPa temperature LR, temperature LR anomaly, and layer-mean wind illustrates that the 99 warm-season EML days are characterized by a northeast displacement of steep LRs > 7.5 K km−1 from the Intermountain West toward the upper Midwest (Fig. 13a). This displacement is characterized by 850–600-hPa temperature LRs that are >1.5 K km−1 above normal (i.e., closer to dry adiabatic) over eastern South Dakota, northeast Nebraska, Minnesota, Iowa, and Wisconsin and >1.0 K km−1 below normal (i.e., farther from dry adiabatic) over locations in the Pacific Northwest. The development of steep LRs at MPX on EML days is characterized by composite 72-h backward air parcel trajectories in the middle and upper troposphere that originate from near the U.S. West Coast, travel across the Intermountain West ahead of an upper-level trough, and terminate over the upper Midwest (Fig. 13b). Air parcels in the lower troposphere originate from over the central and southern plains, travel poleward, and terminate over the upper Midwest. These air parcel trajectories collectively illustrate a modest veering lower-tropospheric (925–600 hPa) wind profile that is indicative of differential warm air temperature advection, which can sustain or promote steeper midtropospheric LRs (Banacos and Ekster 2010).

A composite time–longitude analysis of twice daily (0000 and 1200 UTC) 250-hPa meridional wind anomalies and 700–500-hPa temperature LRs (Fig. 14), similar to Fig. 7, illustrates that warm-season EML days at MPX occur on average in association with RWT amplification over the central North Pacific and dispersion across North America. The composite-mean RWT amplifies over the central North Pacific near 170°E ~7 days prior to the EML day at MPX and propagates across the eastern North Pacific with a group velocity of ~15 m s−1 that increases to ~23 m s−1 over North America; the phase speed of the composite-mean RWT remains quasi constant at ~2–4 m s−1. The Rossby wave group velocity and phase speed are graphically approximated for a reference latitude at 45°N from the time and longitude information in Fig. 14. The increase in RWT group velocity is consistent with the baroclinic generation of eddy kinetic energy following downstream baroclinic development (Chang 1993), but may also be related to a reduction in the variance (e.g., smearing) of the population mean meridional wind anomalies as the time approaches day −0. Composite latitude-averaged LR values increase to >7.5 K km−1 at 0000 UTC along longitudes over the Intermountain West as the ridge axis of the dispersing RWT advances eastward on day −5. The corridor of enhanced LR values > 7.5 K km−1 is transported eastward toward MPX downstream of a trough along the U.S. West Coast that produces statistically significant LR anomaly values > 1.5 K km−1 over the upper Midwest (Fig. 13a). This eastward transport of the EML progresses downstream in association with the dispersion of the RWT following the wave phase speed of ~2–4 m s−1. The composite-mean RWT, as quantified by statistically significant 250-hPa meridional wind anomalies, persists for ~10 days from day −7 to day +2 and spans ~9000 km from 170°E to 60°W. The duration and extent of the composite RWT is comparable in duration and extent to the RWT observed on 23 June–3 July 2011 (Fig. 7).

b. Sensible weather impacts

The 99 EML days at MPX featured at least one severe report on 63 days over southeast Minnesota and western Wisconsin (Fig. 15a). These 63 severe weather EML days contained at least one severe wind report on 53 days (84%) (Fig. 15b), a severe hail report on 48 days (76%) (Fig. 15c), and a tornado report on 20 days (32%) (Fig. 15d). The frequency of severe weather reports over southeast Minnesota and western Wisconsin on EML days is significantly (~50%–90%) higher than severe weather reports on days for a similar-sized climatological population (Figs. 15a–d). The likelihood for a higher-than-normal frequency of severe wind and hail reports (tornado reports) is highest on the day before, the day of, and the after (the day of) an EML day at MPX. These 63 days produced 1246 total severe weather reports, which represented ~7% of all severe weather reports during the June–August 1974–2007 period (not shown). Approximately one-third (26 of 99) of EML days did not contain at least one severe report over southeast Minnesota and western Wisconsin. Further analysis is required in order to better understand which EML days are associated with severe weather and which are not; however, a limiting factor may be related to the presence of an adequate lifting mechanism to overcome significant CIN and a capping inversion that is often related to the EML, similar to what was shown in section 4.

The 99 EML days at MPX are also associated with above-normal surface air and dewpoint temperatures at MSP, likely owing to lower-tropospheric warm-air advection, midtropospheric subsidence, and a lower-tropospheric capping inversion inhibiting vertical growth and mixing of the boundary layer (Figs. 15e–g). The average daily maximum temperature at MSP on EML days at MPX is 32.4°C (+4.2°C anomaly) with 88 of 99 days (89%) having a daily maximum temperature that was higher than climatology (Fig. 15e). The average daily maximum temperature at MSP is significantly elevated with respect to climatology for the 5-day period centered on the EML day. The warmer-than-normal air mass is also associated with an average daily minimum temperature that remains above normal for the period from day −1 to day +2 with the highest values of 18.7° and 18.6°C (+2.9°C and +2.8°C anomalies, respectively) on the day of and day after an EML day (Fig. 15f). These elevated daily minimum and maximum temperature values are associated with above-normal (+3.3°C) daily average dewpoint temperatures > 17.9°C (Fig. 15g) and an above-normal number of cooling degree days (CDDs; calculated using a base of 65°F) prior to, during, and after the EML day (Fig. 15h). The total number of CDDs during the 3-day periods centered on all the EML days during JJA each year accounted for, on average, ~14% of the respective annual CDDs.

6. Discussion

An 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 EML over the upper Midwest. This EML served as an important, but likely insufficient, ingredient in the development of two MCS events and influenced a period of record warmth over the upper Midwest United States that led to a myriad of severe weather reports and daily temperature records, respectively. The presented case study identifies that the recurvature and extratropical transition of an early season TC (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 synoptic-to-intraseasonal time scales (e.g., 5–15 days) in the form of severe weather (i.e., severe winds, hail, tornadoes) or extreme temperatures. Further analysis that is beyond the scope of the current investigation is required in order to identify the variability in the multiscale upstream precursors linked to RWT amplification and dispersion that are responsible for EML days over the Midwest and which fraction of these RWT events are linked to the recurvature and extratropical transition of early season TCs over the western North Pacific. The results presented herein are likely not typical of all severe weather outbreaks over the Midwest United States given that western North Pacific TC recurvature and extratropical transition frequencies are low and increasing during the June–August period as compared with autumnal values (see Fig. 7 in Archambault et al. 2013), and that the frequency of RWT events over the North Pacific is lowest during the June–August period (see Fig. 4 in Souders et al. 2014).

There are several other examples in the recent literature that discuss the influence of RWT amplification over the North Pacific (both stationary and propagating Rossby waves) that can influence the subsequent variability in the occurrence of high-impact weather over North America on different time scales. For example, Archambault et al. (2013) similarly identify the role of autumnal western North Pacific TC–extratropical flow interactions in the amplification of preexisting RWTs that may lead to synoptic-scale high-impact weather over North America that occur on 3–7-day time scales. On longer time scales, Teng et al. (2013) identify the role of warm-season transient-eddy–jet stream interactions in the production of RWT amplification and dispersion that modulates the life cycle of heat waves over North America that occurred on ~20-day time scales, while Thompson and Roundy (2013) investigate the vernal, more stationary, Rossby wave response that links anomalous tropical convection associated with the Madden–Julian oscillation to U.S. violent tornado outbreaks on 15-day (and longer) time scales. Studies by Schubert et al. (2011) and Chen and Newman (1998), in addition to some of the studies listed in section 1, provide case study examples of quasi-stationary Rossby wave amplification over the North Pacific that played key roles in the development of U.S. heat waves and associated droughts.

The presented case study also provides an example of how steep LRs in the midtroposphere may not always be associated with severe convection (or any convection) owing to the likely presence of a strong lower- to middle-tropospheric capping inversion or the absence of a sufficient meso–synoptic-scale lifting mechanism. In this example of an important, but not always sufficient, condition for severe convection, the strength of the capping inversion [often quantified by the 700-hPa air temperature (e.g., Bunkers et al. 2010) or CIN (Colby 1984)] may simultaneously inhibit severe convection and promote the development of high surface air and dewpoint temperatures. Junker et al. (1999) and Bunkers et al. (2010) identify that 700-hPa air temperatures > 10°–12°C typically indicate that a strong capping inversion could inhibit severe convection in the absence of a strong meso–synoptic-scale lifting mechanism. In the presented case study the advection of an EML from the Intermountain West was associated with a strong capping inversion over the upper Midwest as quantified by 700-hPa air temperatures at MPX of 14.4°C at 1200 UTC 30 June 2011 and 16.8°C at 0000 UTC 1 July 2011 (Fig. 12). According to Bunkers et al. (2010), these 700-hPa temperatures are almost never associated with severe convection across the upper Midwest. However, in the presence of sufficient lift and resulting convection, steep LRs favor the downward transport of vertical momentum (e.g., Wakimoto 1985), which may help explain the large number of observed severe wind reports during the late June and early July 2011 case study and the large fraction (84%) of severe weather EML days associated with severe wind reports in the composite analysis.

The sensible weather impacts from heat wave events and organized MCSs are both collectively influenced by enhanced low-level moisture (e.g., Kunkel et al. 1996; Bentley et al. 2000). Across the upper Midwest, the source region for this moisture is attributed to the advection of moisture along a low-level jet stream from the Gulf of Mexico or the southern and central plains or attributed to local sources related to the evapotranspiration of wet soils and irrigated agriculture (Kunkel et al. 1996; Karl and Knight 1997). Composite analysis of eight heat wave events by Bentley and Stallins (2008) illustrates that on these days, elevated surface dewpoint temperature anomalies > 3°–5°C are commonly associated with synoptic patterns and thermal profiles characterized by a strong capping inversion (e.g., 700-hPa air temperatures > 10°C). Perusal of archived thermal profiles on days with heat waves across the upper Midwest United States (e.g., 12–16 July 1995, among others) illustrates that these strong capping inversions typically lay beneath EMLs that have been transported east from the Intermountain West (not shown).

7. Concluding summary

The purpose of this study was to jointly provide an overview of the antecedent upstream and in situ meso–synoptic-scale conditions that led to the development of high-impact weather events over the upper Midwest United States during late June and early July 2011 and to place these results into historical context. The two high-impact MCSs, record warmth, and a myriad of severe weather reports all occurred in an environment characterized by steep midtropospheric LRs. These steep LRs were part of an EML that formed in conjunction with the advection of steep surface-based LRs from the Intermountain West to the upper Midwest United States by an RWT that was amplified over the western North Pacific by a recurving and extratropical-transitioning TC. The EML provided an important, but likely insufficient, ingredient for severe convection over the upper Midwest United States. The observed MCSs initiated along the periphery of the upper-tropospheric anticyclone and the region of steepest midtropospheric LRs where sufficient lower-tropospheric moisture and meso–synoptic-scale lifting mechanisms were able to overcome CIN beneath the capping inversion of the EML. In the region of steepest midtropospheric LRs, hundreds of daily record high maximum temperatures and daily record high minimum temperatures were broken or tied over the upper Midwest owing to inhibited vertical mixing of the boundary layer beneath the strong capping inversion of the EML.

A composite analysis of 99 EML days at MPX illustrated that steep LRs over the upper Midwest are often accompanied by LR advection from the Intermountain West along midtropospheric west-southwesterly flow embedded within a Rossby wave over North America. This Rossby wave is, on average, associated with an RWT that was amplified over the central North Pacific 7 days prior to the EML day and disperses across North America beyond 2 days after the EML day. These EML days are associated with an increased likelihood of severe weather, especially in association with severe wind reports, as compared with climatology, and typically are associated with above-normal maximum temperatures, minimum temperatures, daily average dewpoint temperatures, and cooling degree days.

The results of this study suggest that the EML is an important ingredient in the occurrence of warm-season high-impact weather (e.g., severe convection and record warmth) over the upper Midwest when combined with sufficient meso–synoptic-scale lifting mechanisms. The frequency of EML days over the upper Midwest and therefore the likelihood of high-impact weather appear to be modulated, in part, by RWT amplification over the North Pacific, which may provide advanced guidance in forecast applications. Future work is aimed at investigating the role of steep midtropospheric LRs and the EML and their relationship to high-impact weather at other locations across the United States.

Acknowledgments

A majority of this study was funded by the College of Arts and Sciences and the Center for the Environment at Plymouth State University and by the Hobart and William Smith Colleges Provost’s Office. Initial work on this project by JMC, NDM, and TJG occurred during the completion of their Ph.D. research at the University at Albany/SUNY with support from Dr. Lance Bosart and Dr. Daniel Keyser and funding from National Science Foundation Awards ATM-0304254, ATM-0553017, and ATM-0646907. Part of this research was completed by MEH and NDM as part of the 2013 Hobart and William Smith Colleges summer research program. Comments from two anonymous reviewers greatly improved the quality of this manuscript.

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Footnotes

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