Previous work has demonstrated that high mid-level lapse rates, or elevated mixed layers (EMLs), are an important ingredient for the development of intense deep, moist convection. Along with contributing to a favorable environment for the development of severe mesoscale convective systems, EMLs can also be associated with extremely high surface air temperature (> 40°C) and dew point temperature (> 25°C) values during urban heat wave events over the continental United States (US). The former typically occurs over the northern High Plains of Nebraska and the Dakotas as the EML reaches the surface in association with sensible heat-driven boundary layer mixing. The latter can typically occur over the Great Lakes region and upper Mississippi valley as shallow moist boundary layers develop beneath EMLs either through northward moisture advection from over the Gulf of Mexico or by in situ growth through evapotranspiration processes from underlying vegetation. The low-level moisture slowly pools beneath the EML because the inversion at the base of the EML mitigates boundary layer mixing into the mid-troposphere.

The purpose of this presentation is two-fold. The first part of the presentation will introduce an objective climatology of EMLs over North America during 1974–2007 in order to assess the spatial and temporal variability of these features. With the climatology providing broader perspective, the second part of the presentation will examine the evolution and importance of EMLs during selected heat wave events that affected urban areas over the northern Plains and Great Lakes regions [e.g., July 1995 heat wave over Chicago, IL (CHI), and Milwaukee, WI].

Preliminary results from the climatology highlight a warm season maximum in high lapse rates over the Intermountain West that expands poleward during March through August. These high lapse rates tend to advect eastward over the northern Great Plains and Great Lakes region during June through August in conjunction with strong westerly flow aloft over the northern US on the poleward side of the climatological upper-level anticyclone over the southern-central US. Cursory examination of the July 1995 CHI heat wave case study suggests that an EML was advected eastward to over the northern Great Plains and Great Lakes region during 9–14 July 1995. This EML was important in contributing to surface dew point temperature values > 25°C over the greater CHI area, by inhibiting the vertical mixing of near-surface moisture, produced in situ via evapotranspiration processes, into the midtroposphere. These high surface dew point temperature values, combined with air temperature values >35°C, contributed to ~800 casualties throughout the greater CHI area as previous work has indicated.