The Glass Ceiling: Analysis of 19 July 2006 Western Iowa Null Severe Weather Event Daniel R. Chavas, University of Wisconsin-Madison, Madison, WI AOS 453 Final Case Study 13 May 2007 ABSTRACT On July 19th, 2006 strong near-surface warm moisture convergence over western Iowa to the southwest of an intensifying warm front boosted temperatures above 90F with dewpoints as high as 85F in some locations. Meanwhile, a very dry, generally well-mixed air mass was in place aloft. The combination of these two ingredients resulted in extreme levels of instability that, coupled with a nearly idealized wind profile and strong frontogenetical forcing, produced conditions ripe for a severe weather outbreak. However, strong warm air advection associated with the southwesterly low-level jet just above the moist boundary layer acted to intensify and deepen the capping inversion, inhibiting convective initiation. Analysis of the effects of evapotranspiration from crops is also performed. 1. Introduction Too much instability can sometimes be a self-limiting condition for convective initiation. Such was the case during the 19 July 2006 null severe weather event over northwestern Iowa. A dominant upper ridge permitted the development of a deep, hot, dry well-mixed layer at the surface over the Great Plains, while near-surface winds advected moisture northwestward from the Gulf of Mexico as a weak surface cyclone developed over the north-central Great Plains. As warm frontogenesis intensified, staggering indices of convective instability and supercell potential were observed by late afternoon as temperatures soared into the mid-90s F and dewpoints rose into the mid-80s F. However, as observed directly by the author and his companion, not a single significant cumulus cloud was observed throughout the afternoon and evening, as a shallow boundary layer “dome” of what was effectively fog (visibilities were reduced to 1-2 miles) due to the extreme moisture content trapped beneath a very strong inversion created oppressively humid near-surface conditions. The author seeks to assess the precise mechanisms that trapped this unstable air mass so close to the surface and how, despite the presence of anomalously strong forcing for vertical motion associated with intense warm frontogenesis combined with the arrival of a robust mid-level trough, not even a single convective cloud was able to form. Furthermore, a simple formulation is employed to assess the effect of evapotranspiration on boundary layer equivalent potential temperature values and the associated potential forconvection. 2. Data Analyses and 6-hour forecasts are taken from 6-hourly Eta model runs and 3-hourly RUC model runs from 0000 UTC 19 July to 0000 UTC 20 July 2006. Radar is taken from the archived NEXRAD local radar reflectivity data. Satellite images are taken from the GOES-8 satellite image archive. 3. Synoptic Overview The synoptic set-up for the 19 July 2006 null severe weather event includes many of the key ingredients that typically contribute to a truly explosive outbreak. At 1200 UTC (6a local time) on the 19th, a strong westerly jet stretches across the United States-Canada border, as depicted in the Eta model analysis 250 hPa plot (fig. 1), including a 90-knot maximum wind speed located along the northwestern border of North Dakota, northern Minnesota, and reaching the western edge of Lake Superior. In the vicinity of the jet exit region, the jet streak curves clockwise across central and southeastern Wisconsin. Curiously, this places the null event location interest just to the south of a region strong upper-level ageostrophic convergence and associated subsidence in the right exit region of the jet, which would be expected to suppress convection. However, the shear associated with the jet maximum would also be expected to enhance the initiation and maintenance of supercellular activity. Figure 2 depicts the 700 hPa large-scale flow, which includes a dominant Fig 1. 250 hPa heights, wind speeds, wind barbs from 1200 UTC 19 July 2006 Eta model analysis. anticyclone over southeastern Kansas. The circulation associated with this feature provides moderate northwesterly winds, with speeds at approximately 20 knots. Thus, there exists moderate speed and directional shear between middle and upper levels, but, more importantly, the flow throughout this layer is westerly and of significant magnitude to efficiently tilt an updraft and advect convective outflow away from the updraft core, creating the classic cumulonimbus anvil that is characteristic to a powerful supercell. A strong shortwave is visible in the 3160m height contour over the west-central Dakotas, and a second high-amplitude shortwave trough is visible in the 3200m height contour over eastern Colorado. These two shortwave features combine to create one of the two strong dynamical forcings for vertical motions during this null case. Finally, two moist pockets are located to the northwest and to the south of Iowa, with a channel of very dry air in between. Given the flow at 700 hPa based upon the height contours, one would expect the moist pocket to the northwest to be advected just to the north of Iowa, while the tongue of very dry air would be advected overtop of the region. This dry channel clearly indicates that the free troposphere will be dominated byFig 2. 700 hPa 1200 UTC 19 July Eta model run. (a) wind speeds (fill, kts), heights (m), wind barbs (kts); (b) relative humidity (fill, %) and potential temperature (K). very low θe air, suggesting high convectively instability of moist low-level air. The 850 hPa flow is depicted in figure 3, where the flow has shifted towards a more southwesterly pattern. Furthermore, a small 40-knot low-level jet maximum is visible across the eastern border between South Dakota and Nebraska and stretching into northwestern Iowa. The existence of this feature indicates the availability of strong speed shear at low-to-mid