Identification of Subtle Factors that Result in the Development of Persistent Severe Storms near Upper Level Ridges

During severe weather episodes on 10 October 2007 and 15 June 2008, destructive hail up to 7 cm in diameter and 45 m s-1 winds accompanied long tracked supercells over the west Texas High Plains. These storms were responsible for major property and agricultural damages and a number of injuries. Both of these events occurred as subtle middle and upper tropospheric waves propagated through the northern periphery of subtropical ridges. In both cases, northwesterly flow aloft acted to increase deep layer wind shear and to steepen lapse rates sufficiently to support long tracked and particularly destructive supercell thunderstorms.

Favorable environments for supercells can occur over the western High Plains when a moist and unstable low level airmass advects northwestward beneath the periphery of an upper level ridge. While significant severe storms have previously been documented to occur within similar patterns, many forecasters are prone to underestimate the severe weather potential near a subtropical ridge aloft. This underestimate may be the result of conceptual models which support a perception of relatively weak atmospheric wind fields and associated weak shear, and warm middle tropospheric temperatures that yield increased stability. Instead, deep convection that develops near upper level ridges may be driven by more subtle atmospheric features such as low amplitude shortwave troughs, weak surface boundaries, and upslope low level flow.

This study will examine the severe storm environments associated with the 10 October 2007 and 15 June 2008 supercell cases in west Texas. An analysis of numerical weather model output and observational data will be used to highlight the subtle meteorological features that contributed to a favorable environment for the evolution of these damaging events. The goal of this study is to raise forecaster awareness of local patterns dominated by synoptic scale ridging, yet supportive of organized supercells over the western High Plains.