The 09 December 1999 thundersnow event in west Texas

A case study of heavy snow and snow pellets accompanied by thunder over Lubbock, TX (KLBB), is presented. A total of seven inches of snow and snow pellets fell at KLBB in just over five hours; the first hour and a half featured precipitation accompanied by two periods of thunder as reported by the surface observer. While attributed by forecasters in real-time to the release of conditional symmetric instability, the post-mortem analysis of this event reveals a substantial layer of convective instability in the lower- to mid-troposphere over west Texas. Coupled with the upward vertical motion and copious moisture in the region, upright convection due to the release of the convective instability would appear to have been the culprit. Furthermore, the proximity of the convection to the cyclone center places it in a region of cyclonic shear. As slantwise convection resulting from the release of CSI prefers regions of anticyclonic shear, it seems even more likely that the resulting convection was of an upright, convectively unstable nature.

This case study supports our larger contention that there are two preferred regions of thundersnow in a mid-latitude cyclone: 1) north of the leading warm front, where the release of CSI is more likely, and 2) northwest of the surface low center, manifested as upright convection from the release of elevated convective instability created by the trowal airstream. We will show that the case in question fell under the latter class.