Mesoscale analysis of wintertime nonmesocyclone tornadogenesis in northwest Texas: 27 December 2007

On 27 December 2007, a complex storm system produced a wide variety of high-impact weather across the Texas South Plains. The primary impacts included: intense convective snow showers, strong wind gusts up to 23 m s^-1, critical fire weather conditions, and two brief non-mesocyclonic tornadoes near Lubbock. While the tornadoes produced only minor damage, they are notable for the fascinating meteorological environment in which they formed.

On this day, a cold front became stationary just to the north of Lubbock, and created a strong temperature gradient across the area. As height falls spread over the region, a surface low developed near Clovis, New Mexico, and moved parallel to the frontal boundary across West Texas. Deep mixing and a strong pressure gradient south of the surface low resulted in strong southwesterly winds and very dry boundary layer air. The leading edge of the well-mixed air was demarked by an eastward-moving dryline that intersected the stationary front. By afternoon, isolated convection initiated over the southwest Texas Panhandle; along and just to the north of the cold front. These storms brought snowfall rates in excess of five cm hr^-1. Farther south, convergence along the leading edge of the dryline led to the development of a broken line of cumulus congestus towers which moved across the central South Plains. Between 2300 and 0000 UTC, two nonmesocyclonic tornadoes were observed and photographed across the northern and eastern portions of Lubbock. Radar presentation of the parent cells merely depicted a faint “fineline” in both the reflectivity and velocity moments.

A preliminary mesoanalysis performed using data from the West Texas Mesonet indicated that baroclinity, convergence and vertical vorticity were maximized in the vicinity of the two pre-existing boundaries. Rapid Update Cycle model soundings indicated that 0-3 km lapse rates of 7-9°C km^-1 were present with little convective inhibition. Previous research has identified that this type of environment is supportive of nonmesocyclone tornadogenesis. In this case, low ambient temperatures during the winter (historically, no tornadoes had previously been recorded in or near Lubbock during December) and a lack of radar-detected deep convection presented a unique challenge to the operational forecaster.