The historic Christmas 2004 south Texas snow event: Diagnosis of the heavy snow band

On 24 and 25 December 2004, a rare, narrow banded heavy snow event occurred over portions of south Texas. Snow amounts within this heavy band ranged from eight to 13 inches (20-33 cm), which were amounts not observed in south Texas since the late 1800s. To investigate the cause for the rare heavy snow event, the availability of moisture, lift and stability were examined. Although the observed snowfall totals were of record proportions for many locations, the synoptic and mesoscale features present during this case seemed typical to many other heavy banded snow events in the refereed literature.

Cross sections of saturated geostrophic equivalent potential vorticity (hereafter EPV), Petterssen frontogenesis, saturated equivalent potential temperature (θ_es), and relative humidity were constructed perpendicular to the band to examine the time evolution in the vertical of the listed meteorological parameters. For the entire event (approximately 12 hours), the cross sections indicated negative values of EPV just above an axis of frontogenesis. At the beginning of the event, the θ_es surfaces in the cross sections were decreasing with height just above the frontogenesis axis, which indicated that upright convection from the release of conditional instability (CI) was dominant within the heavy snow band region. Three to four hours into the event, the cross sections still showed the θ_es surfaces decreasing with height, but the depth of the decrease was shallower, and was no longer located just above the frontogenesis axis. By four to six hours into the event, the surfaces were no longer decreasing with height, exhibiting more conditionally stable conditions. This may have indicated the presence of conditional symmetric instability (CSI) within the band region. Therefore, sometime midway into the event, there appeared to be a transition from the release of CI through upright convection, to the release of CSI through slantwise convection within the heavy snow band.