THE HISTORIC CHRISTMAS 2004 SOUTH TEXAS SNOW EVENT: DIAGNOSIS OF THE HEAVY SNOW BAND

摘要

On 24 and 25 December 2004, a rare, banded heavy snow event occurred over portions of south Texas. Maximum snow amounts of 20-33 cm (8-13 inches) were reported in the band region. Such amounts had not been observed in south Texas since the late 1800s.Moisture, lift, stability, and thermal profiles for the event were examined. On the synoptic scale, the event was associated with a strong low-latitude upper- level trough (26-28°N) and jet stream maximum of 72-77 m s"1 across northern Mexico and southern Texas. This trough and jet maximum combined to produce large scale lift over an already sufficiently moist south Texas region. At lower levels, a deep freezing/sub-freezing air mass was present as far south as northern Mexico. Confluent upper-level flow located northeast of the snow region, helped to maintain a deep cold air mass by producing northerly flow and cold advection at lower levels. It was surmised that the rarity of the event was due to the combination of the very low- latitude upper-level trough, and the deep cold air mass. Anomaly calculations confirmed that both the 500 hPa heights and 850 hPa temperatures within the trough over north central Mexico were around four standard deviations below normal during the event. To diagnose the snow band region, cross sections of saturated equivalent potential vorticity (EPV*], Petterssen frontogenesis, saturated equivalent potential temperature (θ_(ex)), and relative humidity (RH) were constructed perpendicular to the snow band. During the entire heavy snow period, the cross sections indicated the presence of negative EPV* located just above an axis of mid-level frontogenesis. However, the 9 surfaces in the cross sections showed a transition from the release of conditional instability (CI) and upright ascent at the beginning of the event, to the release of conditional symmetric instability (CSI) and enhanced slantwise ascent about halfway through the event, as the θ_(ex) surfaces became more horizontal.