Severe thunderstorms of 3 April 2004: an examination of a dry-season severe weather event in the Borderland

(Poster Session)

Severe thunderstorms of 3 April 2004: an examination of a dry-season severe weather event in the Borderland

Michael P. Hardiman and Joseph A. Rogash National Weather Service, El Paso, TX

On 3 April 2003 a rare severe thunderstorm event occurred in south-central New Mexico and extreme west Texas. Hail up to 2.50 inches in diameter accumulated to a depth of four inches on US 62/180 west of the township of Cornudas, TX, causing damage to vehicles and a few minor accidents. Golf ball-sized hail accumulated over 12 inches in the town of Chaparral, NM, damaging roofs, windows, and siding on homes in a narrow band on the north side of town.

Such an event is unusual for this region in April, which is typically characterized by dry conditions (normal monthly rainfall 0.26 inches at ELP), with the most notable sensible weather being frequent wind storms with blowing dust. Since 1955 there have only been six days in April with severe-criteria (3/4 inch or larger) hail reported within the current El Paso NWS County Warning Area.

The synoptic scale features included a closed low pressure system located over central Arizona in the middle and upper-troposphere. This disturbance induced unusually strong southerly wind components above the 700 mb layer with maximum middle-tropospheric wind speeds of 60 kts across the area. In addition, the southerly flow aloft advected some moisture from the Gulf of California into the region, in contrast to the normally prevailing westerly winds which usually transport very dry air across the southwestern United States. At the surface, southeasterly winds ahead of an approaching cold front from the northeast provided weak moisture transport in the lower boundary layer. Quasi-geostrophic forcing attendant with the upper low cooled the middle troposphere across central New Mexico and far western Texas, with 700 to 500 mb lapse rates of 8°C km-1 and wet bulb zero heights of only 5700 feet AGL. As a result, as surface temperatures warmed to near normal during the afternoon, and with surface dewpoints above average, afternoon CAPEs increased to 2100 J kg-1 indicating unusually high convective instability present over the region for April. Surface heating combined with upslope flow over sloping terrain most likely initiated deep convection since little convective inhibition was present. The strong mid-level flow provided sufficient shear for updraft rotation and both radar and photographic analyses indicated the largest hail was produced by supercell thunderstorms.