An example of a left-split supercell producing 5-inch hail: The Big Spring, Texas Storm of 10 May 1996

Left-split supercell thunderstorms have been the subject of only a limited amount of research over the years. On 10 May 1996 the city of Big Spring, Texas was pummeled by a left-split supercell thunderstorm that produced tennis ball or greater size hail over half of this city of 25,000 people. This case study discusses the synoptic setting as well as characteristics of the storm as depicted on the Midland, Texas WSR-88D radar.

An upper level shortwave trough was approaching the area, while at the surface a pre-frontal trough extended northeast to southwest between Big Spring and San Angelo, Texas. Storms initiated both north and south of the surface trough. The evening sounding taken at Midland revealed a straight-line hodograph with sufficient shear, indicating splitting storms could be expected. Indeed, storms did split across the region with left-split storms surviving north of the boundary, while right-split storms survived south of the boundary. The pairs to these splitting storms in each area quickly dissipated.

Low-level winds in the Big Spring area were northerly prior to the storm. The hodograph, adjusted for conditions near Big Spring, showed negative storm-relative helicity and a sufficient vertical wind profile to maintain storms with a persistent meso-anticyclone. Alternatively, a hodograph depicting conditions in the San Angelo area showed positive helicity and favored persistent mesocyclones. Supercell thunderstorms developed in both areas with strong mesocyclones in the San Angelo area and strong meso-anticyclones in the Big Spring area. North winds north of the boundary and southeast winds south of the boundary enhanced storm-relative inflow in both areas.

Stable conditions are often found north of a surface boundary, however, high time resolution weather data from the USDA Agricultural Research Station in Big Spring suggested that surface conditions were quite unstable. Also, the north wind supplied good inflow to this northeastward moving storm and induced strong vertical shear through the lowest 6 km.

An analysis of WSR-88D VR/Shear on the Big Spring storm showed that a deep, persistent anticyclonic rotation was detected for at least two hours. Also, a significant overhang was present in the updraft region on the northern flank of the storm.

There were significant challenges to forecasters in real-time anticipation (or even appreciation) of the storm behavior in the Big Spring area. Since the WSR-88D mesocyclone algorithm does not search for meso-anticyclones, it is incumbent upon the forecaster to be alert to the possibility for these types of circulations. Also, strong values of instability, storm-relative inflow, and wind shear can occasionally all be found north of a surface boundary. A thorough knowledge and use of a modified hodograph would greatly enhance a forecaster's anticipation this type of storm behavior.