The Monitoring of 0-5 km Storm Relative Environmental Helicity Leading Up to and During the Occurrence of an HP Supercell Over Deep South Texas


Paul A. Yura and Mark E. Jackson

National Weather Service

Brownsville, Texas


National Weather Service
20 S Vermillion Rd.
Brownsville, TX 78521
Phone: (956) 504-3084

Fax: (956) 982-1766

e-mail: paul.yura@noaa.gov

Previous studies have well documented the correlation between subcloud layer veering of low-level winds and thunderstorm structure. Strong veering which may lead to high Storm Relative Environmental Helicity (SREH) has been shown to support supercell development, while unidirectional low-level winds and low SREH tend to be associated with multicellular structure and squall line evolution. As a result, the use of SREH can be a highly useful parameter during warning decision making situations and has become increasingly popular in operational severe thunderstorm forecasting. However, it's important to understand the various factors which influence SREH. Of obvious influence is the varying storm motion over time and space. Oftentimes overlooked, however, is the effect of changing ambient low-level winds in the proximity of the thunderstorm. It is therefore critical that forecasters firequently monitor these faceors when determining SREH values in anticipation of thunderstorm structure.

This study examines the development of a heavy precipitation (HP) supercell which affiected purtions of deep south Texas the afternoon of May 11, 1996. Due to their rarity in deep south Texas, it is important to understand their formation and behavior. This thunderstorm formed under northerly upper-level flow, a regime conducive to severe weather events in this region. The storm developed a strong, persistent mesocyclone and bounded weak echo region (BWER) during its lifecycle, with radar animations depicting HP supercell charactenstics, including a forward flank notch (FFN) and front flank mesocyclone. As the storm moved south the parent mesocyclone produced a Tornado Vortex Signature (TVS) and substantial wind damage in and around the city of La Joya, Texas.

Of importance is that 0-5 km SREH values calculated from the morning sounding taken at Brownsville, Texas did not indicate the potential for rotating updrafts. It will be shown that when storm motion and, most importantly, afternoon wind data from the KBRO WSR-88D were utilized, 0-5 hem SREH values substantially increased to levels which supported mesocyclone and supercell development. This paper provides an example of the importance to modifying the hodograph with current wind and storm motion data, thus providing forecasters with valuable information regarding the likelihood or non-likelihood of supercell development and possible turnadic activity. Velocity Azimuth Display (VAD) information from the WSR-8SD data, high resolution model data, as well as new satellite-derived sounder products, provide forecasters with ample opportunities to view a more accurate SREH prior to and during the event.

Following a brief description of the synoptic environment leading up to the convective development, the presentation focuses on the evolution over time of the Iow-level wind field, and subsequent SREH values, in proximity to the HP supercell investigated. Radar reflectivity and velocity data will be shown to describe its structure. The study concludes with a discussion of various sources and techniques available to forecasters in Ihe modernized National Weather Service using SREH to anticipate monitor thunderstorm structures.