Observations of a Thermal Boundary and its interaction with the 27 April 2011 EF-5 Hackleburg Tornado

The 27 April 2011 super outbreak can be separated into three distinct events: an extensive early morning quasi-linear convective system (QLCS), a shorter midday QLCS confined to northern Alabama, and afternoon tornadic supercells. The midday QLCS, in particular, produced a significant cold pool and associated thermal boundary over northern AL and NE Mississippi. This thermal boundary produced clouds and showers during the afternoon hours that reinforced the cool air while a capping inversion in the air mass south of the boundary experienced warmer temperatures via solar heating that reinforced the thermal boundary. By mid-afternoon this boundary started to propagate northward as indicated by NOAA, FAA, DOD, university, and general public network surface observation stations located over Alabama and Mississippi. The cool side of this boundary was characterized by a shallow near surface stable layer that extended only a few hundred meters AGL with an elevated unstable layer of 2500 J/kg. It is believed this boundary produced a low-level thermally driven circulation that was able to provide baroclinically-generated vorticity, which is thought to be critical in the formation of intense tornadoes. This boundary was well sampled by UAH and NWS/DoD facilities, including WSR-88D radars at Hytop, AL, and Columbus, MS; the Advanced Radar for Meteorological and Operational Research (ARMOR) C-band dual polarimetric radar, the Mobile Alabama X-band (MAX) dual polarimetric radar, the Mobile Integrated Profiling System (MIPS, which includes a 915 MHz Doppler wind profiler and microwave profiling radiometer), and Mobile Meterological Measurement Vehicle (M3V), and a balloon sounding acquired at 2120 UTC.

Through integrated radar measurements from the above radars a comprehensive wind profile time series can be produced. These wind profiles reveal the evolution and horizontal variability of storm-relative helicity as the boundary approached and passed each radar. In addition, single and dual-doppler analyses from MAX, ARMOR, and KHTX are used to relate storm structure and strength with proximity to the boundary. The EF-5 Hackleburg supercell appeared to be significantly influenced by this boundary. This storm formed in eastern Mississippi and traversed north Alabama before dissipating in south central Tennessee. It produced a tornado that remained on the ground for approximately 212 km. Surface data indicate that the storm formed on the cool side of the boundary and remained on that side for the duration of its time in Alabama. Other notable storms that formed on or near this boundary include the Cullman EF-4, Rainsville EF-5, Jackson County EF-4, and Smithville EF-5.