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

The historic 27 April 2011 Super Outbreak can be separated into three distinct events: the early morning quasi-linear convective system (QLCS) with an embedded mesoscale convective vortex (MCV), a shorter midday QLCS, and afternoon tornadic supercells. The two QLCS's, plus continued development of showers along and north of the boundary, maintained cool temperatures (~16 C) north of the boundary via cloud shading and rainfall evaporation for most of the day. The regions south of the boundary experienced solar radiation and temperatures exceeding 25 C by early afternoon. This set up a prominent, quasi–stationary thermal boundary. The cool side of this boundary was characterized by a shallow near surface stable layer that extended only a few hundred meters with an elevated unstable layer having a CAPE value >2000 J/kg. The boundary likely produced a low-level thermally driven circulation that was able to provide baroclinically generated vorticity, which is believed to be critical in the formation of intense tornadoes. Since this boundary was within the heart of the UAH observational network, fine-scale structural details of the boundary were sampled by three radars,(KHTX WSR-88D, the Advanced Radar for Meteorological and Operational Research (ARMOR) C-band dual polarimetric radar, the Mobile Alabama X-band (MAX) dual polarimetric radar), and in particular instruments comprising the Mobile Integrated Profiling System (MIPS), and the Mobile Meterological Measurement Vehicle (M3V). The MIPS 915 MHz wind profiler and the microwave profiling radiometer documented the temporal evolution of the wind and thermodynamic profiles on the cool side of the boundary, as well as the more humid air mass south of the boundary after the passage of the Hackleburg tornado, which moved 15 km northwest of the MIPS.

Analysis of the MIPS and radar data are placed in the context of the boundary location as determined from an assortment of surface station measurements, satellite imagery, and ground-based images of the Hackleburg tornado, which was surrounded by very low cloud base (200-300 AGL). The Hackleburg tornado was the most energetic of the day, remaining on the ground just north of the boundary for about 212 km. Another distinguishing feature of the Hackleburg storm was the rapidity (~50 min) in which it attained supercell properties. The Smithville, MS, tornado also formed along this boundary in NE MS. Other notable storms that formed on or near this boundary include the Cullman EF-4, the Rainsville EF-5, and the Jackson County EF-4 tornadoes. 57.71.66 on 7-10-2012-->