Thursday, 25 October 2018
Stowe & Atrium rooms (Stoweflake Mountain Resort )
A severe quasi-linear convective system (QLCS) impacted north Alabama during the evening hours of 3 April 2018. Extensive wind damage occurred with the QLCS, as well as numerous recorded wind gusts at or above 26 m/s (50 kt, 58 MPH). Environmental soundings ahead of the QLCS indicated a well-mixed convective boundary layer with steep low-level lapse rates and weak inverted-v profiles. Mixed-layer lifted condensation level (MLLCL) values generally ranged from 1300-1800 m AGL, much higher than climatologically favorable values for tornadoes across the southeastern United States. Despite these observations, analysis of research radar data and extensive post-event survey efforts by personnel from the University of Alabama in Huntsville’s Severe Weather Institute – Radar and Lightning Laboratories (UAH-SWIRLL) and the University of Oklahoma (OU) found at least 18 distinct tornado tracks of EF0-EF2 intensity. Automatic surface observing stations (ASOSs) at both Huntsville (KHSV) and Decatur (KDCU) were directly impacted by separate tornadoes at EF0 intensity, with KHSV measuring a peak wind gust of 31 m/s (70 MPH) and KDCU measuring a peak wind gust of 32 m/s (72 MPH). Five of the tornadoes impacted the city limits of Decatur (pop. ~55,000), damaging or destroying over 100 homes and causing at least one serious injury, the worst storm damage in the city since 1974. At least 8 of the tornadoes were associated with definitive or likely polarimetric tornado debris signatures (TDSs), and the maximum observed gate-to-gate shear was over 40 m/s. The tornadoes featured path lengths ranging from approximately 0.3 to 14.4 km and maximum path widths ranging from approximately 80 m to as much as 740 m. Damage outside of the tornadoes appeared limited in scope and intensity as observed by survey teams, despite an environment that appeared favorable for widespread, non-tornadic convective wind damage.
This presentation serves as an overview of the 3 April event. The evolution of the pre-storm environment is examined through the VORTEX-SE array of profiler and rawinsonde observations. A detailed overview of the damage analysis process is given. The event is placed in climatological context, highlighting the rarity of such an event relative to past tornado environments across north Alabama. The implications of these environmental and storm-scale observations on the anticipation and detection of QLCS tornadoes is discussed.
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