Research and Operational Challenges Posed by the 18 November 2017 High-Shear/Low-CAPE QLCS Tornado Outbreak in North Alabama

A classic high-shear/low-CAPE (HSLC), quasi-linear convective system (QLCS) tornado event impacted the Tennessee Valley region of north Alabama during the late afternoon and early evening hours of 18 November 2017. An exhaustive survey by personnel from the University of Alabama in Huntsville’s (UAH) Severe Weather Institute – Radar and Lightning Laboratories (SWIRLL), spanning approximately 75 man-hours in the field over 10 days, revealed 14 separate tornado tracks, 2 of which reached significant (EF2) intensity. Three people were injured, with all injuries occurring in the significant tornadoes. Twelve of the 14 tornadoes occurred within 2 long-lived mesovortices/line breaks within the QLCS. The first vortex tracked approximately 75 km across northwestern Alabama, producing 3 tornadoes (1 EF0, 1 EF1, and 1 EF2) and 2 injuries. The second and most substantial mesovortex/line break developed across northwestern Alabama, near the Mississippi/Alabama state line, and moved approximately 210 km across northern Alabama, dissipating along the Alabama/Georgia border. This vortex was responsible for 9 of the 14 tornadoes (2 EF0, 6 EF1, and 1 EF2) and 1 injury, with tornadic damage found across 7 north Alabama counties. In addition to the 14 tornado tracks, 2 areas of damage were surveyed that were found to be produced by vortices of sub-EF0 intensity. Both of these vortices featured tight circulations observed at close range with two polarimetric radars, and one had a depression in correlation coefficient consistent with debris.

An event of this magnitude was not anticipated in advance by UAH-SWIRLL personnel. As suspicion grew during the morning and early afternoon that an event worthy of deployment may occur, based on of surface temperature and humidity trends, as well as high-resolution model guidance indicating the potential for more robust convection, a skeleton deployment was quickly assembled, which featured placing the Mobile Alabama X-band (MAX) radar and the Mobile Meteorological Measurements Vehicle (M3V) in the field while operating the Mobile Integrated Profiling System (MIPS) and Mobile Lidar and Sounding system (MoDLS) at SWIRLL. A sounding crew also operated at SWIRLL, launching 2 balloons prior to the arrival of the QLCS, and the Advanced Radar for Meteorological and Operational Research (ARMOR) was in operation at Huntsville International Airport. Despite the hasty deployment strategy, a bevy of noteworthy observations were made. The MAX radar, placed 18 km northwest of the ARMOR radar, observed 5 tornadoes and 1 sub-tornadic vortex within 20-km range, while the ARMOR radar observed 3 tornadoes and 1 sub-tornadic vortex within 20-km range. Parts of 3 different tornadoes, including the entirety of an EF1 tornado, occurred within the high-resolution ARMOR-MAX dual-Doppler domain, which featured a short 18-km baseline. The M3V gathered 1-Hz resolution surface data in the southern periphery of the most-prolific mesovortex, where it sampled a 25 m/s wind gust approximately 6.8 km south of an EF1 tornado.

This event also posed significant challenges to the official forecasting and warning process. While a slight risk area was issued in the Day 1 Convective Outlook by the Storm Prediction Center for northwestern Alabama, the slight risk did not encompass most of the tornadoes, the slight risk was only valid for damaging non-tornadic winds, and no severe thunderstorm or tornado watch was issued for any part of north Alabama. Additionally, no tornado warnings were issued by NWS Huntsville during the event.

This presentation showcases several of the observations made by UAH-SWIRLL instrument platforms during the tornado outbreak. These observations are framed in addressing several of the challenges posed by this event. Environmental observations via the MIPS and/or the balloon soundings will be used to demonstrate the evolution of the pre-storm environment. The radar observations from ARMOR and MAX will be showcased, with an emphasis on their roles in aiding understanding of the magnitude of the event. The importance of detailed surveys after QLCS events in order to properly frame many of the storm-scale research questions will be highlighted. Broader questions will be posed regarding polarimetric tornado detection, the detection of sub-EF0 vortices, and the commonality of different causes of wind damage in HSLC QLCS events. Finally, the scientific observations will be merged with observations made during interviews with tornado victims during the survey process to discuss questions in effective QLCS tornado warning methodology.