A Significant Tornado in a Heterogeneous Environment During VORTEX-SE

On 1 March 2016, an EF-2 tornado occurred in the southwestern Birmingham, Alabama suburb of McCalla, destroying or damaging 26 homes. This tornado was examined as part of the Verification of the Origins of Rotation in Tornadoes in the Southeast (VORTEX-SE) experiment. The environment over most of central Alabama on 1 March 2016 appeared to be only weakly supportive of severe local storms, with very low CAPE and only moderate wind shear. A Severe Thunderstorm Watch was issued only 18 minutes prior to the tornado, and a Tornado Warning had negative lead time. However, both SPC mesoanalysis, and what would normally be an excellent proximity sounding in space and time, greatly underestimated the CAPE. Furthermore, SPC mesoanalyses underestimated wind shear and helicity by a factor of 3. Large spatial heterogeneities and rapid evolution in dewpoint and wind shear were produced that were not resolved by the 40 km SPC mesoanalysis, nor even the proximity sounding. These heterogeneities were associated with a thermal boundary, frontogenetical processes, and antecedent light showers ahead of the supercell storms.

Supercell interactions with thermal boundaries have received attention in the literature, however the frontogenetical nature of the boundary on 1 Mar 2016 caused large mesoscale heterogeneities in wind shear, associated with a thermally-direct circulation. The frontogenesis, along with antecedent showers, also helped produce a local maximum in dewpoint, and therefore CAPE, through moisture flux convergence and associated moisture pooling. The gradient in CAPE was extremely large, with changes of two orders of magnitude over a distance of only 30 km. NASA Lightning Mapping Array data show a peak in lightning activity as the convective system, initially a weakly-organized quasi-linear convective system (QLCS), moved near the boundary and became cellular. Furthermore, it appears that wave features may have interacted with the storm around the time of tornadogenesis.

The main focus of this paper is on the boundary layer heterogeneities, in both wind shear and instability, that preceded the rapid development of an EF-2 tornado in an environment that, according to standard synoptic and mesoscale analyses, did not appear conducive to any tornadoes (SPC mesoanalysis showed CAPE of 250 J kg^-1, 0-1 km SRH near 125 m² s^-2). Meso-gamma scale analysis of CAPE and VAD wind profile analysis of helicity using multiple NEXRAD radars (that showed CAPE greater than 800 J kg^-1 and 0-1 km SRH near 400 m² s^-2 in a very localized region) would have increased the probability that a Tornado Warning would be issued before the tornado occurred. Opportunities to improve operational mesoscale analyses in real time will be discussed, including the wide array of stations now available through the FAA, RAWS, CWOP, and even personal weather stations (PWS).