Observational Summary of the Effects of the Northeastern Alabama Plateaus on the Near-Storm Environment of Tornadic Storms during VORTEX-SE

As part of VORTEX-SE, the University of Alabama in Huntsville’s Severe Weather Institute-Radar and Lightning Laboratories (UAH-SWIRLL) has been investigating the possible role of the Sand Mountain and Lookout Mountain plateaus in tornadogenesis distribution patterns across northeastern Alabama. This area has been shown in Lyza and Knupp (2018; hereafter LK18) to be a regional statistical maximum of tornadogenesis, but physical explanations for this maximum have been unclear. LK18 presented several hypotheses for processes that the plateaus may force on the near-storm environment that could increase the probability of tornadogenesis for a storm encountering the plateaus. These hypotheses include:

(1) Enhanced storm-relative helicity (SRH) owing to acceleration and backing of low-level flow over the plateaus;

(2) Enhanced convergence over the northwestern edge of Sand Mountain, owing to the development of a downslope wind acceleration and subsequent restoration of flow northwest of the slope; and

(3) Lowering of lifting condensation level (LCL) heights atop the plateaus relative to the surface.

To investigate these hypotheses, numerous intensive operation periods (IOPs) were conducted in northeastern Alabama between Fall 2016 and Spring 2019 with deployment of UAH-SWIRLL assets, as well as numerous additional assets during the Spring 2017 and Spring 2018 VORTEX-SE field campaigns. This presentation will summarize findings from these IOPs, including confirmation that SRH can be higher atop the plateaus under certain severe storm scenarios, LCLs tend to be 100-200 m lower atop the plateaus, and low-level flow atop the plateaus accelerates into a downslope wind maximum in at least some severe storm environments. Avenues of further necessary research to clarify the mechanisms by which the plateaus may modulate tornado potential are discussed, and how these findings can be used operationally are highlighted.