Variability in High-Shear, Low-CAPE QLCS Environments in the Southeastern United States

Abstract High-shear, low-CAPE (HSLC) quasi-linear convective systems (QLCSs) in the southeastern United States pose a challenge to operational forecasters. Prior studies have investigated bulk environmental ingredients associated with the severe weather produced by these systems; however, there is spatial and temporal variability in HSLC severe weather from QLCSs that has yet to be explained. This study sought to improve the conceptual model for HSLC QLCSs by characterizing heterogeneities in their environments. In pursuit of this goal, QLCS-relative analyses that incorporate hourly data spanning the full duration of 79 HSLC QLCS events occurring across the southeastern United States were created using archived operational High-Resolution Rapid Refresh (HRRR) model data. “All case” composites reveal that HSLC environments are not uniformly “low CAPE,” and HSLC QLCSs may be partially fed by elevated “most unstable” parcels. Additionally, comparisons of different QLCS regimes show greater bulk shear magnitudes along the poleward portions of the QLCS convective lines during tornadic hours, which may explain the poleward bias of QLCS tornado reports noted by prior studies. A strong correlation was also identified between low-level nocturnal stabilization and the enhancement of both the low-level jet and low-level vertical wind shear. These enhancements could account for the prevalence of nocturnal tornadoes produced by these systems. Finally, the potential response to diabatic heating (i.e., low-level flow enhancements) may contribute to increased bulk shear magnitudes during the mature phase of these systems, which could help address the noted time lag between system formation and the occurrence of the first tornado reports. Significance Statement Severe convective storms occurring in limited buoyancy (CAPE) present challenges to operational forecasters. These low-CAPE environments can produce severe hazards, including tornadoes, and have the tendency to occur during the cool season and overnight hours. Here, we analyze proxy data to investigate variability in the environments preceding severe squall lines occurring in limited buoyancy. Our results reveal differences in the prestorm environments of these systems that may help address the prevalence of nocturnal tornadoes produced by these squall lines as well as why tornado reports tend to be biased poleward and occur several hours after the formation of these systems.