Identifying Discriminating Environmental Features Between High Shear/Low CAPE Severe Convection and Null Events

In the mid-Atlantic and southeastern United States, many severe weather events are characterized by large 0-6 km shear (≥ 35 kts) but low surface-based Convective Available Potential Energy (CAPE; ≤ 500 J kg-1). High shear/low CAPE (HSLC) events remain a challenge for operational meteorologists at the National Weather Service (NWS) Storm Prediction Center (SPC) and local NWS Weather Forecast Offices due to their propensity to occur within climatologically unfavorable times and environments. Previous studies by SPC have indicated that over one quarter of all tornadoes that occur in the United States are associated with low CAPE, while a considerable fraction of those are significant (EF2 or greater on the enhanced Fujita scale). However, existing research on HSLC severe convection only spans the last two decades, and few studies have focused on the mid-Atlantic and Southeast.

The purpose of the present study is to develop a parameter-based climatology in order to discriminate high-end HSLC severe weather episodes from null events. We will detail the temporal and spatial variability of HSLC events, investigating annual, diurnal, and regional cycles of severe HSLC convection. Additionally, we will describe the thermodynamic and kinematic differences between HSLC events that produced severe convection compared to those that resulted in non-severe convection, including ingredients that are skillful in determining the dominant convective mode and associated hazards. Finally, we will discuss the operational utility of existing composite parameters—such as the significant tornado parameter and vorticity generation parameter—during HSLC events and the practicality of developing a composite parameter specifically for marginally unstable environments.