Conditioning and Evolution of High Shear, Low CAPE Severe Environments

High shear, low CAPE (HSLC) severe weather events are prevalent in the Southeast and Mid-Atlantic United States. HSLC events have a low probability of detection, high false alarm rates, and are often poorly forecasted. A key to improving forecasting for HSLC thunderstorms is clarifying the mechanisms that modulate the convective environment over time. Recent studies suggest that both synoptic forcing and mesoscale processes play a crucial role in the evolution of HSLC environments. The specific goal of this research is to diagnose how instability develops and increases over time within different HSLC environments.

In order to study the synoptic and mesoscale influences on convective development, real-data simulations of several HSLC events are performed using WRF-ARW at convection-allowing resolution. Analyses of simulations reveal that surface based CAPE and low-level lapse rates increase significantly in a short time period leading up to convection. In many cases, these increases are coincident with strong temperature and moisture advection near the surface and continuous positive vertical motion, all of which are likely attributable to synoptic forcing. Substantial increases in southerly low-level flow also occur, likely involving diabatic generation of low-level potential vorticity as a result of latent heat release. The resulting increases in low-level wind shear combined with increases in moisture and instability provide an environment favorable for sustaining HSLC severe convection. Budgets and comparative analyses will be used to demonstrate the differences in forcing among different environments.