On the usage of composite parameters in High-Shear, Low-CAPE environments

Mid-latitude High-Shear, Low-CAPE (HSLC) environments (defined here as those associated with 0-6 km bulk shear vector magnitude ≥ 35 kt and surface-based CAPE ≤ 500 J/kg) are responsible for a considerable fraction of severe weather events in the United States, particularly across portions of the Southeast, Mid-Atlantic, and Ohio Valley during the cool season and overnight. However, such environments are also commonly non-severe, and the ability to discriminate between severe and non-severe HSLC environments remains limited. Individual event case studies have noted synoptic scale, mesoscale, and storm-scale features characteristic of severe HSLC environments, but these case studies have yet to be synthesized into a unified conceptual model. One particular challenge noted by forecasters within the aforementioned case studies is the poor skill exhibited by CAPE-driven composite parameters (e.g., significant tornado parameter, energy-helicity index, Craven-Brooks significant severe parameter) during HSLC episodes, with little to no discernible difference between values in severe versus non-severe HSLC environments. Recently, some alternative HSLC composite parameters have been developed, although they are still experimental and subject to some operational contingencies. This presentation will review prior HSLC case studies and climatologies, focusing on the strengths, weaknesses, and caveats of utilizing composite parameters for operational guidance in these environments.