Radar Climatology of Tornadoes Occurring in High Shear/Low CAPE Environments in the Mid-Atlantic and Southeast

Issuing accurate tornado warnings with sufficient lead time in environments characterized by strong vertical wind shear (0-6 km bulk shear vector magnitude ≥ 35 kts) but low Convective Available Potential Energy (CAPE ≤ 500 J kg^-1), also known as high shear, low CAPE (HSLC) environments, can be a challenge for forecasters. Radar reflectivity and velocity signatures associated with HSLC tornadoes are often weaker than those associated with tornadoes in environments with greater instability. Appearances of tornadic HSLC storms on radar can vary markedly at times from the typical signatures associated with classic supercells in the Great Plains. Rotation may be difficult to detect, especially far from the radar, due to the typically shallower storm depths and smaller mesocyclones and mesovortices associated with HSLC tornadoes. Also, numerous weak areas of rotation may be present at the same time, many but not all of which are non-tornadic, making it difficult for forecasters to discriminate between tornadic and non-tornadic circulations.

This study focuses on tornadoes occurring in the county warning areas (CWAs) of several Southeast and Mid-Atlantic National Weather Service Weather Forecast Offices (WFOs) participating in the Collaborative Science Training and Research (CSTAR) project on HSLC convection. These WFOs include Blacksburg, VA, Columbia, SC, Charleston, SC, Greer, SC, Huntsville, AL, Morehead City, NC, Peachtree City, GA, Raleigh, NC, Sterling, VA, Wakefield, VA, and Wilmington, NC. 302 HSLC tornadoes from 107 HSLC severe weather events identified by these WFOs between 2006 and spring 2011 were included in this study; tornadoes were included based on the CAPE and shear values at the nearest 40 km grid point in the hourly mesoscale analysis produced by the Storm Prediction Center (SPC). An analysis of convective modes for these tornadoes, based on data from the SPC's convective modes database, will be presented. Also, a climatology of azimuthal shear values associated with tornadic and non-tornadic mesocyclones and mesovortices in HSLC environments will be shown, with an emphasis on characteristics of these two types of circulations that can help discriminate between whether they are tornadic or non-tornadic. Finally, an analysis of other radar signatures that have value in indicating the likelihood of a HSLC tornado will presented.