Quasi-Linear Convective System and Supercell Tornado Environments in Central Alabama

126 severe weather events in the county warning area (CWA) of the National Weather Service Birmingham forecast office (NWS BMX) since November 2005 were manually classified by primary convective mode, using archived radar data from the National Climatic Data Center (NCDC). Environmental parameters from archived Storm Prediction Center (SPC) mesoscale analysis data were contained in a preexisting database at NWS BMX. These data were examined for 47 discrete supercell events and 50 quasi-linear convective system (QLCS) events. The effectiveness of near-storm environmental characteristics as predictors of QLCS and supercell tornado production was assessed. Parameters analyzed included basic measures of instability and wind shear as well as composite indices commonly used in severe weather forecasting. Various other combinations of parameters were also tested; those producing highly significant results are discussed.

In discrete supercell cases, 0-1 km storm-relative helicity (SRH) was found to be the most useful individual parameter for discriminating between tornadic and non-tornadic events. Among widely known composite indices, significant tornado parameter (STP) and energy-helicity index (EHI) performed well in supercell cases. A new combination of parameters proved to be the most accurate in identifying supercell events as producers or non-producers. For QLCS cases, no single environmental parameter yielded significant results except 0-3 km convective available potential energy (CAPE). EHI was found to be the most reliable of the widely accepted composite indices in QLCS events. Two new combinations of parameters, involving multiple measures of instability and low-level wind fields, were most effective in separating QLCS producers from non-producers.