Environmental conditions associated with weak tornadoes across southern Virginia and northeast North Carolina in 2003 and 2004

During 2003 and early 2004, 36 tornadoes occurred within the National Weather Service (NWS) Weather Forecast Office (WFO) Wakefield, Virginia (AKQ) county warning area (CWA). All were classified as weak, with ratings of F0 or F1 on the Fujita scale. Most of the tornadic storms were not supercells, and radar signatures were more subtle and short-lived, with generally weaker, shallower and more transient velocity and reflectivity signatures, than in typical tornadic supercells. In most cases, the storms were low-topped, with echo tops of 4 to 11 km, and many did not contain lightning during their lifetimes. In addition, many of the tornadoes were not well anticipated by the WFO AKQ, or by the NWS Storm Prediction Center (SPC). These types of tornadoes are common in the AKQ CWA, and an improved understanding of the environments in which they form is critical to improving the forecaster’s ability to warn for them.

All the cases of tornado development across the WFO AKQ CWA during 2003 and the first half of 2004 were examined, with respect to their storm environments around the times of tornadogenesis. Model soundings from the Rapid Update Cycle (RUC) model were analyzed for the times just prior to tornadogenesis, and near the location of tornado development. Surface, radar, satellite and model data near the time of tornadogenesis were also analyzed.

It was found that, although the tornadoes formed in a variety of different environments, with respect to convective available potential energy (CAPE), vertical wind shear, and synoptic and mesoscale patterns, common characteristics were identified, including very low lifted condensation levels (LCLs) associated with near saturated storm inflow, and surface-based CAPE greater than zero. In addition, mesolows associated with frontal waves, and/or surface boundaries were present in the majority of these events, which likely played an important role in the tornadogenesis processes.

This presentation will examine the conditions that favored the development of weak tornados in 2003 and early 2004. Suggestions for the warning forecaster are given for recognition of possible tornadic environments, and radar signatures given different thermodynamic and shear profiles. It is a focus of this study to improve probability of detection and lead time for Tornado Warnings, which are emphasized in the NWS performance measures.