A Study of the Pre-storm Environment of Tornadic Quasi-linear Convective Systems

Tornadoes are known to evolve from a variety of different parent storm types, including supercells, squall lines, and other convective systems. Trapp et al. (2004; hereafter, TTGB04) classified reported tornadoes in the United States from 1998–2000 by parent storm type: cell, quasi-linear convective system (QLCS), or other. It was found that tornadoes from QLCSs, such as squall lines or bow echoes, composed 18% of the reported tornadoes during the period of study. Because of this, it is imperative for the safety of human life and property that an adequate warning time is given for QLCS tornadoes. Part of the tornado warning problem is due to rapid QLCS tornado development. It is possible for rotation to develop and form a tornado in only a few radar volume scans. Trapp et al. (1999) found that the majority of tornadoes that formed within QLCSs had nondescending tornadic vortex signatures (TVSs). These non-descending TVSs appeared, in the mean, just 5 min prior to the time the associated tornado began. This makes forecasting QLCS tornadoes an extremely difficult task for the operational meteorologist. Unlike the well-known hook echo associated with supercell mesocylones, there are no widely accepted radarbased indicators of QLCS tornadoes (Trapp et al. 1999). The large areal extent of many QLCSs makes it difficult to determine the precise location where the greatest tornado threat may exist. According to Atkins et al. (2004), QLCS mesovortices are less likely to be detected than supercell mesocyclones, especially at larger distances from the radar. Additionally, although much work has been done to determine the environments in which supercells may become tornadic, little effort has been made to determine the environmental conditions favoring QLCS tornadogenesis. These conditions could aid forecasters in making the decision whether or not to issue a tornado warning for a QLCS. This study seeks to investigate pre-tornadic environments by examining soundings near tornadic cells and