Investigation of an Unusual Storm Structure Associated with Weak to Occasionally Strong Tornadoes over the Eastern United States

The purpose of this paper is to document an unusual type of tornadic thunderstorm that has been observed in the western Carolinas, northeast Georgia and other parts of the eastern United States in recent years. Most occurred in the "cool season" and developed in an environment characterized by marginal instability and strong, lower tropospheric wind flow. A summary of the environmental conditions present with three different cases that occurred in areas extending from northeast Georgia to southeastern Pennsylvania is presented. Surface and upper air data, as well as the closest representative soundings, are used to derive an average atmospheric profile for the storms. The WATADS software is used to evaluate storm structure through the use of WSR-88D archive II data. Where archive II data are not available, WSR-88D archive IV radar images are used. Our work reveals that the signature typically evolved from a linear MCS into an S-shaped inflection point and eventually into two separate line segments with the southern member accelerating ahead of the line. Weak to occasionally strong tornadoes are found to occur in the early stages, either the result of, or in association with, the break. The work documents that such tornadic events often occur with little or no association with straight-line wind damage as is often the case with other types of bow echo storms. The lack of a reflectivity core suspended above the updraft, and weak rotational velocities along the break, indicate that the tornadoes were not the result of embedded HP supercells. This suggests that tornado-genesis results from a different physical process than found with conventional supercell and comma-head tornadoes. It is hoped that an awareness of the synoptic and mesoscale environment favoring the development of these tornadoes will lead radar operators to anticipate this sequence of evolution, resulting in short, but effective, lead times.