It remains an open question how tornadoes could develop in low-shear, high-CAPE environments like the one that characterized the 27 May 1997 Jarrell, TX event. Previous observations have been made of tornadoes in similar environments but our understanding of the mechanisms that lead to their formation in such cases is lacking. One way to explore these mechanisms is to examine the nature of updraft evolution and organization. We feel that this approach will allow us to identify probable sources of vorticity generation.

Several questions will serve to focus this work. Did discrete cells developing on or near a preexisting northeast-southwest oriented wind-shift boundary (presumably as a result of interaction between the storm’s gust front and the preexisting boundary) and moving as isolated updrafts through the storm account for the development of updraft rotation? Or did discrete cells “merge” with a main/background updraft thereby suggesting a vorticity consolidation process previously modeled in an HP supercell? Perhaps the character of cell development and storm evolution periodically changed from multicellular (discrete upstream development and discontinuous propagation) to unicellular (“secondary” development manifested as updraft maintenance and continuous propagation). If this change occurred was it tied to the strengthening of updraft rotation and therefore likely representative of a change from cell development forced by surface convergence to cell development/maintenance forced by mid-level pressure deficits owing to the interaction between the updraft and the environmental shear? Did this change coincide with a change in the location of cell development and overall storm propagation; namely a shift from an upwind location on the boundary to a location adjacent to the strongest cell and perpendicular to the shear vector?

Although discrete updraft evolution would be consistent with the convective mode expected from the low-shear, high-CAPE environment associated with this case, it is not consistent with the observed tornado motion: it is not clear how southwestward moving tornadoes lasting up to 30 minutes can be associated with updrafts moving northeastward in the mean wind. It is also not obvious how the short-lived “air-mass” cells could produce vertical vorticity of mesocyclonic (let alone tornadic) strength. While the updraft/vorticity consolidation theory could account for the southwestward motion of the tornadoes (since the main/background updraft would propagate in the direction of updraft development) the inherent steadiness of the storm belies the nature of air mass convection. However, this steadiness could account for the intensity of the mesocyclones/tornadoes. Storm organization that vacillates between multicellular and supercellular would perhaps provide the best explanation for the intensity, longevity, and motion of observed mesocyclones/tornadoes but is an organization that is the most incongruent with the expected convective mode.

To address these issues in relation to the Jarrell event we are beginning to carry out a detailed analysis using level II radar data from the WSR-88D’s at New Braunfels, TX (EWX) and Fort Worth, TX (FWS). Both were strategically located to capture the full evolution of the parent storm of the Jarrell, TX tornado. Results will be presented at the conference.