A potentially valuable WSR-88D severe storm pre-cursor signature in highly dynamic, low CAPE, high shear environments

During several events over the 2005 and 2006 severe convective seasons, a “reflectivity tag” or “reflectivity bulge” has been noted moving quickly up the backside of a linear cluster of potential severe storms. This feature generally moved more quickly than embedded cells within the line, and often appeared to be linked to a jet streak in the water vapor imagery. As this tag passed an embedded cell with a persistent rotational couplet, a tornado or straight-line severe wind swath was sometimes produced shortly thereafter. The environments that appear to be most conducive for this feature are highly dynamic, and contain low Convective Available Potential Energy (CAPE) and relatively high shear.

The Evansville, Indiana tornadic storm of November 6, 2005 is shown as an example of a reflectivity tag. When WSR-88D 0.5 degree elevation reflectivity and velocity data from Paducah, Kentucky is looped, a reflectivity tag was apparent moving northeast along the backside of the line. The convective cell that eventually produced the tornado can be identified within the line as a mesocyclone, which persisted for several hours across southeast Missouri and southern Illinois without tornadogenesis taking place. However, within a WSR-88D scan after the reflectivity tag moved past the cell, tornadogenesis occurred.

An example of a reflectivity bulge is demonstrated using the Newton, Illinois tornadic storm of November 15, 2005. Despite high environmental shear, the only tornado that occurred in central or southeast Illinois was associated with an embedded supercell concurrent with a reflectivity bulge moving up the line.

One possible mechanism leading to the development of these signatures is gravity wave-convective storm interaction. Although many studies (e.g. Stobie et al., 1983; Balachandran, 1980; Koch, 1979) have highlighted this interaction, it is only recently that temporal, spatial, and data display resolution in WSR-88D and satellite imagery have reached the point where evidence of these waves can be identified in real-time, and used as a potential input into operational warning decisions. It is hoped that the recognition of these features through the use of rapid looping and detailed meso-scale analysis may provide operational meteorologists with another valuable tool in the forecasting of severe local storms in these challenging environments.