Assessment of anticyclonic supercell environments using close proximity soundings from the RUC model

Anticyclonic, left-moving supercells (defined relative to the mean shear vector) are a concern for severe local storms forecasters primarily because of their tendency for prolific large hail production. Such storms can be quite persistent, lasting several hours and, on rare occasion, producing tornadoes. Typically, such thunderstorms also move to the left of the lower tropospheric (lowest 3 km layer) hodograph; however, we document variations in low level wind profiles that sometimes yield positive net SRH in the 0-1 or 0-3 km layers.

Following the "close proximity sounding" methodology of Thompson et al. (2003) for right-moving supercells, we analyze the environmental characteristics of at least 27 left-moving supercells of both splitting and discrete origins, utilizing Rapid Update Cycle (RUC) proximity soundings for each storm. For this study we limit the set to storms with surface-based CAPE, since our separate sample of elevated storms remains far smaller in size than this surface-based set. Observed storm motions were gathered for each case in the same way as for Thompson et al. (2003). These in turn are compared to hypothetical storm motions that were derived using the left-moving storm algorithm of Bunkers et al. (2000 and 2002), which Edwards et al. (2000) independently verified as the most reliable method of approximating supercell motion. Also, we compare and contrast shear, buoyancy and derived parameters for this subset of storms, both by their origin (discrete versus splitting) as well as to our right-moving supercell subset. Finally we devise a Left-moving Supercell Composite Parameter (LSCP) as a complement to the existing SCP for right moving storms, in order for forecasters to quickly diagnose environments suitable for the development and maintenance of anticyclonic supercells.