Climatology of High-Impact Supercells over Northeastern Wyoming and Western South Dakota

High-impact supercells—defined here as those producing both giant hail ≥ 7 cm (2.75 in) in diameter] and significant winds [≥ 33 m s^–1 (65 kt), or proportional wind damage, within 3 hours of each other in the same storm]—represent a considerable threat to life and property across the northern plains (and elsewhere). Dating back to 1996, over two dozen such events have been observed in the NWS Rapid City’s county warning area. The ultimate evolution of each supercell varies, and broadly can be categorized as a) merging with an upstream quasi-linear convective system (QLCS), b) evolving into a QLCS, or c) remaining relatively discrete throughout its lifetime.

High-impact supercells tend to occur in meteorological summer, with only three occurring in either spring or fall. Typically, summertime high-impact supercells develop in the afternoon over higher elevations of eastern Montana or along the northern Black Hills within northwest or zonal flow in mid- and upper-levels. Events occurring in the shoulder seasons developed downstream of strong upper-level troughs and tended to enter the area from the south. During the summer, synoptic-scale forcing preceding the development of convection appeared relatively innocuous, though most cases were associated with at least a subtle jet streak aloft. Observed sounding analysis reveals that environments supporting high-impact supercells over northeastern Wyoming and western South Dakota are represented by anomalously large CAPE and deep-layer shear, and to a lesser extent, anomalously large storm-relative helicity and vertical equivalent potential temperature differences.