WSR-88D Observations of an Extreme Hail Event Impacting Western South Dakota on 20 June 2015

On 20 June 2015, an intense supercell thunderstorm with hail to at least 6 inches (15.2 cm) in diameter impacted western South Dakota in the vicinity of Nisland. The storm produced widespread damage to vehicles, homes and businesses, with news reports of hailstones penetrating the shingle roofs of homes and the metal roofs of barns. The storm occurred within 100 km of the KUDX WSR-88D (located near Rapid City). This study examined the character and evolution of the Nisland hailstorm from 0020‒0230 UTC, as seen by KUDX. Overall storm intensity was assessed via one reflectivity and two velocity derived parameters: maximum expected size of hail (MESH), storm-top divergence (STD), and mid-altitude rotational velocity (MRV). During the time period 0‒20 min prior to the 6 inch hailstone report, all three parameters attained magnitudes typically associated with a supercell capable of producing giant hail. However, only the STD suggested the potential for an extreme hail event, reaching a maximum value of 138 m s^-1 (the MESH and MRV maximums were 107 mm and 35.6 m s^-1, respectively). Also examined were low-altitude measures of several dual-polarization parameters above the locations of the six severe hail reports associated with the storm from 0020–0230 UTC. The results show the large hail was associated with high horizontal reflectivity (Z_H ≥ 54 dBZ), low differential reflectivity (Z_DR < 2 dB), and low-to-moderate cross-correlation coefficient (ρ_HV from 0.87‒0.94) and specific differential phase (K_DP from 0.4‒2.3° km^-1). Results from an analysis of mid-altitude polarimetric signatures such as negative Z_DR and reduced ρ_HV near and above the Z_DR column of this storm will also be presented.