Rapid-Scan Dual-Polarization WSR-88D Observations of an Oklahoma Hailstorm Producing Extremely-Large Hail

On 31 May 2013, an outbreak of severe thunderstorms occurred across central OK, resulting in one of the most extreme hail and tornado events recorded to date. Numerous hailstones from 100 mm to 160 mm in diameter were observed across a ~50 km² domain on the west side of El Reno, OK from 2300 UTC – 2330 UTC, originating from a large supercell that was simultaneously producing the EF-3 El Reno tornado, the widest tornado ever documented, with a maximum damage width of 4.2 km. The extreme hail fell within 60 km of the KOUN WSR-88D located in Norman, OK. Unlike most WSR-88D radars, KOUN is able to rapidly scan limited volumes of space via traditional "sector scans." This allowed for higher time-resolution (compared to other WSR-88D) dual-polarization radar observations of the supercell storm that produced the giant hailstones. The advantages of using dual-polarization parameters for identification of giant hail are evident in that most of the hailstones fell in locations where the low-altitude reflectivity was less than 50 dBZ, whereas the low-altitude differential reflectivity (Z_DR), cross-correlation coefficient (ρ_HV), and specific differential phase (K_DP) had values indicative of very large hail. In the case of the observed 160 mm hailstone, the higher time-resolution of the KOUN data, compared to the co-located KCRI WSR-88D scanning in Volume Coverage Pattern (VCP) 212, shows in much better detail the descending hail core that contained this near record-size hailstone for the state of Oklahoma.