High-Resolution Phased Array Radar Observations of an Oklahoma Hailstorm Producing Extremely-Large Hail

On 31 May 2013, an outbreak of severe thunderstorms occurred across central OK, resulting in some of the most extreme tornado and hail events recorded to date. These included an EF3 tornado with a maximum damage width of 4.2 km (2.6 mi), along with hailstones up to 160 mm in diameter. Prior to 31 May 2013, the five largest verifiable hailstones in the United States ranged from 203 mm to 152 mm in diameter. That would rank the 160 mm hailstone observed on 31 May 2013 as the fifth largest in the U.S. (fourth largest being 175 mm in diameter). The 160 mm hailstone fell on the west side of El Reno, OK at ~2305 UTC, placing it within 60 km of the National Weather Radar Testbed Phased Array Radar located in Norman, OK. This allowed for high-resolution (in both space and time) Doppler radar observation of the supercell storm that produced the 160 mm hailstone, from the time of storm initiation (first echo ≥ 15 dBZ) at 2119 UTC through the storm's movement across El Reno (2300 UTC – 2310 UTC). The character and evolution of this supercell, from storm initiation through occurrence of the 160 mm hailstone, was examined via a variety of radar-based reflectivity and velocity parameters. Following an initial period of explosive growth (first 40 min after initiation), the storm reached maximum intensity around the time of the 160 mm hailstone. Radar-predicted Maximum Expected Size of Hail (MESH) peaked at 121 mm at 2245 UTC, whereas both storm-top divergent outflow and mid-altitude rotational velocity peaked within 5 min of 2305 UTC, reaching maximum values of 150 m s^-1 and 85 m s^-1, respectively. The storm-top divergent outflow signature at times had inbound velocities > 100 m s^-1, exceeding the previously documented maximum Doppler-radar velocity of 97 m s^-1 (Fort Cobb storm's outflow signature observed on 18 June 1973).