A Catalogue of Extremely Damaging Hailstorms

Hail damage represents a substantial portion of insured losses. Average annual hail losses in the U.S. total $1.65 billion. Since 1995, there have been 56 hailstorms that have caused greater than $100 million in insured property losses, including seven events that have exceeded $1 billion. The top three damaging hailstorms since 1995 occurred on 5 October 2010 in Phoenix, AZ ($3.07 billion), 28 April 2012 in St. Louis, MO ($1.66 billion), and 5 May 1995 in Dallas/Ft. Worth, TX ($1.63 billion).

Common ingredients were present in the synoptic and mesoscale environment for the three events. There was an anomalous trough or closed 500-hPa low to the west and a slowly moving surface boundary over the area. The warm side of the boundary had ample convective available potential energy and 0­­–6 km vertical wind shear to support supercells. Supercells initiated near and along the boundary and propagated parallel to it, leading to storm longevity. Furthermore, the Phoenix, AZ and St. Louis, MO events were characterized by multiple episodes of storms, leading to large areal coverage of damaging hail. The combination of longevity and coverage over a populated area led to the very high damages.

Radar-derived hail kinetic energy (HKE) and maximum expected size of hail (MESH) are catalogued for each event, showing the timing and coverage of the potential for damaging hail. To resolve sampling gaps between successive radar scans, a morphing and advection algorithm is used to generate smooth swaths of HKE and MESH. Storm Prediction Center hail reports overlap well with HKE and MESH swaths. The complete catalogue of hailstorms may be used to assess hail risk by 1) evaluating expected losses if an identical hailstorm would occur today and 2) evaluating possible maximum losses, where swaths are moved over different insured assets with varying exposure and vulnerability.