Evaluation of National Weather Service Radar Hail Detection Outputs

Hail detection outputs from the National Weather Service (NWS) network of more than 150 Doppler radars form the backbone of many commercially available post-storm hail swath products. At the Insurance Institute for Business and Home Safety (IBHS), a field program was designed to collect and measure hailstone swathes from a variety of states where severe weather and hailstorms occur most often. The detailed ground-truth observations recorded during the IBHS Hail Field Program (2012–2018) were ideal for evaluating the hail detection capabilities and the width of the hail swath by using quality observations of maximum hail size at the ground. Using the field measurement database, analyses were conducted with the objective to determine the performance of NWS Maximum Estimated Size of Hail (MESH) and the usefulness of other hail index products, including the dual-polarization based hydrometeor classification algorithm.

The field catalog includes over 3,500 hailstones from more than 60 different thunderstorms. In most cases, hail measurements were made at multiple locations across the swath of hail. The radar products were evaluated following a conventional skill scoring methodology using: probability of hail detection, false alarm rate, critical success index, and bias score. The hail detection outputs performed well with predicting hail when it was observed on the ground but performed poorly in all other areas, especially with discrete size estimation.