Hail Warning Decision Guidance

National Weather Service (NWS) forecasters will have several new hail diagnostic attributes available to them in Advanced Weather Information Processing System (AWIPS; Wakefield 1998), beginning with the Operational Build version 6 (OB6). In a warning decision environment, it is essential that forecasters have the best guidance available to them. Therefore, the purpose of this study is to compare these new hail diagnostic radar parameters with legacy radar parameters to determine which are the “best predictors” for hail warning guidance. This study is the first step in developing official NWS Hail Warning Guidance training information for warning forecasters. A total of 11 hail producing storm events were analyzed. The events chosen had geographic diversity across the United States and included both the warm and cool seasons. For each individual hail report, the values of 17 different hail diagnostic parameters were recorded. Each attribute was compared to ground truth reports and then statistically analyzed. Statistical analysis included the calculation of various correlation coefficients, as well as analyses of probability of detection (POD), false alarm rate (FAR), critical success index (CSI), and the Heidke skill score (HSS) for varying forecast decision thresholds, as well as different severe hail criteria (i.e., not just 2 cm diameter). Results indicated that the new high-resolution hail diagnostic radar parameters outperformed the legacy hail diagnostic parameters. Suggestions for future work to complete the development of NWS Hail Warning Guidance are offered.