Using ARL as the reference elevation, a very large overforecasting bias is apparent. Because of similar concerns for the data from Tucson, no attempt was made to generate a reliability diagram for that site. The cell-based VIL values corresponding to each severe hail report are quite high, with values around 50. The observed cell-based VIL values were quite low on this day, particularly for severe hailstorms occurring during the month of August. Note the first hail report, with a corresponding maximum VIL of only 14.
Similar results occur for the two Arizona storm days.
The analysis results presented above clearly show a large overforecasting bias for the POSH parameter, as it is currently calculated in Build 9. Specifically, the equation used to generate the warning threshold, which is a function of the melting level above some reference height, is producing values that are likely too small. One way to increase the WT values is to use a reference height closer to mean sea level. From the results shown in Figs. Although changing the reference elevation to MSL should lead to overall improvements in HDA performance, it is possible that, for some high elevation sites, this adjustment may lead to an actual underforecasting bias.