ProbHail and ProbWind v3 in Florida

A slow-moving cold front converged with a sea-breeze boundary to produce some strong storms in a weakly-sheared environment. One forecaster at the HWT remarked how the ProbHail values in version 3 (PHv3) seemed more reasonable to him in this environment, compared to ProbHail version 2 (PHv2).

Figure 1: ProbSevere contours, MRMS MergedRef, and NWS severe thunderstorm warnings in the Florida panhandle. The highlighted storm produced severe wind reports

We can see that despite the favorable MESH, satellite growth rate, and thermodynamic parameters in the storm highlighted in Figure 1, PHv3 was only 11% at this time, while ProbWind v3 (PWv3) was 60%. This storm later produced multiple reports of downed trees and power lines. While ProbWind v2 correctly had high probabilities (PWv2 = 81%), PHv2 was heavily overforecasting (PHv2 = 85%). The machine-learning models in ProbSevere v3 (gradient-boosted decision trees) were able to more accurately discriminate between the wind and hail threats in this situation. For this storm, the low effective bulk shear (12 kt), high wetbulb 0C height (13.4 kft) and high PWAT (2.1 in) all detracted from the PHv3 probability. 

Several storms in the Florida panhandle did produce severe hail reports, including the storm highlighted in Figure 2. In general, PHv3 was in the 25-40% range for storms that produced hail in this environment, and perhaps could help forecasters identify hail threats compared to other storms in the area not producing hail, which had PHv3 probabilities in the 0-15% range. The higher MESH (1.5 in) and slightly better effective shear (19 kt) contributed to higher PHv3 for this storm.

Figure 2: ProbSevere contours, MRMS MergedRef, and NWS severe thunderstorm warnings in the Florida panhandle. The highlighted storm produced severe hail and wind reports.