Probability of Severe Hail (POSH) - Probability of Severe Hail (POSH) - Warning Decision Training Division (WDTD)
Probability of Severe Hail (POSH)
Short DescriptionThe probability of 0.75-inch diameter hail occurring.
Primary UsersNWS WFO
Vertical temperature profile from the current operational NCEP/EMC mesoscale model (i.e., the RAP as of 2014).
Spatial Resolution: 0.01o Latitude (~1.11 km) x 0.01o Longitude (~1.01 km at 25oN and 0.73 km at 49oN)
Temporal Resolution: 2 minutes
A linear correlation of the height of the melting level and Severe Hail Index (SHI) was produced by looking at a number of storm cases (Witt et al. 1998):
where WT (J m-1 s-1) is the warning threshold and Ho (km) is the melting level measured above radar level (ARL). If WT < 20 J m-1 s-1, then WT is set to 20 J m-1 s-1.
The severe hail probability function is given by:
where POSH is the Probability of Severe Hail (%). POSH values < 0% are set to 0%, and POSH values > 100% are set to 100%. Despite the continuous nature of POSH, actual algorithm output probabilities are rounded off to the nearest 10%, to avoid conveying an unrealistic degree of precision. Note when SHI = WT, POSH = 50%.