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Probability of Severe Hail (POSH)

Probability of Severe Hail (POSH)

Short Description

The probability of 0.75-inch diameter hail occurring.

Subproducts

None

Primary Users

NWS WFO

Input Sources

Severe Hail Index (SHI)

Vertical temperature profile from the current operational NCEP/EMC mesoscale model (i.e., the RAP as of 2014).

Resolution

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

Product Creation

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%.

Technical Details

References

Witt, A., M. D. Eilts, G. J. Stumpf, J. T. Johnson, E. D. Mitchell, and K. W. Thomas, 1998: An enhanced hail detection algorithm for the WSR-88D. Wea. Forecasting, 13, 286-303.