Line plots are False Alarm Ratio, Probability of Detection and Critical Success Index. The performance results were used to determine the optimum population density threshold at which to score the reliability of the POSH parameter. If the original equation produces overforecasting bias, it is possible that a simple change in the slope or intercept of the equation may improve the reliability of the POSH parameter. Reliability diagrams were constructed for the POSH parameter, calculated at the population density threshold which produced the best performance statistics. Observed relative frequency is frequency of severe hail observations when POSH equals given value.
Solid line represents perfect reliability.
Open circles represent reliability of original POSH equation with no population density constraint. Solid black circles represent reliability of original POSH equation when scored only over areas of high population density. Red circles represent reliability of adjusted POSH equation when scored over areas of high population density. At KMLB, the reliability of the POSH parameter improves with increasing population density threshold, and the adjusted POSH equation improves the reliability further. Observed relative frequency is frequency of severe hail observations when POSH equals given value.