A verification measure of categorical forecast performance equal to the total number of correct event forecasts (hits) divided by the total number of events observed. Simply stated, it is the percent of events that are forecast. The ideal value is 1 (100%).

POD = Hits / (Hits + Misses)

A verification measure of categorical forecast performance equal to the number of false alarms divided by the total number of event forecasts. The ideal value is 0 (0%).

FAR = False Alarms / (Hits + False Alarms)

Also called the threat score, is a verification measure of categorical forecast performance equal to the total number of correct event forecasts divided by the total number of storm forecasts plus the number of misses. The CSI is not affected by the number of non-event forecasts that verify. However, the CSI is a biased score that is dependent upon the frequency of the event. The ideal value is 1 (100%).

CSI = Hits / (Hits + False Alarms + Misses)

A skill corrected verification measure of categorical forecast performance similar to the success ratio but which takes into account the number of correct random forecasts (chance hits + chance correct rejections). The HSS is equal to the total number of correct forecasts minus the correct random forecasts divided by the total number of forecasts minus the correct forecasts due to chance. This skill score falls within a (-1, +1) range. No incorrect forecasts give a score of +1, no correct forecasts give a score of -1, and either no events forecast or no events observed give a score of 0.

HSS= (Hits + Correct Negatives - Chance)/(Hits + False Alarms + Misses + Correct Negatives - Chance)

Chance = (1 / Number of Forecasts)*((Hits + Misses)*(Hits + False Alarms)+(Correct Negatives + Misses)*(Correct Negatives + False Alarms))