Climatology based on a 1991 - 2020 Normal Period

Heidke Skill Score

The Heidke Skill Score (HSS) compares how often the forecast category correctly match the observed category, over and above the number of correct "hits" expected by chance alone.

This score utilizes the number of correct and incorrect category hits. The values range from -50 to 100. A score of 100 indicates a perfect forecast and a score of -50 indicates a perfectly incorrect forecast. Scores greater than 0 indicate improvement compared to a random forecast and indicate skill.

For monthly and seasonal forecasts, the equal chances (EC) forecast category are included in scores.

The equation for the score is:
HSS (%) = 100 * (H - E) / (T - E)

where H = Number of correct forecasts, E = Expected number of correct forecasts (1/3 of total), and T = Total number of valid forecast-observation pairs.

The equation for the score including EC forecasts is:

HSSwithEC (%) = HSS * coverage

where coverage = number of non EC forecasts/Total.