The term "skill" in reference to forecasts means a measure of the performance of a forecast relative to some standard. Often, the standard used is the long-term (30-year) average (called the the climatology) of the parameter being predicted. Thus, skill scores measure the improvement of the forecast over the standard.

CPC uses the Heidke skill score, which is a measure of how well a forecast did relative to a randomly selected forecast. A score of 0 means that the forecast did no better than what would be expected by chance. A score of 100 depicts a "perfect" forecast and a score of -50 depicts the "worst possible" forecast. The dashed lines in the skill graph indicates the average skill score for all forecasts and for "Non-CL" forecasts. "CL" refers to climatology or a forecast of equal chances of Above, Near Normal, and Below Normal temperature or precipitation. "Non-CL" refers to all forecasts where enhanced above normal or enhanced below normal temperatures or precipitation are predicted. "Percent Coverage" is refers to the percent of the forecast region where enhanced above or below temperature or precipitation is predicted.