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.