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.