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One way to evaluate forecast skill is pattern matching—an overall correlation "score" that describes how well the actual winter precipitation (such as the precipitation for winter 1997-98, a strong El Niño year, at left) matched individual model forecasts (top right), the NMME average (middle right), or the typical El Niño pattern (lower right). A score of 1 means a perfect match, a score of 0 means no matches at all, and a score of -1 means an inverse match, or a mirror image, such as you might expect to see during a La Niña winter. NOAA Climate.gov image, adapted from original by Michelle L'Heureux and Nat Johnson.