Description: An Anomaly Correlation Coefficient (ACC) score of 1.0 during a particular year indicates that the 5-day forecasts made during that year were, on average, highly accurate (i.e., essentially perfect) across the Northern Hemisphere. The closer the GFS (black line) is to an ACC score of 1.0 during a particular year, the more accurate the GFS's 5-day forecasts were that year. Colored lines represent other weather models, according to the key. An upward trend in a line from left to right indicates that 5-day forecasts have generally improved from 1984 to 2023.

Description: An Anomaly Correlation Coefficient (ACC) score of 1.0 during a particular year indicates that the 5-day forecasts made during that year were, on average, highly accurate (i.e., essentially perfect) across the Northern Hemisphere. The closer a weather model is to an ACC score of 1.0 during a particular year, the more accurate that weather model's 5-day forecasts were that year. An upward trend in a line from left to right indicates that 5-day forecasts have generally improved from 1984 to 2023. GFS = black line. ECMWF = red line.