Description: An Anomaly Correlation Coefficient (ACC) score of 1.0 indicates that the forecast was highly accurate (i.e., essentially perfect) across the Northern Hemisphere. A model is said to have "useful skill" when the ACC score is greater than or equal to 0.6. This plot shows the average forecast day (i.e., forecast length) when the ACC score falls below 0.6 and the GFS loses useful skill. For example, GFS forecasts had less than 6 days of useful skill in 1989 and had greater than 8 days of useful skill in 2024.