Description: Anomaly Correlation Coefficient (ACC) measures the skill with which GEFS forecasts capture the large-scale weather pattern. Specifically, an ACC score of 1.0 indicates that the anomalies in the GEFS forecast perfectly represented the observed anomalies. The thin red and black lines represent the average forecast lead time when GEFS forecasts attained an ACC score of 0.8 and 0.6, respectively, during a given month. The thick red and black lines represent the 13-month running average of the forecast lead time when GEFS forecasts attained an ACC score of 0.8 and 0.6, respectively. An upward trend in the lines from left to right indicates that large-scale forecast skill has improved over time.