This is a wintertime diagnostic, so updates are paused until November.
About the forecasts
These forecasts use the 46-day 101-member extended-range run of the ECMWF Integrated Forecasting System (IFS). The forecast data are obtained from the S2S archive via the ECWMF Public Datasets service and interpolated to a 1.0° grid. The latest available forecast is from 2 days beforehand. Details on the method and its physical interpretation can be found in Messori et al. (2022 and corrigendum) and Schutte et al. (2025). The index is defined for December–March. A shorter 5-day running mean is used for the benefit of more available forecast dates. Anomalies are computed here with respect to a 1 January 1979–31 December 2025 ERA5 climatology. Note that no mean-state hindcast bias correction is performed, which may affect the index at longer lead-times.
If you have any queries, issues, or suggestions, please let me know. The script which produces these plots is currently scheduled to run at 0800 UK time.