Ensemble mean chiclet plot (last 90 runs)
PT = Pacific Trough, PR = Pacific Ridge, AKR = Alaskan Ridge, GH = Greenland High, NO = No Regime
Bars show the proportion of ensemble members assigned to each regime. Squares across the top show the attribution of the ensemble mean, which often tends toward No Regime at long lead-times. The weather regime index (WRI) is the standardised amplitude of each pattern.
About the forecasts
These forecasts use the 35-day 31-member 00Z run of the NCEP Global Ensemble Forecast System (GEFS). The regimes are computed using 00Z 500 hPa geopotential height (Z500) anomalies in the domain 20–80°N, 180–330°E. The forecast data are obtained from the NOAA Operational Model Archive and Distribution System (NOMADS) on a 0.5° grid and bi-linearly interpolated to a 1.0° grid. Specific details on the regimes are available here and in Lee et al. (2023) and Lee and Messori (2024). The regime centroids are computed over 1979–2025 using ERA5. The forecast Z500 data are smoothed with a 5-day centred running-mean prior to regime assignment; this differs slightly from the 10-day filter used in the original definition for the benefit of more available forecast dates. Note that no mean-state hindcast bias correction of the Z500 field is performed, which may affect the regime assignment at longer lead-times.
The four regimes are Pacific Trough (PT), Pacific Ridge (PR), Alaskan Ridge (AKR), and Greenland High (GH). If the anomalies are closer to climatology than a regime centroid by Euclidean distance, then “No Regime” is assigned. A map of the average Z500 anomalies associated with the regimes is available here.
If you have any queries, issues, or suggestions, please contact me. The script which produces these plots is currently scheduled to run at 0700 UK time.