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These show how the probabilities of each of the four regimes (NAO+, NAO-, Block, Atlantic Ridge (ATR), plus No Regime) evolve on a daily basis as the ensemble members progress through the sub-seasonal range forecast period.  It gives an indication of the likelihood of a change of regime, and the potential timings of such a change.  Comparison with the cumulative histograms from recent sub-seasonal range forecasts can also give insights into the consistency of the forecast sub-seasonal range evolution and probabilities, but flip-flops and trends should be viewed with caution.  The histograms enable the user to assess rapidly the confidence to be given to forecasts issued to the consumer.

One of the four regimes is assigned only if:

• the minimum distance between the forecast anomalies and any of the four regimes lies within an "average value", and
• that distance is significantly different from the distance to other regime types.

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Regime allocation 

Regimes and Regime Frequency Histograms for the sub-seasonal range are derived using 4-regime scheme.

A particular regime is allocated for each ensemble member of the sub-seasonal forecast.  The process is:

• Select the regime with the smallest distance between the ensemble member anomaly forecast and each of the four SUBS-M-Climate anomaly regime types.
• Assign the regime type associated with this smallest distance to the ensemble member if this smallest distance is both:
  • smaller than a defined threshold, and
  • significantly different from the remaining three distances.
• Assign "No Regime" if either or both criteria are not satisfied

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Real time example of Sub-seasonal Range Weather Regime Frequency histogram.

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