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Regime Frequency Histograms

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 extended 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 extended range forecasts can also give insights into the consistency of the forecast extended 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.

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Fig8.2.7.2: An example of a somewhat complicated change of regime (from NAO+ (blue) on 24 March to Blocking (red) on 10 April) together with other inferences that may be made from a retrospective forecast cumulative weather regime frequency diagram (DT 21 March 2019).  The indicated verification (blocks with black borders low down) largely confirms the greatest forecast probabilities.  However an observed  interlude of NAO- (green) from 3 to 5 April was not really captured by the ensemble probabilities and the forecast change to Blocking (red) was somewhat premature.  The probabilities of each regime type tend to become less diverse with forecast lead time, which corresponds to increasing uncertainty further into the future.

Regime Projection or Phase Space diagram

These show the distribution of the forecast Euro-Atlantic flow type presented diagrammatically in NAO/BL phase space in two-dimensional probability density function format.   Three forecast periods are shown (days 5-11, 12-18, 19-25).  The x-axis is for NAO+/-, and the y-axis for BLO+/-.  An objective assessment of each ensemble member forecast, during each seven day period, supplies a measure of both the NAO and BL components for each member (i.e. 7 days x 51 ensemble members = 357 assessments in total).  The mean assessment for each member during the 7-day period can be plotted as an empirical orthogonal function (EOF) in NAO/BL space, with colouring subsequently applied to denote frequencies within the ensemble in this phase space.  A sequence of such diagrams for several lead times from a set of ensemble forecasts gives an indication of the changing probabilities of each forecast type.  See Fig8.2.7.3.

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