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The regime charts show the distribution of the forecast Euro-Atlantic flow type presented .  These are shown diagrammatically in NAO/BL phase space in two-dimensional probability density function format.  The x-axis is for NAO+/-, and the y-axis for BLO+/-.  An objective assessment of each ensemble member forecast, during each 24hr period, supplies gives a measure of both the NAO and BL components for each member forecast in each 24hr period.   These are plotted on the diagram, colour coded according to forecast lead time.  See Fig8.1.17-1.

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Fig8.1.17-1: NAO-BLO phase space diagrams diagram (regime projection diagram) for the medium range period Day0 to Day8 from the medium range ensemble (DT 00UTC 23 April 2019).  Shown are the  The analysis for the previous 8 days (is shown as black spots and black line) and the .  The ensemble mean trajectory (is shown as a red line).  Coloured markers show the ensemble members at 24-hour intervals (colours and times shown above the chart).  No clear assessment of regime type should be made for those few members which lie within the central circle.  The spread of the cluster at a given time is a measure of uncertainty at that time.

In Fig8.1.17-1 the Medium Range forecasts show a weakening BLO+ type before a transition, mostly by around Day7, to a moderate NAO- regime type.  Some ensemble members make the transition more fairly quickly; others . Others fail to move from the BLO+ type, though these nevertheless do nevertheless show a trend towards NAO-.  Comparing the latest regime projection diagram with those from previous medium range forecasts allows an assessment of run-to-run consistency.

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Rough practical guide to transitions

A study of The frequency of transitions from one circulation type to another has been derived from a large number of reanalyses (36 years of ERA-interim data) gives an indication of the frequency of transitions from one circulation type to another (giving .  This gives a “climatology” of transitions )between types. 

The results show:

• Transition to BLO+:
  • NAO+ and persistence are the most probable precursors for BLO+.

• Transition to NAO­–:
  • BLO+ and, to a rather less extent, persistence are the most probable precursors for NAO–.  This characteristic is clear even in the longest range forecasts. Usually  Usually a strong breaking cyclonic wave south of Greenland favours destruction of BLO+ and subsequent . Subsequent more minor eddies tend to establish the NAO–.  See Fig8.2.7-7 for an example.
  • NAO+ and BLO– are very unusual precursors for NAO–.

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• Transitions to BLO+ and NAO– are slightly more frequent than transitions to NAO+.
• NAO+ somewhat favours transitions into BLO+; BLO+ favours transitions into NAO–.
• Transitions from blocking conditions are more confidently predicted than transitions into blocking. 
• The probability of persistence (for more than 12 >12 days) of NAO- is about twice what it is that for other circulation types.

The At all forecast ranges, the model statistics and relative frequencies , at all forecast ranges, compare well with those from the analysis, indicating that .  This indicates the IFS is well able to simulate transitions , and suggesting that suggests model bias in this context is not a major problem.

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• Read more on the predictability of changes in large scale flow.
• Read more on diagnosis and verification of high-impact weather forecasts.
• Read more about the use of "Weather regimes" at ECMWF (2020 article)

See also the section regarding Extended Range Regime Charts circulation patterns and regimes which gives further information on regimes within the Extended Range.Read more about the use of "Weather regimes" at ECMWF (2020 article)