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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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Real time example of Medium Range Two-dimensional Regime diagram.   

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.

Transitions between circulation patterns

Sometimes the transition from one regime to another may be fairly clear.  

Image Added
Fig8.1.17-2: NAO-BLO diagram for the medium range showing transition of circulation pattern with time.  Colours indicate the elapse of time.  The initial NAO+ circulation pattern becomes a BLO+ circulation pattern by T+72hr, and finally becomes a NAO– circulation pattern by about T+168hr.

Rough practical guide to transitions

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).  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 a strong breaking cyclonic wave south of Greenland favours destruction of BLO+. 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–.

• Transition to BLO–:
  • NAO+ and persistence are the most probable precursors for BLO–.
  • Transitions to BLO– are much less common than other transitions. Hence there is rather less confidence in the associated statistics.

• Transition to NAO+:
  • Persistence is the most probable precursor for NAO+.  However, BLO– and BLO+ are also

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• • significant precursors for transition to NAO+. Transitions into NAO+ do not appear to have a preferred path.

In general:

• 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 >12 days) of NAO- is about twice that for other circulation types.

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

Additional Sources of Information

(Note: In older material there may be references to issues that have subsequently been addressed)

• 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 circulation patterns and regimes which gives further information on regimes within the Extended Range.