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  • The proportion of previous forecasts that are "better" than the latest ones increases with lead-time:
    • at short lead-times a small but significant proportion appear better (~15% at Day2),
    • at longer lead-times a larger a larger proportion appear better (~40% at Day6).  (Fig7.2-5).
  • There is only a very small correlation between forecast jumpiness and the quality of the latest forecast (Fig7.2-6).
  • Beyond about Day3 the ensemble mean, by using results from all ensemble members, provides more consistent forecasts than the ensemble control.  This benefit gradually increases with forecast range.  
  • The frequency of a flip (single jump) is very similar for both the ensemble mean and ensemble control.
  • The frequency of flip-flopping occurs clearly less frequently in the ensemble mean than in the ensemble control.

    • Persson and Strauss (1995), Zsótér et al. (2009) found:

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  • the connection between forecast inconsistency (flip-flopping etc) and forecast error is weak,
  • the average error of the ensemble mean relates quite strongly to the absolute spread in the ensemble.  
  • on average, larger spread implies larger errors (this does not apply to the ensemble median or ensemble control, even if they happen to lie mid-range within the ensemble).

    • ...

    Table 7.2-1:The percentage of cases when >2mm/24hr has been observed when up to three consecutive ECMWF runs (T+84hr, T+96hr and T+108hr) have forecast >2mm/24hr for Volkel, Netherlands October 2007-September 2010.  R indicates where such rain has been forecast and has occurred.  Similar results are found for other west and north European locations and for other NWP medium-range models.

    Image Modified

    Fig7.2-7: An example of notable jumpiness in weekly ensemble mean anomaly of 2m temperature.  Sequential sub-seasonal range forecasts DTs 00UTC 13 to 24 Nov 2024; VT for week 24-30 Nov 2024.  

    • In Great Britain: the forecasts from 13 to 18 Nov were colder than normal, 19 to 21 November were warmer than normal, 22 Nov colder than normal, 23 Nov warmer and finally 24 Nov close normal.
    • In southern Scandinavia: the forecasts from 13 to 18 Nov were colder than normal, 19 to 24 Nov were warmer than normal.
    • In southeastern Europe and southwest Russia: the forecasts on 13 Nov and 16 Nov were warmer than normal, at other times to 18 Nov were nearer normal, 19 to 24 Nov were much colder than normal .

    Weakness of an Intuitive Approach towards likely outcomes

    ...

    Table 7.2-2: Efficiency of intuitive techniques to assess outcomes given a series of forecast values from a series of sequential deterministic forecasts.


    Image Modified

    Fig7.2-8: The graphs show sample schematic forecasts of 12UTC temperature over four successive NWP model runs: Jumpy (top) and Trend (bottom).  The histograms show the forecasts made by the students using their own techniques.   Spread was low with the jumpy forecast case since the oscillations remained fairly steady throughout, and the next forecast could be higher or lower without changing the range of the oscillation much. The spread was high with the trend forecast case illustrating the point that the next forecast may well be higher than the one before but destroy the trend, or lower than the one before, continuing the trend.

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    (FUG Associated with Cy49r1)