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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).  (Fig7.2.4).
  • There is only a very small correlation between forecast jumpiness and the quality of the latest forecast (Fig7.2.5).
  • 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).

    • ...

    Fig7.2.7:An example of Cumulative Density Function (CDF) produced by a sequence of ensemble forecasts for precipitation at Zaga in Slovenia verifying for the 24hr 00UTC 27 to 00UTC 28 April 2017.  All show a very high extreme forecast index (EFI).  Note the four earlier CDFs (blues) showed a moderate slope indicating a spread of forecast precipitation intensities, and then jumped to a steeper slopes (purple and red) with lessening of spread of precipitation intensities.  Here the forecast showed a steady trend towards heavier precipitation with a jump to very heavy precipitation.  A forecaster would have been unwise at the time of the T+60 to 84hr forecast (rightmost dashed blue line) to think that this significantly wetter forecast overall was too much of  jump from the trend to be believed.

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