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