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The ensemble is designed to give an assessment of uncertainty.  However, occasionally the uncertainty becomes very large, with additionally a potential for very high or low forecast values to occur.  An example is shown in Fig8.1.9.7-12 where ensemble members showed widely differing yet potentially extreme hot or cold temperatures.  These cases are relatively rare, usually associated with a sharp upper trough and associated front separating widely dissimilar airmasses.  Users should accept the uncertainty and tailor their forecasts to reflect the impact on users.  Certainly they should not take middle course.  Some customers may require an indication of very high or very low temperatures, even if at a low probability. 

Fig8.1.9.7-12: EFI and SOT chart  VT 22-23 July 2012 (T+156-180) from ensemble DT 00UTC 15 July 2012, with associated Cumulative Distribution Function (CDF) and meteogram with M-climate for the marked position.  The uncertainty is clear on the meteograms.  The CDF shows some ensemble members indicating temperature values above M-climate maximum and some below M-climate minimum and thus the SOT will show values for both the warm and cold tails.  This is shown on the chart where the SOT lines (black) overlap.  The airmasses are clearly very different as there is a large positive EFI indicative of high temperatures over eastern Europe and a large negative EFI indicative of low temperatures over western Europe.

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