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It is important to avoid over-interpretation of the ensemble mean, in particular underestimation of the risk of extreme weather events.   To aid visual interpretation by the user, ensemble mean output should be presented together with a measure of the ensemble spread.  The ensemble mean and the probabilities relate naturally to each other and can be most effective when shown together.  So, for example, the ensemble mean of the MSLP (or 1000hPa) presented together with gale probabilities will put the latter into a synoptic context that will help interpretation (see Fig6.1.-1).

Fig61Fig6.A1-1: 1000hPa forecast from 12UTC 13 August 2010 T+156hr to 00UTC 16 August 00 UTC T+96 h, all valid at 00UTC 20 August 2010.   Full lines are the 1000 hPa geopotential EM overlaid by the probabilities of wind speeds >10m/s.  Probabilities are coloured in 20% intervals starting from 20%.

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