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The M-Climate is derived from a set of medium range re-forecasts.  These are created using the same calendar start dates over several years for data times either side of the time of the medium range ensemble run itself.  The re-forecast runs are at the same resolution as the medium range ensemble (currently 9km) and run over the 15-day medium range ensemble period.   

There is merit in examining the real-time performance of a forecasting system.   But the sample sizes created for one system are far too small to conclude anything about its true performance levels.  Re-forecasts are used to increase the available data to produce a model climate.   The results of forecast system may be compared with this model climate.  

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• For shorter ranges, the priority is the best possible capture of the climatological distribution of the tails (e.g. extreme forecast index (EFI) and shift of tails (SOT)).
• For longer ranges, the priority is the correct representation of seasonal cycles.

(FUG Associated associated with Cy50r1)