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In total, each set of re-forecasts consists of 20 years x 9 runs x 11 ensemble members = 1980 re-forecast values.  These are available for each forecast parameter, forecast lead-time, calendar start date, location, at forecast intervals of 6 hours.  These are used to define the M-climate.The large number of re-forecasts is justified because it is essential to minimize sampling noise in the M-climate tails for EFI and SOT computations.  

The M-climate is used in association with the ensemble forecast:

• to present the 15-day ensemble meteograms with the medium range climate (M-climate)
• to deliver the extreme forecast index (EFI) and shift of tails (SOT) products 
• to highlight significant forecast departures of 2m temperature, wind speed, cloudiness and precipitation from the norm for a given location and time of year.  

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If for the same lead-time, one compares, the M-climate quantile plots (e.g. for a Thursday 00UTC run), and a run 24hrs later, they will be slightly different.  This limitation of twice weekly updates to the M-climate can be significant.  It can be particularly evident in spring and autumn when mean temperatures are changing most rapidly day by day.

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• For shorter ranges, the priority is the best possible capture of the climatological distribution of the tails (e.g. for EFI and extreme forecast index (EFI) and shift of tails (SOT).  This can be better achieved using a re-forecast span of 5 weeks (1980 re-forecast values).  
• For longer ranges, the priority is the correct representation of seasonal cycles.  This can be better achieved by using a span of 1 week (660 re-forecast values).  The tails should not be so prone to having a reduced sample size.

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