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• seasonal forecast verification metrics are based on the re-forecasts.
  
• re-forecasts allow computation of the S-M-climate which allows actual forecasts to be converted into an anomaly format.   Forecasts in terms of anomalies relative to a model climate (rather than relative to the observed climatology) means that some calibration for model bias and drift into the products is incorporated.

These are used entirely for skill and reliability assessment, and verification maps are accordingly provided on the web. 

Seasonal re-forecasts for verification

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• to highlight significant monthly anomalies of forecast parameters (2m temperature, 500 hPa height, 850 hPa temperature, mean sea level pressure, precipitation) from the norm for a given location and time of year.  

Selection of seasonal re-forecasts

The set of re-forecasts is based on using the same calendar start date (the 1st of each month) as the seasonal run in question.  Re-forecasts are created using the same calendar start dates during the 36 year period: Jan 1981 to Dec 2016.  Currently, this is not updated. 

The set of re-forecasts is made up from:

• a set of re-forecasts using the same calendar start dates for each of the 24 year period: Jan 1981 to Dec 2016. 
• each re-forecast is from an 25-member ensemble (all perturbed members) run over a 7-month forecast period for monthly runs, and a 13-month forecast period for quarterly runs.

In total, each set of re-forecasts consists of 36 years x 1 runs x 25 ensemble members = 900 re-forecast values.  These are available for each forecast parameter, forecast lead-time, month, location, at forecast intervals of 6 hours.  This is used to define the S-M-climate.