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ECMWF re-forecasts are produced on the fly. This means that every week a new set of re-forecasts is produce to calibrate the real-time ensemble forecast of the following week using the latest version of IFS. The ensemble re-forecasts consist of a 5-member ensemble starting the same day and month as a Thursday real-time forecast, but covering the past 20 years. For instance the first re-forecast set archived in the S2S database was the re-forecast used to calibrate the real-time forecast of 1st January 2015 (a Thursday). This set consisted of a 5-member ensemble starting on 1st January 1995, 1st January 1996, ... 1st January 2014 (20 years, 5 member ensemble = 100-member climate ensemble). The re-forecast dataset is therefore updated every week in the S2S archive, and re-forecasts covering all the 4 seasons will only be available at the end of 2015.
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