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In order to estimate and compensate for any model drift the model output is compared with the corresponding model climates (M-climate for medium range, ERSUBS-M-climate for sub-seasonal range, S-M-climate for seasonal forecasting) for the current forecast date.  This is derived using the same model construction as the ensemble from a number of perturbed forecasts based on calendar dates surrounding the date of the current ensemble run using historical data from several years.  Systematic errors are then corrected during post-processing after the forecast is run.

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