A method of forecasting in which a computer simulation is run numerous times, each with slightly different initial conditions. (The European Centre for Medium-Range Weather Forecasts currently calculates 51 versions for its global ensemble forecast.) If the forecasts remain similar 7–10 days ahead, it may be assumed that the weather is in a quasi-stable state and that predictions are likely to be reasonably accurate. If, however, the forecasts deviate after a few days, conditions are more likely to break down suddenly and unpredictably. Ensemble forecasting therefore provides a method of estimating the probability that particular forecasts will be correct.
Subjects: Meteorology and Climatology.