The economic value of the European Centre for Medium-Range Weather Forecasts (ECMWF) operational ensemble prediction system (EPS) is assessed relative to the value of a perfect deterministic forecast. The EPS has substantial relative value throughout the medium range. Probability forecasts derived from the EPS are of greater benefit than a deterministic forecast produced by the same model. Indeed, for many users, the probability forecasts have more value than a shorter-range deterministic forecast. Based on the measures used here, the additional information in the EPS (reflecting the uncertainty in the initial conditions) provides a benefit to users equivalent to many years' development of the forecast model and assimilation system.
The impact of ensemble size on forecast value is considered. The difference in performance between ensembles with 10 and with 50 members may appear relatively small, based on standard skill measures, yet the larger ensembles have substantial benefit to a range of users. Further increases in ensemble size may be expected to provide additional value.