SHORT-RANGE ENSEMBLE FORECASTING

Short range forecasts do not use any probabilistic information. Although such forecasts are more and more skilful, some events remain unpredictable. Météo-France wishes now to use probabilistic forecasts at short range in order to get an assessment on extreme events risks. At the time being, ensemble forecasting seems to be the more simple and efficient tool to achieve these goals. Ensemble prediction is already used for medium range forecasting (Molteni et al. 1994 ; Toth and Kalnay, 1997). However, methods used in such ensembles can not be directly applied for the short range forecasting (Brooks et alt., 1995). Research has already been performed in this area and a short range ensemble system is operational at NCEP (Tracton et al. 1998; Stensrud et al. 1999). We are presenting here the description of an experimental ensemble developped at Météo-France in the PEACE (Prévision d’Ensemble A Courte Echéance) project. This ensemble is devoted to detect rare severe events such as storms in the short range (24-48h). Emphasis is put on assessing skill of predicting strong mean sea level pressure gradient probabilities.