Atger, Frederic (ECMWF)
Medium-range forecasting of rare or extreme meteorological events, such as intense Mediterranean cyclones and their effects, may appear as a hopeless challenge. Such events are hardly satisfactorily predicted in the short-range; they are related to small scale features well known to become rapidly unpredictable.
On the other hand, current high resolution numerical models prove to be able to forecast intense developments with a great realism in the medium-range, although not necessarily with skill. Performance of the ECMWF high resolution model in the Mediterranean area will be reviewed; usefulness of its guidance in operational medium-range forecasting of intense Mediterranean cyclones will be discussed.
Forecasting extreme events for operational purposes requires a specific approach: even in the short-range, forecast uncertainties have such consequences for the user (e.g. safety) that they must be explicitly specified in the end forecasts. In the medium-range, uncertainties have grown so much that the main concern is not "where and when will it happen" but "is there any chance that it happens". The end forecast should therefore focus on probabilistic aspects rather than on a hypothetical deterministic evolution.
The Ensemble Prediction System (EPS) has been developed at ECMWF to provide forecasters with numerical probabilistic forecasts, i.e. numerical forecasts of forecast uncer- tainties. EPS performance will be reviewed and usefulness of its guidance in operational medium-range forecasting of Mediterranean extreme events will be discussed.