How well does the ECMWF Ensemble Prediction System predict blocking?

Abstract Forecasting atmospheric blocking is one of the main problems facing medium‐range weather forecasters in the extratropics. The European Centre for Medium‐Range Weather Forecasts (ECMWF) Ensemble Prediction System (EPS) provides an excellent basis for medium‐range forecasting as it provides a number of different possible realizations of the meteorological future. This ensemble of forecasts attempts to account for uncertainties in both the initial conditions and the model formulation. Since 18 July 2000, routine output from the EPS has included the field of potential temperature on the potential vorticity (PV) D 2 PV units (PVU) surface, the dynamical tropopause. This has enabled the objective identification of blocking using an index based on the reversal of the meridional potential‐temperature gradient. A year of EPS probability forecasts of Euro‐Atlantic and Pacific blocking have been produced and are assessed in this paper, concentrating on the Euro‐Atlantic sector. Standard verification techniques such as Brier scores, Relative Operating Characteristic (ROC) curves and reliability diagrams are used. It is shown that Euro‐Atlantic sector‐blocking forecasts are skilful relative to climatology out to 10 days, and are more skilful than the deterministic control forecast at all lead times. The EPS is also more skilful than a probabilistic version of this deterministic forecast, though the difference is smaller. In addition, it is shown that the onset of a sector‐blocking episode is less well predicted than its decay. As the lead time increases, the probability forecasts tend towards a model climatology with slightly less blocking than is seen in the real atmosphere. This small under‐forecasting bias in the blocking forecasts is possibly related to a westerly bias in the ECMWF model. Copyright © 2003 Royal Meteorological Society