Flow-dependent verification of the ECMWF ensemble over the Euro-Atlantic sector

This study aims to assess the relative skill of medium- range weather forecasts depending on which flow pattern is in place over the North Atlantic when the forecast is initiated. A key aspect in the evaluation of flow-dependent predictability is that a defined flow circulation pattern must occur with sufficient frequency that statistics of ensemble forecast spread can be gathered. For this reason we use the concept of weather regimes to classify a small number of flow patterns. Consequently, the intra-seasonal variability of the North-Atlantic atmospheric circulation is described as transitions between a small number of recurrent and quasi- stationary states called weather regimes.