ATR, daarna positieve NAO, later in december een behoorlijke kans op blokkade.
Daily probabilities for the occurrence of Euro-Atlantic weather regimes. The coloured bars indicate the proportion of ensemble members forecasting a specific regime for each day of the forecast: positive (blue) and negative (green) North Atlantic Oscillation, Scandinavian Blocking (red), Atlantic Ridge (purple). The spatial patterns of the four Euro-Atlantic regimes are shown here. The members not close enough to any of the 4 weather regimes are represented by the grey bars (members with no regime attribution). Bars with diagonal lines represent the ensemble mean regime attribution. Bars with solid colours and solid black contours represent the verification. The regime attribution is computed in an EOF space using 'calibrated' anomalies. To account for the seasonal evolution the patterns and amplitudes of the regimes are adjusted month by month. A pattern matching algorithm assigns each forecast to the closest regime (using the root mean square metric).
The forecast anomalies are computed with respect to a 20-year reforecast climate. A given regime is assigned only if the minimum distance between the anomalies and any of the 4 regimes is smaller than a defined threshold and if the minimum distance is significantly different from the other distances. The climatological frequencies of the Euro-Atlantic regimes, computed over 29 years of extended winter periods (October to March) of ERA-Interim , are 32% for the NAO+, 26% for the Blocking, 21% for the NAO- and 20% for the Atlantic Ridge. Because the relative frequency of each regime exhibits a pronounced multiyear/decadal variability over the twentieth century, the climatological frequencies are not necessarily representative of different climate periods. Associated with this product there is a table with the list of individual members belonging to each regime.