...
The forecast from around 19 December was for slightly colder than normal (persistence of cold wave?), but gradually picked up the signal for warmer than normal conditions in the forecasts from the following days.
3.4 Monthly forecasts
The plot below shows the verification of 2-metre temperature anomalies for the week starting 26 December. Consistent with the forecast evolution plot, one can see that the forecast from 19 December predicted a cold anomaly for north-western Europe, opposite to what was observed.
3.5 Comparison with other centres
...