The Role of Weather Regimes for Subseasonal Forecast Skill of Cold-Wave Days in Central Europe

Abstract. Weather regimes (WRs) represent the large-scale tropospheric flow and therefore may contain useful information about the subseasonal predictability of cold waves, one of the most severe weather extremes in Central Europe. Firstly, we investigate in how far the succession of WRs during a forecast can be used to explain skill differences of forecasts initialized during different WRs. As an example, we use the skill differences of mean-bias-corrected 14-day reforecasts of the European Centre for Medium-Range Weather Forecasts for the occurrence of wintertime cold-wave days in Central Europe. Reforecasts initialized during the WR Greenland Blocking (GL; characterized by a high pressure system over Greenland) show the best Brier skill while those initialized during the WR Scandinavian Trough (ScTr; characterized by a low pressure system over Scandinavia) show the worst skill compared to a climatological ensemble for the winters 2000/2001–2019/2020. We find, that for forecasts initialized during GL, more often WR succession which follow typical climatological pattern are found during the 14 days of forecasts than for forecasts initialized during ScTr. We suggest that this is one of the main reasons for an increased forecast skill of predictions initialized during GL in contrast to predictions initialized during ScTr. Secondly, we analyze the WR succession for the best (worst) predicted days within the observed cold waves in the winters 2000/2001–2019/2020 independent from the WR present at initialization. We find, that forecast skill is significantly higher, when the European Blocking WR (characterized by a high pressure system over the British Isles and southern Scandinavia) is present a few days before the predicted cold-wave day. These results can be used to assess the reliability of cold-wave day predictions at the subseasonal lead time of 14 days.