Flow dependence of wintertime subseasonal prediction skill over Europe

<p><span>Issuing skillful forecasts beyond the typical horizon of weather predictability</span> <span>remains a challenge actively addressed by the scientific community. This study evaluates winter subseasonal reforecasts delivered by the</span> <span>CNRM and ECMWF dynamical systems and identifies that the level of skill for predicting temperature in Europe </span><span>varies</span> <span>fairly </span><span>consistently</span><span> in both systems. In particular, forecasts initialized during positive NAO phases tend to be more skillful over Europe at week three in both systems. Composite analyses performed in an atmospheric reanalysis, a long-term climate simulation and both forecast systems unveil very similar temperature and sea-level pressure patterns 3 weeks after NAO+ conditions. Furthermore, regressing these fields onto the 3-week </span><span>pre</span><span>vious</span> <span>NAO index in a reanalysis shows consistent patterns over Europe but also eastern North America, thereby revealing</span> <span>a lagged teleconnection,</span> <span>either related to the persistence or recurrence of the NAO+</span> <span>weather regime. Since this feature is well captured by forecast systems, this </span><span>is </span><span>a key mechanism for determining a priori confidence in the skill of wintertime subseasonal forecasts over Europe and North America.</span></p>