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Zusammenfassung:
Severe winter windstorms belong to the most damaging extreme events over Europe with damages costing over several millions of EUR. Hence, the understanding and skilful predictions would be of great value. On the other hand, the seasonal time scale is not only of interest for windstorm studies but is gaining more and more interest in general.
This study investigates extreme event predictions on the seasonal time scale. Therefore, it is using the seasonal forecast model GloSea5, from the UK Met Office, validated with ECMWF ERA5 reanalyses. Windstorms are tracked with an impact-focused algorithm for 23 winter seasons, DJF, 1993-2015.
The windstorm frequency shows skilful predictions over the British Isles and southern Scandinavia while the forecast skill of intensity depends on the measure used. An accumulated intensity of the season shows skilful areas downstream of the North Atlantic stormtrack. The significant spatial area for a normalised intensity decreases slightly, but still shows positive forecast skill.
A multi-linear regression analysis shows that most of the windstorms over the North Atlantic and European area are connected to the NAO index, but there is also a gap of explanatory variance over the North Atlantic for the NAO. The Scandinavian and East Atlantic pattern are filling the gab exactly in this area. Those three large-scale patterns describe all together up to 80% of windstorm frequency and 60% of windstorm intensity variance. Besides large-scales patterns, dynamical factors, like Eady Growth Rate or the Jetstream Location, are also important drivers for seasonal forecasts of windstorm activity.
