Statistics of co-occurring extreme wind speed and snowfall in Finland

High wind speeds and intense snowfall are significant factors that play a crucial role in the safety and operational integrity of critical infrastructures. From a societal perspective, high wind speeds and intense snowfall can severely disrupt transportation networks, cause widespread power outages, and limit access to emergency services. Climate in Finland is characterized by severe and varying winters, and thus Finland experiences risks caused by such conditions. 

Recently, two strong mid-latitude cyclones affected Finland within a short time range, storm Lyly on 1 November 2024 and storm Jari on 20-21 November 2024. High wind speeds and gusts, co-occurring with heavy snowfall caused severe power outages and traffic accidents. Understanding and mitigating the impacts of such conditions are vital for societal resilience, as well as for ensuring the uninterrupted functioning of essential services, including energy production. 

This study investigates the frequency, duration and intensity of these weather phenomena in Finland, with higher emphasis on coastal regions. The occurrence of high wind gusts and snowfall, as well as combined wind speed and snowfall events, were analyzed using ERA5 reanalysis data covering the period from 1960 to 2024. To identify extreme events for snowfall, wind gust, and wind speed, high percentile thresholds were applied, with the 95th and 98th percentiles serving as cutoff values for extreme events. Events where both snowfall and wind speed, or snowfall and wind gust, exceed these thresholds were classified as extreme compound events. 

Statistics of the co-occurrence of extreme hourly snowfall and wind speed, as well as snowfall and wind gusts, were examined using the established thresholds. The primary research questions addressed are: (i) What are the frequency and location of co-occurring high winds and heavy snowfall, (ii) What is the duration of consecutive hours during which compound events persist, and (iii) What is the intensity of these compound events.