Derechos in Finland

Derechos are long-lived convective wind events that cause damage in vast areas. Since little was known before about derecho occurrence in Finland, the purpose of this study was to search for previously unidentified derecho events and describe derecho occurrence and characteristics in Finland. Derechos were identified by comparing radar reflectivity development of convective storms to the maximum wind gust measurements and reported wind damage. Based on results of this study, derechos are not as rare events in Finland as has been previously assumed. In the preliminary dataset derechos have been observed in Finland from June to October. Compared to other types of convective severe weather, derechos are long lasting events, with their duration in Finland ranging in our dataset from 4h 30 min to 9h 30 min. The length of the observed derecho tracks in Finland varied between 400 km and 660 km. The often-used criteria to search and classify derecho cases based on wind gust measurements of 25 m/s or more, seem not to be very effective in Finland. Even many of the major events, with vast wind damage areas identified by emergency reports of wind damage, would end up having few if any ≥25 m/s gusts measured. The challenge of identifying events that cross country borders, move over open sea or low population areas, is also discussed.