In this paper we used reanalysis data, best track data and high resolution simulations with the global model OpenIFS to study a historical windstorm (Mauri) which caused considerable damage in northern Finland on 22 September 1982. It had previously been speculated by forecasters that this windstorm was related to Hurricane Debby, a category four hurricane that occurred over the tropical Atlantic. We find that a very complex sequence of events took place. Hurricane Debby underwent extra-tropical transition which resulted in ridge building and an acceleration of the jet stream but ex-Debby did not re-intensify immediately. Instead ex-Debby travelled rapidly across the Atlantic as a diabatic Rossby wave - a feature which constantly regenerates itself due to diabatic heating - but it still did not strengthen. When ex-Debby approached the UK, it moved ahead of an upper-level trough and rapid re-intensification began. Ex-Debby then underwent 24 hours of rapid deepening before affecting northern Finland on 22 September. We also studied the causes for the strong wind gusts over Northern Finland and found that turbulent mixing behind the cold front and convectively driven downdrafts in the warm sector enhanced the wind gusts. We conclude that Hurricane Debby likely contributed to some extent to the development of storm Mauri but many other factors, such as the interaction with the upper level trough near the UK, were of equal importance.
Our numerical model simulations were performed with a state-of-the-art model yet all forecasts diverged from reanalysis after only two days. This indicates that the whole event was characteristed by intrinsic low predictability and strong sensitivities. To further improve understanding of storm Mauri an ensemble approach should be taken.
Laurila, T. K., Sinclair, V. A. and Gregow, H. (2019), The Extratropical Transition of Hurricane Debby (1982) and the Subsequent Development of an Intense Windstorm over Finland, Monthly Weather Review. DOI: 10.1175/MWR-D-19-0035.1