Violent thunderstorms and heavy rain hit the Helsinki region early Thursday, knocking out electricity and causing minor flooding as many residents prepared for the long Midsummer holiday weekend.
In Espoo, a lightning bolt set a building ablaze around 4am. Lightning also caused technical problems with the capital region Metro system, leading to the cancellation of some train departures.
Yle meteorologist Matti Huutonen said the intense storm system moved off toward the southeast and the northeast later in the morning.
In the late morning, a cottage burned to the ground on Våtskär island in Lovisa after a lightning strike.
"Highly unstable, moist air flowed into Finland from the south, and during the day there will be thunder particularly from North Ostrobothnia to eastern Finland," Huutonen said.
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Heavy showers caused flooding overnight in some spots, and are predicted for Thursday afternoon and evening in eastern and northeastern Finland.
The Finnish Meteorological Institute (FMI) warns of particularly strong thunderstorms in North Ostrobothnia, North Karelia, Kainuu and Savo.
South Savo Rescue Squad said that it responded to at least 21 emergency situations caused by lightning, mostly fires.
"Compared to the average for June, quite a lot of rain has fallen, for instance 26 millimetres in Helsinki today," Huutonen said.
In the Savonlinna area of eastern Finland, rainwater flooded a shop and several cellars, and was half a metre deep on some city streets.