A front of rain and thunderstorms that arrived between Turku and Pori on the west coast spread later into the east and north-east.
Severe thunderstorms may produce strong wind gusts of more than 15 metres per second and rain accumulations of more than 20 millimetres an hour, the FMI says.
Parts of Kainuu and North Ostrobothnia may receive more than 50mm of rain overnight.
There is also a warning of near-gale force winds and thundersqualls in many maritime areas.
By Sunday evening, a couple of outbuildings had been destroyed by fires set off by lightning, but no serious damage was reported.
Local thunderstorms are possible on Monday in central Finland, North Ostrobothnia and Kainuu.
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