Slow-moving thunderstorms bring exceptional rainfall to Finland

Sunday was Finland’s busiest thunderstorm day of the summer, with Seinäjoki recording 107.7 millimetres of rain, Yle reported. More than 80 millimetres fell at the Pelmaa observation station in a two-hour period.

About 5,600 ground lightning strikes were recorded across Finland during the day. Thunderstorms and heavy rain lasted unusually long, particularly in South Ostrobothnia and Ostrobothnia.

“These are exceptional accumulations by Finnish standards. A daily total of more than 100 millimetres is at least considered rare,” said Petteri Pyykkö, a duty meteorologist at the Finnish Meteorological Institute.

The storm was linked to a broader area of low pressure. Its unusual feature in Ostrobothnia was the exceptionally slow movement of the thunderstorms, which remained over the same area for several hours.

Normally, an individual thunderstorm moves on quickly and lasts about 30 minutes, or at most an hour. Slow upper-atmosphere air currents kept the storms moving slowly, while new showers repeatedly formed to the west over the same areas, increasing the rainfall totals, Pyykkö said.

Seinäjoki received its heaviest rain between 5pm and 7pm, when more than 80 millimetres was measured at Pelmaa. The station recorded 46.7 millimetres in one hour, the fifth-highest hourly total in Finland and a record for that station. The previous record was 38.1 millimetres per hour, set in 2022.

The Finnish Meteorological Institute said local rainfall totals may have been even higher in some places because measurements can vary considerably between observation points. Pelmaa also recorded more than 40 millimetres in consecutive hours.

“You do not often get to witness something like this. More than 80 millimetres in two hours is an enormous amount,” said meteorologist Eveliina Tuovinen.

Finland’s highest recorded daily rainfall was 198.4 millimetres in Lahnus, Espoo, in 1944. At the Seinäjoki Pelmaa station, 120.1 millimetres was recorded in one day in 1967, exceeding Sunday’s total.

Systematic hourly rainfall measurements have been taken in Finland only since 2010. The highest hourly total during that period was 59.2 millimetres, recorded at the Pellonpää observation station in Oulunsalo in July 2014.