Rainstorms and flash flooding

When a rainstorm hits, a single day’s rainfall can exceed the usual monthly precipitation. Rainstorms cause flash flooding, especially in cities.

Cities are particularly vulnerable to flooding caused by heavy rains as the ground surface is paved with stones and asphalt. The rainwater cannot infiltrate into the soil, and stormwater drains do not have enough capacity to convey all of it away. Water starts accumulating in underpasses and other low-lying points.

Downpours and the heaviest thundery showers usually only affect limited areas for a short while, perhaps a few hours. The exact place where a rainstorm will strike cannot be predicted.

Longer spells of rain over larger areas are associated with weather fronts, however, and the Finnish Meteorological Institute is usually able to forecast such rains at least a day or two in advance. Even if not particularly heavy, a long spell of rain may cause floods. The Flood Centre jointly managed by the Finnish Meteorological Institute and the Finnish Environment Institute issues flood forecasts and warnings.

The heaviest rains usually come in late summer, with rainfall amounts of up to a hundred millimetres per day. This equals 125 bucketfuls of water over an area the size of a car parking space.