Recurrence of heavy precipitation, dry spells and deep snow cover in Finland based on observations

The recurrence of heavy precipitation, dry spells and deep snow cover were estimated based on observations at about ten stations in Finland during about five decades. The 10-year return levels were assessed by means of the so-called peak over threshold (POT) method. The return levels of the annual maximum snow depth ranged from about 65 cm in southwestern Finland to about 110 cm in Lapland. On average once in ten summers, there is likely to be a 40-day period with at most 10 mm of accumulated rain, and a period of about 75 days with less than 50 mm of rain. The average 10-year return level estimate at a fixed site was 50 ± 8 mm for daily precipitation and 139 ± 9 mm for monthly precipitation. In comparison, additional material, consisting of monthly precipitation data at about 200 stations during the past 50-150 years, suggested that once in a decade the monthly precipitation somewhere in Finland exceeds 240 ± 12 mm. The difference demonstrated the lower likelihood of an extreme event at a certain site compared with the probability that such an event occur somewhere in the country. Climate change may alter these return levels in the future.