Monthly hydrological report December 2013

Record-breaking winter floods due to mild and wet weather

Precipitation in December 2013

December was mild and precipitation was generally high, particularly in Kainuu. At the beginning of the month most of the country was covered with a layer of snow, which then melted in southern and central Finland. The mild weather, melting of snow and heavy precipitation resulted in unprecedentedly high discharges and water levels in some coastal rivers from Satakunta to Pohjois-Pohjanmaa around Christmas time. In some places the highest levels of the whole year were measured at the end of December. Groundwater levels increased in southern and central Finland, but were below the seasonal mean in Lapland. Groundfrost was recorded generally only in the north. In southern and central Finland, mild weather caused melting of the ice layer which had formed over many lakes at the end of November and the beginning of December; by the end of the year many of these lakes were unfrozen. In Lapland, recorded ice thicknesses were typical for the time of year.

Precipitation

Precipitation was particularly high in an area between the lines from Vaasa to Joensuu in the south and Kemi and Kuusamo in the north. Accumulated precipitation in this area was 80−130 mm, two- or almost threefold the seasonal mean. Elsewhere the precipitation figures were mainly 40−80 mm, i.e. close to or above the mean. In many parts of northern Lapland however, the precipitation was below 40 mm. In almost all areas the precipitation fell as rain, snow and sleet.

Snow cover

At the beginning of December most of the country was covered with snow, which then melted as a result of mild weather. The melting was almost complete by the end of the year to the south of a line from Oulu to Joensuu. Further north more snow fell, with above-average accumulation in some areas. By the end of the month, melting of snow extended even to southern Lapland. The water equivalent of the snow cover was over 120 mm in north-western Lapland and in the area around Kuusamo; in other parts of Lapland, Koillismaa and Kainuu the corresponding figures were 40−100 mm.

Water level and discharge

The decrease in water levels continued in Lapland during December, but elsewhere in the country rain and melting of snow increased both water levels and discharges. In an area extending from Satakunta to Pohjois-Pohjanmaa, water levels were exceptionally high, in many cases setting new seasonal records. In some areas the highest water levels of the year were measured during the very last days of December. The December floods did not cause extensive damage, but fields, forest areas and some roads were inundated. Lake water levels were above the seasonal mean at the end of the year in most areas except Lapland. In many central and eastern regions lake levels were higher than ever previously recorded at the close of the year. The mean monthly discharges of the major watercourses varied on both sides of the seasonal mean.

The deviation of the level of lake Pielinen from the seasonal mean at the end of December was +46 cm, and corresponding figures in some other major lakes were: Kallavesi +53 cm, Saimaa +20, Keitele +19, Päijänne +15, Pyhäjärvi, Säkylä +1, Längelmävesi +8, Näsijärvi +6, Lappajärvi +67, Lammasjärvi +41, Oulujärvi +39, Lokka -110 and Inari -20 cm. The discharge of the river Pielisjoki was 125% of the mean for December during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 90%, Kymijoki 132, Karjaanjoki 97, Kokemäenjoki 98, Siikajoki 266, Oulujoki 96, Iijoki 121, Kemijoki 67 and Tornionjoki 87%.

Groundwater level

Groundwater levels increased above the seasonal mean as a result of the mild weather in southern and central Finland, but in Lapland the levels were 5−60 cm below the mean.

Groundfrost

Groundfrost was recorded during December mainly in the north of the country, with depths of 0−10 cm. In southern and central Finland depths of a few centimetres were recorded in some places.

Ice cover

Frosts at the beginning of the month caused freezing over of lakes even in southern Finland, but milder weather after mid-December melted or at least weakened ice layers as far north as Oulu. At the end of the year, most of the lakes of southern and central Finland were completely open, or had only a weak ice cover near the shore. Ice thicknesses at the end of December were 30−45 cm in Lapland and 15−30 cm in Kainuu. Elsewhere, ice thicknesses could be measured only in northern regions of central Finland and in some eastern areas, but the ice covers were very weak.

Maps, graphs and tables

Published 2014-05-09 at 16:36, updated 2014-05-09 at 16:33