by EFAS Meteorological Data Collection Centre

December 2020 was characterised by higher than normal mean sea surface pressure in the eastern parts and below normal in the north-western parts of the EFAS domain. Monthly precipitation totals were below the long-term means especially in the eastern parts of the EFAS domain and above normal in the central Mediterranean Region and northwest Europe. Monthly mean air temperatures were below the long-term means northward of the Caspian Sea, the Iberian Peninsula, Ireland and Iceland, and above the long-term means elsewhere in the EFAS domain.

In the beginning of December 2020, the Azores High was northward of its usual position and another high-pressure system was located in the eastern Mediterranean Region. A very strong high-pressure system was situated above Russia and Kazakhstan, blocking the eastward movement of low-pressure systems to eastern Europe from the Atlantic Ocean during most of December. A weak low-pressure system was located above the North Sea and a strong one southeast of Greenland. The first mentioned low-pressure system moved to the central Mediterranean Region associated with intense precipitation around the Adriatic Sea and the Algerian Coast. The other low-pressure system moved with strong winds via Iceland to Great Britain and Ireland and was associated with high precipitation totals along its track and in the western and central Mediterranean Region. This pattern with the Azores High, a trough over western and central Europe and the high-pressure system over Russia and Kazakhstan lasted for several days. By mid-December, a low-pressure system was cut-off the trough and moved to the eastern Mediterranean Region and was associated with high rain amounts. At the same time, a strong low-pressure system developed over the Atlantic Ocean and moved to Great Britain and Ireland with strong winds and intense precipitation. Another low-pressure system with strong winds moved from the Atlantic Ocean via Scandinavia towards Russia, replacing the high-pressure system for some days till the end of the month. It brought notable precipitation amounts to the central and eastern parts of the EFAS domain. During the same days, a weak and small low-pressure system moved from the Atlantic Ocean via France to the central Mediterranean Region, with high precipitation totals there and at the Balkans. In the last days of December, a very strong low-pressure system developed eastward of Greenland and moved with very strong winds via Iceland to Great Britain and Ireland. It weakened fast as it moved further to Jutland by end of December.

In December 2020, the highest precipitation totals were observed in Great Britain and Ireland, around the Bay of Biscay, and the central Mediterranean Region (Figure 1). No or almost no precipitation fell in the south of the EFAS domain, around the Caspian Sea and the southern coast of the Iberian Peninsula. Monthly precipitation totals below the long-term means occurred in the south and east of the EFAS domain but also in western Scandinavia, central Europe, the southern Iberian Peninsula, and the Baltics (Figure 2). Monthly totals above the long-term means were reported from central Scandinavia, Great Britain and Ireland, around the Bay of Biscay, the central Mediterranean Region and Balkans, and the north-western parts of Africa.

The monthly mean air temperature ranged from -22.7°C to 21.6°C with the highest values in the southern parts of the EFAS domain. The lowest temperature values were reported in the northern, eastern, and mountainous parts (Figure 3). Air temperature anomalies ranged from -7.4°C to 7.3°C (Figure 4). Monthly mean air temperatures below the long-term means occurred around the Caspian Sea, Iceland, Ireland, the Iberian Peninsula, and around the Provence region. Positive monthly mean temperature anomalies were reported in the other parts of the EFAS domain with the largest anomalies in Scandinavia and southeast Europe.