by EFAS Meteorological Data Collection Centre

May 2021 was characterized by higher than normal mean sea surface pressure over the Denmark Strait and the Greenland Sea and slightly higher than normal over the central and western Mediterranean regions and the Atlantic Ocean. Lower than normal mean sea surface pressure occurred over the remaining part of the EFAS domain with one core over Great Britain and Ireland and the other core over southern Scandinavia. Monthly precipitation totals were below the long-term means in the Mediterranean Region, the eastern parts of the EFAS domain as well as in Iceland and parts of Scandinavia, and around or above normal elsewhere in the EFAS domain. The monthly mean air temperature was above normal in the African and south-eastern parts of the EFAS domain as well as in eastern Europe and below normal elsewhere in the EFAS domain.

At the beginning of May, weak surface pressure gradients occurred in the EFAS domain with a small low-pressure system over northern Scandinavia. It moved to the north and dissolved soon after. In the upper-pressure levels, a large trough extended from the north towards the Iberian Peninsula, flanked by a ridge from the southwest to the central Mediterranean region. Intense precipitation occurred in many parts of Algeria due to this constellation for several days. The constellation of the trough and the ridge allowed the formation of a low-pressure system over the eastern Alps, associated with intense rain north of the central Mediterranean Sea. This low-pressure system moved via the Baltic region to the Kara Sea, bringing notable rain amounts along its track. At the same time, a low-pressure system developed over the Atlantic Ocean, to the west of Ireland. It experienced a rapid intensification and caused strong winds in Great Britain and Ireland, the Benelux, and the North Sea and was related to intense rainfalls. It moved, while weakening, via Jutland to northern Scandinavia with significant rain amounts along its track. A small and weak low-pressure system developed again over the eastern Alps caused intense rains northeast of the Adriatic Sea. At the same time, another low-pressure system arose over the Atlantic Ocean. It reached its highest intensity over the ocean and started weakening before it reached Great Britain and Ireland, where it dissolved. The highest rainfall intensity was in western and central Europe, caused by the eastern flank of the associated upper-level trough. The largest rainfall amounts were received at the Cévennes, Jura and western and southern Alps. Also, at the same time, an upper-level low-pressure system was cut off over the Black Sea. A related surface low-pressure system developed soon after and remained in this region associated with notable precipitation sums. Later, it moved westwards and joined with the other low-pressure system over central Europe. A high-pressure ridge squeezed between these two low-pressure systems. It transformed later into a high-pressure system over western Russia. Another low-pressure system developed over the Atlantic Ocean and moved across the North Sea towards northern Scandinavia. This system had weak gradients but, like the connected upper-level trough, a large spatial extent. The frontal zone was located above the northern Mediterranean coast. It caused large-scale rains in western, central, and south-eastern Europe. A low-pressure system moved under intensification from the Atlantic Ocean to Great Britain and Ireland and caused strong winds in the same regions as the one at the beginning of the month. It dissolved soon over the North Sea while another low-pressure system moved from the Labrador Sea to Great Britain and Ireland. While weakening, it moved via Jutland to the Gulf of Bothnia. In the last days of May, high pressure occurred in the majority of the EFAS domain with mostly undefined circulation patterns. A weak upper-level low-pressure system occurred over the Balkans associated with intense precipitation.

In May 2021, the highest precipitation totals were observed around the Alps and the Carpathian Mountains, the Baltic region, southern Scandinavia and Great Britain and Ireland (Figure 1). No or almost no precipitation fell in the African and Middle Eastern parts of the EFAS domain. Monthly precipitation totals above the long-term mean occurred over parts of Algeria as well as in central and eastern Europe (Figure 2). Monthly totals below the long-term mean were reported over Iceland, northwest Scandinavia, around the Mediterranean Sea, the Middle East, in and to the northeast of the Caspian Depression.

The monthly mean air temperature ranged from -9°C to 34.8°C with the highest values in the southern and eastern parts of the EFAS domain. The lowest temperature values were reported in the northern and mountainous parts (Figure 3). Air temperature anomalies ranged from -6.4°C to 10.7°C (Figure 4). Monthly mean air temperatures below the long-term means occurred in central and northern Europe, while positive temperature anomalies appeared in the remaining part of the EFAS domain.