by EFAS Meteorological Data Collection Centre

September 2020 was characterised by roughly normal monthly mean surface pressure in the EFAS domain, only lower than normal around Spitsbergen. Monthly precipitation totals were below the long-term means in many regions in the EFAS domain, especially in the southeast. Monthly mean air temperatures were below the long-term means in parts of northern and western Europe and Africa as well as in the east of the EFAS domain, while positive anomalies occurred in the other parts.

At the beginning of September 2020, the Icelandic Low and Azores High were around their usual positions while another high-pressure system was located over western Europe and Scandinavia, a low-pressure system over the North Russian Lowlands and an upper-level low-pressure system above the northern Adriatic Sea. The upper-level system moved towards central Europe causing intense precipitation at its eastern edge and disappeared, while the high-pressure system moved via Scandinavia towards Russia. The Azores High extended to central Europe with the development of a new core over western Russia. The Icelandic Low intensified, causing continuous heavy rain in Great Britain and Ireland, and around the North Sea. Later, it moved to Spitsbergen. Also at the beginning of August, intense precipitation events associated with flash floods were reported from Tunisia and Algeria. From the aforementioned low-pressure system, an upper-level trough developed over central Europe, was cut-off and moved to the western and central Mediterranean Sea, causing heavy precipitation with associated flash floods. During the same days, a low-pressure system moved from the Atlantic via Iceland to Scandinavia including the formation of secondary low-pressure systems, which brought notable precipitation amounts there and eastward of Scandinavia. A weak high-pressure ridge developed from the Azores and extended to eastern Europe.  By the middle of the month, a high-pressure system developed over central and eastern Europe moving slowly eastward and an old low-pressure system intensified again over the West Siberian Plain. A low-pressure system formed over southern Scandinavia, experienced a rapid intensification and moved to western Russia, associated with strong winds and continuous heavy rain. At the same time, a high-pressure system developed over the North Sea and a low-pressure system moved from the Atlantic Ocean to the west of the Iberian Peninsula and later the Bay of Biscay with notable rain amounts in southwest Europe. In addition, the short-lived tropical storm ‘Alpha’ developed west of Portugal and Medicane ‘Ianos’ (also named ‘Cassilda’) brought strong wind and heavy precipitation including flash floods to Greece and Crete, and some coastal regions in Libya. A low-pressure system moved with strong wind via Iceland and northern Scandinavia to the West Siberian Plain. Weak pressure gradients were found in the remaining EFAS domain, allowing several convective precipitation events in the western and central Mediterranean Region. An upper-level trough with several low-pressure cores at the surface developed over Great Britain and Ireland and extended to the central Mediterranean region, whereas one of the cores moved to Scandinavia. It was cut-off, with the centre over the Alps, weakened and moved to the eastern Carpathian Mountains by the end of the month, associated with repeated continuous heavy precipitation in central and eastern Europe, and the Balkans. Another low-pressure trough developed over Iceland and extended to Great Britain and Ireland, also associated with intense precipitation.

In September 2020, the highest precipitation totals were observed around the central Mediterranean region, northern Carpathian Mountains, and along the western Scandinavian coast (Figure 1). No or almost no precipitation fell in the south-east of the EFAS domain and north-west of the Caspian Sea. Monthly precipitation totals below the long-term means occurred in many parts of the EFAS domain, especially in the east and south-east, but also at the southern Iberian Peninsula, central Europe and the Baltic Bridge (Figure 2). Monthly totals above the long-term means were reported in parts of the Mediterranean region, eastern and northern Europe, and the northern Iberian Peninsula. Two erroneous spots with high precipitation totals are in Syria/Jordan and Morocco/Algeria due to single erroneous data points passing the automated quality control procedures as they were still below the thresholds.

The monthly mean air temperature ranged from -4.8°C to 36.7°C with the highest values in the southern parts of the EFAS domain. The lowest temperature values were reported in the northern and mountainous parts (Figure 3). Air temperature anomalies ranged from -6.3°C to 8.6°C (Figure 4). Monthly mean air temperatures below the long-term means occurred in Iceland, Great Britain, Ireland, and in some regions in Scandinavia, the Iberian Peninsula, northern Africa, and eastern Europe. Positive monthly mean temperature anomalies were reported in other parts of the EFAS domain.