Many temperature anomalies in northern hemisphere spring
Much of Europe, the United States of America, north-west Russia and parts of Japan had a much colder than average spring (1 March to 1 June), which ended with heavy rain in some European countries. By contrast, the Arctic region was considerably warmer than normal, as was a large area covering most of central and northern Africa (except Morocco and western Algeria), the Eastern Mediterranean, southern Russia and much of China.
This climate pattern started already in February, with cold Arctic air moving far to the south over Europe and the USA, and also wetter conditions over the eastern USA and western and southern Europe due to inclusion of the cold air in low pressure circulation. Corresponding sea level pressure anomalies showed higher pressure than usual over the Arctic and lower pressure than usual over mid-latitudes. This is a typical pattern of a negative phase of the so-called Arctic Oscillation (AO). The Arctic Oscillation had several phases of outstandingly negative values during the first half of spring.
In the second half of the spring, the Arctic Oscillation changed to a positive and then to a neutral phase. However, during the cold spell in late May there was still a similar pressure pattern with high pressure over Greenland and low pressure systems to the south, which caused Arctic air masses to flow to western and central Europe.
According to provisional figures, the UK reported the 5th coldest spring in national records dating back to 1910 and the coldest spring since 1962. France also reported below average temperatures from 10 May.
Germany reported its coldest spring since 1987, with an average temperature of 6.7 degrees Centigrade, 1 degree below the 1961-1990 average. Germany had its second wettest May since the beginning of records in 1881, with 178 percent of the average May rainfall.
Switzerland also recorded below average temperatures with a big deficit of sunshine compared to the long term average. In Austria, spring was one of the seven wettest on record. May was one of the three wettest of the past 156 years (along with 1962 and 1965), with twice as much precipitation as the long-term average. Austria received as much rain from 30 May-2 June as it would normally receive in one and a half to two months.
The Czech Republic declared a red alert in the western part of the country 3-4 June for flooding, which also affected some southern parts of Germany particularly badly.
According to the Finnish Meteorological Institute, May was unusually, in many places even exceptionally, warm. The anomalies were particularly great in Lapland. The last days of the month were particularly warm in Northern Finland, where temperature records for May were broken at several observation stations. Hot days (over 25° C) numbered nine in May, or the same as in 2010. The statistical average for hot days in May is three.
Surface air temperature anomalies (1981-2010 reference) for spring 2013 (1 March – 25 May 2013). Source: NOAA/ESRL
Unusually cold week 19-25 May
For the week of 19-25 May, the NOAA Climate Predication Centre noted average temperatures more than 5°C below the long-term values. The cold spell affected large parts of western and central Europe and even parts of North Africa. Minimum temperatures in central Europe reached around the freezing point; some were new local records for the last May decade. Snow fell even in elevations of less than 800m in central Europe and the Alps. The cold spell extended to northwest Africa (Morocco and Algeria) with a snow fall reaching 10 centimeters on 22nd of May at 1800 meters elevation over some places of the Atlas Mountains in Morocco, which is unusual in this period of the year.
The weather situation was related to a strong flow of polar air to western and central Europe (Fig. 4 and 5). In southern Europe, the cold air was taken up by the circulation of a low pressure area centered over Italy, causing widespread and intense snowfall, particularly in the Alpine region.