Cold Air Outbreak - 10 to 20 December 2009

Summary and Conclusions

The Cold Air Outbreak during the period from 10th till 20th December 2009 had a huge impact on everyday lives of many people in many European countries. From a meteorological point of view different physical processes were responsible for snow and extreme weather conditions, such as very low temperatures and strong wind.

The synoptic situation over Europe was characterized by a long lasting high pressure belt from Greenland towards Russia. Along the southern flank of the anti-cyclone over Russia there was a continuous flow of cold and dry air over Poland, Germany and the Netherlands into England. On the 16th/17th and also on the 20th December depressions developed over Scandinavia. With the high pressure system over Iceland and Greenland, these depressions receive a southerly impulse and, as moving over the relatively warm sea water, picked up moisture. As the depressions moved over the colder land surfaces they produced severe snowfall. During the same period the Mediterranean region was undergoing strong cyclonic activity. Most of the depressions were entering the Mediterranean Sea from the Atlantic Ocean and moving further to the east. Some of them were deepening over Genoa bay and, accompanied with strong cold advection from the north, brought heavy precipitation - rain and often snow.

Snow on the 16th/17th December in the Netherlands was a consequence of retreating of a high pressure system towards Iceland and changing of upper level flow to a southerly direction. The low over southern Scandinavia moved towards the southwest and deepened over the North Sea and the Netherlands. Dry stratospheric air protruding downwards and in combination with the warmer temperature of the North Sea have had a destabilizing effect explaining the snow showers experienced and also seen in the vicinity of the depression.

An upper level low which was situated over Germany, played a role for the snow on the 20th. This ULL had cooled the air significantly also in the lower layers (-10 °C at 850 hPa). The active depression was situated west of Scandinavia, near the Norwegian coast. The second depression, found over the North Sea south of Norway, got the impulse from the first one, and moved rapidly to the east. It was with the passage of this trough, especially in the western and central part of the Netherlands, that most snowfall was recorded (10-15 cm). There were also some thunderstorm reported, associated with the extremely cold upper level temperature (-38 °C at 500 hPa) over the North Sea.

Snow on the 18th December in England had a different origin. The relatively warm North Sea and the colder air above provided the ingredients for Lake-Effect Snow. Due to the Lake-Effect the most snow has fallen over South-East England.

The precipitation in Croatia was very frequent in the period between 14th and 19th December, due to strong cyclonic activity in the Mediterranean Sea.

An upper-level low which caused the snowfall in the Netherlands and the neighboring countries (on the 16th and 17th) indirectly played the role for the snow in Croatia. Namely, that upper-level low moved southward and in the combination with the upper-level low over Germany formed one broad low over Central Europe with a trough axis extending towards Genoa Bay.

On 18th the strong westerly wind caused depression to move to the North Italy, followed by a pronounced maximum of cold advection. The strong dipole of cold and warm advection over Genoa Bay and North Adriatic is a sign of the ongoing cyclogenesis which resulted with heavy snowfall recorded on 19th over north Italy and Istria in the northern Adriatic. By the end of the day cold air covered the whole country with snow still falling over the most parts of the inland. The skies cleared in the night and the combination of clear skies, snow on the ground and strong cold advection caused extremely low temperature in the morning of the 20th December.