The north of Germany is facing an explosive border weather situation at the weekend that has not existed for a long time. An air mass boundary exactly above Germany separates the icy polar air in the north from the very mild spring air in the south.
Where the air masses meet, there can be heavy precipitation: In the northern half of the permafrost on Sunday large amounts of snow are expected, while it is raining in southern Germany.
In the Berlin-Brandenburg region, 5 to 20 centimeters of snow are possible from Sunday to Monday, locally even up to 40 centimeters to the southwest. Where cold and warm air meet, there is always a lot of wind, which means that a full-blown snowstorm can occur at freezing minus five degrees.
It is still unclear whether the snowfall area will spread between North Rhine-Westphalia and East Germany or from the Münsterland over the Hanover area to Saxony. And then at least a week of permafrost follows.
This year the polar vortex is unusually unstable
The prerequisites for such a cold air ingress were already in place at the beginning of the year. The background is an unusually unstable polar vortex this year. The huge depression that circulates in the stratosphere high above the Arctic in winter collapsed for the first time in January.
As a result, the jet stream, a band of strong winds in the atmosphere, also became unstable. It begins to lurch and so cold air can penetrate far to the south. A stable polar vortex, on the other hand, usually creates a strong jet stream that holds the cold air together over the Arctic and thus clears the way for warmer air masses from the Atlantic to Europe.
The climate physicist Marlene Kretschmer from the University of Reading in Great Britain told Tagesspiegel that the ingress of cold air, as far as it can currently be assessed, is related to the state of the polar vortex. “We know that the probabilities of such weather conditions increase very strongly when the polar vortex is weak,” said Kretschmer.
After the collapse of the polar vortex at the beginning of January, such an event occurred twice. “After a short recovery, the vortex became weak again, these weak phases favor weather conditions as we are currently seeing.”
The vortex split in January
In January the eddy was split up, a so-called split event that some meteorologists and scientists are now expecting again. Marlene Kretschmer is currently assuming a shift in the eddy, but the effects on the weather are already similar.
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The collapse of the polar vortex is accompanied by sudden warming in the stratosphere at an altitude of 10 to 50 kilometers – a so-called major warming, which is characterized by easterly winds at high altitudes. On the ground, this increases the likelihood that icy polar air will travel from the Arctic to the south.
Such situations are observed around seven times in ten years; in extreme cases, such as 2013, there may be severe frosts in Germany until April, including permafrost.
But the consequences of the event could also hit Scandinavia and regions east of it more severely. There are many other drivers of the weather in Western Europe. “In Germany, the effect of weak polar vortex phases varies greatly from event to event,” says Kretschmer.
The effects can last up to two months
Extreme processes in the stratosphere are only of relatively short duration, but they can influence our weather for several weeks. The effects from the stratosphere can last up to two months.
The climate physicist Kretschmer is also fundamentally concerned with the question of whether global warming could contribute to an accumulation of extreme winters in temperate latitudes. To what extent climate change plays a role in the current event is difficult to assess.
Some experts now assume that the polar vortex will no longer recover this winter, which would allow cold waves into spring. However, Marlene Kretschmer thinks it is too early for such statements. At present, the data would not provide that. So everything is still open.
