It seems that this is directly caused by the arctic temperature rise: https://en.wikipedia.org/wiki/Sudden_stratospheric_warming

Relevant quote: Following a sudden stratospheric warming, the high altitude winds reverse to flow eastward instead of their usual westward. The eastward winds progress down through the atmosphere and weaken the jet stream, often giving easterly winds near the surface and resulting in dramatic reductions in temperature in Europe.

see also (in german): http://www.meteoschweiz.admin.ch/home/aktuell/meteoschweiz-b...