It is basically the result of a process called stratospheric warming, where a massive bubble of warm air over the Pacific made it's way up to the stratosphere 40 km up, and was replaced by cold air.

The result was that the global temperature dropped by one deg C in only 8 days. This has given way to the high pressure to establish itself over Russia and now Central Europe, pushing this cold air all over the place.

The story on the stratospheric warming can be found at [link to wattsupwiththat.com]