PG reader Randy-Random obviously needs to catch up on the subject of polar vortex splits and a revision lesson at the beginning of the year was always a good idea at school, so we'll confidently leave the fuss about individual snowflakes on individual mountains to our colleague Oracle and look at the bigger picture again.
As our planet is slightly tilted on its axis, it is not only cold but also dark at the poles in winter. Of course, only at the respective winter pole. (Meanwhile, it is cold and bright at the other pole.) At the moment, it is winter at the North Pole and the radiation balance is particularly negative. This means that it is getting colder and colder because the sun is not shining. Cold air is denser than warm air, which is why a lot of air is currently flowing towards the ground at the North Pole. This causes the air pressure on the ground to rise, while it sinks at altitude. The result is a so-called high-altitude low, also known as a polar vortex. In contrast to dynamic lows, which are caused by air mass movements, the polar vortex is a thermal low, i.e. one that is formed by temperature differences. Like all other lows, the polar vortex rotates anti-clockwise in the northern hemisphere (viewed from above). The southern polar vortex, i.e. the one over Antarctica, usually looks really quite swirly and round, as the temperature contrasts are sharper due to the land-water arrangement there and the polar vortex is more or less enclosed by extremely pronounced westerly winds. In the north, on the other hand, several low-pressure centers often form, wafting around in a loose network. There is often a center over Baffin Island and one over northeast Siberia. This low-pressure formation over the polar region is a kind of motor for our winter weather. As it rotates anti-clockwise, "the weather" moves from west to east. (Prize question: Which way does "the weather" move in the southern hemisphere?) The closer the pressure centers are to each other, or the rounder the polar vortex is, the straighter the west-east flow. An intact, strong polar vortex therefore tends to provide us with rapidly changing, Atlantic-influenced weather. If a strong wedge forms somewhere and transports warm air far to the north, the polar vortex can be disrupted or even split into two completely separate centers. This upsets the entire circulation because the usual west-east highway is suddenly diverted and snakes around. A polar vortex split therefore often brings us meriodonal weather patterns with a strong tendency to persist. When there are demonstrations on the Inntal highway because of the new toll sticker and you have to take winding detours, you can't get anywhere either.
This map from today (Wednesday, 4.12.13) shows the pressure distribution over the northern hemisphere from a Eurocentric bird's eye view. (The North Pole is in the middle, Europe is at the bottom, Asia is largely upside down). The polar vortex is a large purple blob and in our area the current runs from west to east without any major waves.
In the forecast for Friday, a wedge is pushing northwards over the Atlantic, while a trough is spreading over Central Europe, which will hopefully provide significant fresh snow in areas of northern congestion, but as I said, we'll leave that to the oracle.
Another week later, on Friday, December 13, at least the ECMWF model sees a decent split. The wedge has cut through the polar vortex and shifted to the east. This would block the highway and it would remain dry, relatively warm and sunny in the Alps on a quasi-permanent basis. This map is particularly unattractive in this respect, but the general trend after the weekend is pretty clearly in this direction.
