WeatherBlog 13 2017/18 | Outlook, Polar vortex, Miscellaneous

There was some fresh snow yesterday, and there is still some today, especially on the eastern southern slopes of the Alps. It could be quite a nice refresher there, but in all likelihood it will remain below alarm levels. In the north, it was relatively warm in typical foehn snow drifts and windy at least in the summit areas.

In the course of today, Wednesday, the unproductive precipitation in the east will also spread to the northern Alps and it will remain rather cloudy until around noon on Thursday before it clears up again. Friday looks fairly sunny, then more unsettled again at the weekend. More unsettled in the sense of a few clouds, not in the sense of exciting precipitation or similar.

The polar vortex

We haven't taken a closer look at what the polar vortex is up to for a while, so here's a quick recap: In contrast to dynamic lows, which are formed by air mass movements, the polar vortex is a thermal low, i.e. one that is formed purely by temperature differences. In winter, it is not only dark at the North Pole, but also cold. The cold air sinks to the ground and a high-altitude low forms: the polar vortex.

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500hPa geopotential and surface pressure from Tuesday 6.2. A small low pressure trough near Spain will provide a foehn-like flow and some fresh snow in the south. Remnants of the low pressure trough to the north. — 500hPa geopotential and surface pressure from Tuesday 6.2. Small low pressure area near Spain will provide a foehn-like flow and some fresh snow in the south. Remnants of the low pressure trough to the north.

meteociel.fr

The polar vortex, like all other lows, rotates anti-clockwise in the northern hemisphere. At the transition between the cold, polar air masses in the north and the milder air masses in the mid-latitudes, the strong pressure differences create a powerful high-altitude current, the so-called jet stream.

The winter low pressure formation over the polar region and the jet stream blowing around it are a kind of engine for our winter weather. The jet stream runs from west to east due to the direction of rotation of the low. Accordingly, the weather comes from the west and moves eastwards. (Prize question: What is it like in the southern hemisphere?)

A "normal" polar vortex is more or less round, whereby there are often two or more pressure centers, especially in the northern version due to the irregular land-water distribution. If there are two (or more) such centers that are completely separated, this is referred to as a polar vortex split. For some time now, the polar vortex has been quite "disturbed", i.e. very little round, with a center in the eastern half of the North American continent or the western Atlantic and a changing structure of other low pressure centers "opposite" in the direction of eastern Siberia. The American low pressure center and the very cold air it contains is the reason for the westerly weather that has prevailed in the Alpine region recently. A possible polar vortex split is now emerging in the weather models' crystal ball, in which the aforementioned dipole pattern would intensify again, possibly even with a major warming. More on this next week, if it becomes more concrete.

In the end, there were always windows of sunshine and clouds. — Recently, windows of sunshine, clouds again and again.

LH

In other news

February 5 was World Meteorologists' Day. Congratulations to all colleagues. World Meteorology Day is on March 27, so these are two different celebrations. February 5 was the birthday of John Jeffries (1745-1819), a weather pioneer from the USA. Among other things, he made a name for himself in the still very young field of meteorology by being the first to record the changes in temperature, air pressure and humidity with altitude using a hot air balloon. This happened during a flight over the English Channel with the pioneer of hot-air ballooning Jean-Pierre Blanchard. They allegedly had to drop ballast to avoid landing in the water, which is why they threw their pants overboard.

Last Friday (2 February) was Candlemas, better known as Groundhog Day. Groundhog Punxsutawney Phil saw his shadow on this occasion, ergo it remains six more weeks of winter. Wiarton Willie, an albino marmot from Canada, agrees with Phil. Staten Island Chuck and Pierre C. Shadeaux, the latter a nutria (South American coypu), however, did not see their shadows. So it's possible that spring is coming early in some areas after all.

Finally, here's Punxsutawney Phil's important forecast as a video: