The world of winter sports fans is famously divided into two camps: season ticket holders and road trippers. The former usually live in relative proximity to this or that ski resort and are happy to indulge in the occasional boredom of a fixed commitment, while the latter are flighty and faithless - always on the lookout for the perfect spot with the perfect conditions. Last week, the weather rightly caused excitement in both groups. The season ticket holders of the Northern Alps were watching the skies with their fingernails crossed, hoping that the predicted northerly storm would finally arrive. Meanwhile, the road trippers tried their hand at logistical masterpieces and effective cost-benefit compromises between driving time through the Alps and the amount of fresh snow on both sides.
The jet makes the weather
In the mid-latitudes of the northern hemisphere, in other words the Alps, the weather is largely determined by the position of the jet stream: A band of strong winds in the upper atmosphere that forms at the air mass boundary between polar cold air and warm subtropical air. This band does not run evenly around the globe, but in a more or less irregular wave motion, in the direction of the Earth's rotation (west to east). In the resulting troughs filled with cold air, low-pressure areas and frontal systems embedded in them form. The direction of flow, the direction from which the snow clouds hit the mountains, is always decisive for the recently so interesting accumulations. If we imagine the trough as a U-shape over the Alps, the air flows along it, first to the south, then to the east and finally to the north. For example, in areas on the front side of the trough (right side of the U), there is congestion on the south side of the Alps (air flows up the U "up") and this is often accompanied by Föhn in the north. Rapidly successive, opposing weather conditions that have such a positive effect on the snow situation everywhere as the events of the last few days are rather rare. Usually only one side gets something, while the other goes away empty-handed and at best looks enviously over the main Alpine ridge while the Föhn rages. So what has happened? A look at the weather maps tells us more.
The accumulated precipitation of the last week
The picture shows the reason why MSP and TGR turned simultaneously in Alagna from the weekend of February 7 and 8: Western Italy lies at the front of the trough in a strong southerly flow.
Three days later, in the map from Tuesday, February 10, we can see how this low pressure system has moved eastwards: it now lies over Greece and Turkey. Meanwhile, a new low pressure system has moved from the northern Atlantic down to the southwest over France. Our U has lost its shape a little, or rather we have two merging Us. The Alps are clearly in a westerly flow (air flows to the east). It is now snowing heavily in corresponding accumulation areas, such as small, unnamed treeskiing areas in Valais.
This new low then develops more and more to the south as it moves further eastwards over a large area. The Alps slowly move to the back of the trough, or the left side of the U and into a northerly flow (air flows down the U). The northern congestion is here!
Snowy winter in the south
This winter, the south is shining with amounts of snow that have not been seen for decades. A series of Mediterranean lows and weather conditions in which the aforementioned U comes to rest relatively far to the southwest are bringing one monster dump after another to the ski resorts from western Italy to the Karawanken. In the record winter of 2005/2006, on the other hand, there was a seemingly endless series of north-westerlies and the northern side of the Alps was covered in snow. How such large-scale patterns come about is largely unclear. What is certain, however, is that the chances of snow are better, at least within certain limits, when there is already snow. This makes it easier for a thick inversion layer to form, especially in the valleys. If moist air from a warm front slides up on top of this, it not only snows further down, but usually also significantly more. Apart from that, enough theory, the weather will do what it wants anyway. The next trough with great potential for northern congestion is just around the corner and it's time to use your season tickets or go on a road trip. If you don't come at the right time, you'll have to ride what's left!
