On the edge of a Western European trough, the Alps are caught in a strong southerly current. While it snows on the southern slopes of the Alps, at least at high altitudes, the Föhn dominates in the north. How does the Föhn work again? And what's next for the weather
Current situation
The last few days have been extremely windy in some foehn regions, with gusts of over 100 km/h, making it difficult for the few open ski resorts to operate lifts. In the southern Western Alps, on the other hand, PowderAlert is in effect, with new snow forecasts fluctuating by a meter or two every day. The general weather situation "Trough Western Europe" (TRW) is responsible for both. Here is a major weather situation classification if anyone wants to take a closer look at TRW and colleagues.
As the name suggests, there is a trough over Western Europe. The Alpine arc lies in front of the trough (east of the trough) in a strong flow from the south. The direction of flow results from the direction of rotation of the trough: In the satellite film from Monday, the low pressure vortex can be seen nicely, as well as the dusty clouds upwind of the Alpine arc and clearings downwind.
Storms can pick up a little extra moisture over the warm Mediterranean, which is then discharged in the südstau. The precipitation in the south is quite heavy, with the Fiorino station (west of Genoa), for example, recording over 500mm in 24 hours.
Hairdryer
On the other side of the Alps, the remaining snow is disappearing and if it wasn't so windy, you could sunbathe here and there. Föhn winds can develop wherever air masses meet a mountain range more or less at right angles, as in the current case from the south to the Alps, which run more or less from west to east. Foehn is not a downslope wind in the usual sense, such as glacier winds or downslope winds (katabatic winds), which are caused by gravity (cold air is heavier and wants to go down), but rather an overflow effect, such as a stream flowing over rocks. This creates waves and other turbulence that can be felt quite some distance after the stone or the mountain.
There are various foehn theories that describe mathematically and conceptually how, where and why the air rises or flows over the mountain and how, where and why it comes down again on the other side. The most widespread concept in German-speaking countries is that of the so-called "Swiss Föhn". Here it is assumed that precipitation always occurs upwind. The air then flows downwind into the valley and warms up dryadiabatically, i.e. it becomes warmer than it was upwind. This is the thermodynamic foehn theory as found in almost all German weather books.
However, precipitation does not always occur upwind and the high wind speeds downwind cannot really be explained in purely thermodynamic terms. An alternative theory is the hydraulic "Austrian Föhn". In this case, the air upwind does not rise up the mountain, but remains there. Only the higher layers of air flow over the remaining lower air and then over the mountain. The foehn air is treated here like a shallow water flow, i.e. a single, rather shallow layer that can be described using the shallow water equation. (Animated simulation with shallow water equation: Five drops of water fall into a small bathtub and trigger waves that are thrown back at the edges of the tub).
There are also various ideas as to why exactly the foehn air descends into the valley downwind. An overview can be found here, or here. The idea that the hairdryer is eating the snow is enough for skiing.
Outlook
The foehn-like southerly current is weakening, although it will continue a little further in the usual foehn corridors. In the south of the Western Alps, it will continue to rain or snow heavily at high altitudes. The TRW will gradually be cut off and the high pressure areas on its sides will form a bridge to the north of the trough remnant. This will result in a high over low situation in which colder air can slowly penetrate towards the Alps from the east. The end of the Föhn phase is likely to be reached on Friday, it will become cooler and more unstable again on the northern slopes of the Alps. The exact development is still very uncertain and we prefer not to lean out of the meteorological window for now.
Finally...
...a brief look at the Arctic: the sea ice extent remains extremely low. Incidentally, it doesn't look particularly icy in the Antarctic this year either. Here you can play with an interactive illustration.
This video shows the development of sea ice in the Arctic up to October 2016, not only in terms of extent, but also the age of the ice. Logically, the older the ice, the more summers it has survived and the thicker it tends to be. The brighter the white in the video, the older the ice:
