“Avalanches in the mountains and floods in ski villages”: an atmospheric river will deliver a monthly amount of precipitation to the Alps within 48 hours | Science and the planet

Heavy rain is expected to fall in the French Alps, western Switzerland, Valais and the northern slopes of the Alps until Wednesday afternoon. Due to the very soft subtropical air flow, the snow line is often higher than 2000 metres. During the densest phases, the snowfall line will be between 2,200 and 2,400 metres, in the southwest it reaches 2,600 metres, falling to between 1,800 and 2,000 metres, during the quieter phases. This means that it will rain in most winter sports villages.

The culprit is the so-called atmospheric river. They are filaments (river-shaped) present in our atmosphere at an altitude of 1.5 to 2 kilometers, carrying masses of water vapor and can be thousands of kilometers long. In Western Europe, atmospheric rivers occur on average once every 10 days, although this does not mean that we should always expect heavy rain. When this ridge of moisture reaches the shore and encounters a mountain range, the moisture is pushed upward. This causes heavy rain, which may cause huge amounts of water and consequently floods or avalanches. This is what will happen in the coming days.

In the French Alps (especially Savoie and Haute-Savoie), French-speaking Switzerland and the Bernese Oberland, around 100 to 150 mm of rain is expected in the mountains, especially on slopes exposed to westerly winds. In certain places, especially in the West, as much rain can fall in 48 hours as would normally fall in an entire month. Rainfall of between 50 to 100 mm is expected on the northern slopes of the Central and Eastern Alps on Wednesday afternoon. Rain intensity will decrease starting Wednesday.

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Persistent heavy rains, intensified by melting snow, increase the risk of flooding. After weeks of heavy rain, the ground is already wet, increasing the risk of flooding, especially on the northern and western sides of the Alps at warning levels 3 and 4. Recently, rainfall and melting snow have caused river levels to rise significantly, with some areas falling. Temporarily at level 2 risk achieved. Lake Geneva is expected to reach danger level 2 on Tuesday.

Rivers such as the Isère are closely monitored and have a high chance of overflowing their banks. Local authorities also expect that the intensity and accumulation of rainfall will lead to spontaneous landslides. Above 2000-2500 m, precipitation falls as snow. Due to the unstable conditions, there is a high risk of avalanches here.

What does this mean for winter sports areas? Obviously, rain on snow is not a good sign. In many places the snow cover will become quite thin, melt and lose its quality. Fortunately, weather models show a colder phase from Wednesday and certainly Thursday, with snow showers again on the northern side. As of Saturday the weather appears to be settling down with drier weather. So it remains to be seen exactly how much damage the rain will cause.

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