Snowpack

An exceptionally warm and moist air mass will reach the Swiss Alps on Wednesday and give rise to a very critical avalanche situation.
The strong to storm force westerly to northwesterly wind will transport large quantities of fresh snow at elevated altitudes and, in the regions exposed to heavier precipitation, large snow drift accumulations will form in areas that are not adjacent to ridgelines. The fresh snow and wind slab layers will be prone to triggering, in particular on west, north and east facing slopes. Dry avalanches will be able to penetrate the weak old snow cover. This applies in particular in the regions from central Valais through Ticino to Grisons.
As a consequence of the heavy rain, which will fall up to approximately 2800 m in the west from Wednesday morning and up to approximately 2400 m in the east in the course of Wednesday afternoon, the snowpack will become thoroughly wet. Numerous wet snow avalanches are to be expected, a growing number of which will be large. They will be capable of releasing the old snow.

Observed weather on Tuesday, 28.12.2021

It was mostly very cloudy and snow fell over a wide area. In the south and Upper Engadine the weather became brighter during the day and was partly sunny. The snowfall level during the night was approximately 1000 to 1300 m. As the day progressed it rose for a while in the west and north to approximately 2000 m. Otherwise it was approximately 1000 to 1400 m.

Fresh snow

From Monday afternoon until Tuesday afternoon the following amounts of snow fell above approximately 2000 m:

Lower Valais in the extreme west and north: 20 to 30 cm, on the border to France locally up to 40 cm
Rest of Valais, western part of the northern flank of the Alps, elevated altitudes in the western Jura, northern Prättigau: 10 to 20 cm
Other regions on the northern flank of the Alps, Gotthard region: 5 to 10 cm; elsewhere, a few centimetres

Temperature

At midday at 2000 m: about +1 °C in the north and about -3 °C in the south

Wind

During Monday night in the north, moderate to strong, otherwise light to moderate from the southwest to west
During the day in Valais and the north, moderate to strong, storm force at times, from the west, otherwise mostly light to moderate from the southwest

Weather forecast through Wednesday, 29.12.2021

On Tuesday night the precipitation will ease a little for a while. The snowfall level during the night will be approximately 1500 m. In the early hours of the morning, heavy precipitation will move in from the west. During the day in Valais and the north there will be heavy and persistent precipitation. The snowfall level will rise significantly, to approximately 2800 m in the west and approximately 2400 m in the east. In the inneralpine regions of Grisons it will be approximately 1300 m initially, before rising towards 2000 m in the afternoon.
In central and southern Ticino it will be predominantly dry and partly sunny.

Fresh snow

From Tuesday afternoon until Wednesday afternoon the following amounts of snow will fall above approximately 3000 m:

Western and central parts of the northern flank of the Alps: 40 to 70 cm
Rest of the northern flank of the Alps, rest of the Gotthard region, northern Grisons, northern Lower Engadine: 20 to 40 cm
Elsewhere: smaller quantities, none in the far south

All the precipitation will fall as rain below approximately 2400 m in the west and below approximately 2000 m in the east.

Temperature

At midday at 2000 m: between +5 °C in the west and 0 °C in the far east

Wind

During the night in the west and north, strong, otherwise light to moderate from the southwest to west
During the day at elevated altitudes over a wide area, strong to storm force from the west to northwest

Outlook through Friday, 31.12.2021

Thursday

On Wednesday night, precipitation will fall over a wide area except in the south. A further 20 to 40 cm of snow will fall in the north and east, and as much as 50 cm will fall in the regions on the northern flank of the Alps where orographic lifting occurs. Smaller amounts will fall elsewhere. The snowfall level will rise on Wednesday evening towards 2500 m, including in the east. During the day the east will be mostly cloudy, and the precipitation will cease as the day progresses. In the west the weather will become increasingly bright, and in the afternoon partly sunny. The south will be mostly sunny. It will be very mild in the mountains. The wind will be strong at elevated altitudes and strong to storm force on the main Alpine ridge, from the northwest.
The avalanche danger will decrease in the northwest as the day progresses, probably to level 3 (considerable). In southern Valais it is likely to remain level 4 (high). In the regions exposed to heavier precipitation on the northern flank of the Alps and in the east, the avalanche danger will be level 4 (high). Otherwise it will not change significantly.

Friday

After a clear night, New Year’s Eve will be sunny and exceptionally mild in the mountains. The northwesterly wind will ease to become moderate at elevated altitudes. The danger of dry avalanches will decrease. As a consequence of the mild weather and solar radiation, wet snow slides and avalanches are to be expected on south facing slopes that are interspersed with rocks.