Gersau

The abundant fresh and drifted snow in the west and south is prone to triggering, and in many areas is also lying on a weak snowpackSnow deposited on the ground in a multiplicity of layers.EAWS glossary. In many places, the old snow surface had become faceted and loose before the fresh snowfall. In addition, there are distinct weak layersSnowpack layer in which the crystals are poorly bonded and the layer has the potential to fail. Typical crystals in dry weak layers causing avalanches are : precipitation particles; surface hoar; faceted crystals; hollow crystals; graupel. Generally >1mm in size, loose grains with few bonds Wet weak layers can be formed by all snow crystal/grain types.EAWS glossary that are prone to triggering in the middle and lower part of the snowpack, especially south of a line from the Rhône to the Rhine. As a result, fractures can propagate over long distances. Extensive avalanches may start in particular from catchment areas which have not seen any previous releases, taking the entire snowpack with them.

North of the line from the Rhône to the Rhine, snowpackSnow deposited on the ground in a multiplicity of layers.EAWS glossary structure is often somewhat more favourable, but there are weak layersSnowpack layer in which the crystals are poorly bonded and the layer has the potential to fail. Typical crystals in dry weak layers causing avalanches are : precipitation particles; surface hoar; faceted crystals; hollow crystals; graupel. Generally >1mm in size, loose grains with few bonds Wet weak layers can be formed by all snow crystal/grain types.EAWS glossary deeper in the old snowpackSnow layers deposited from earlier precipitation, prior to fresh fallen snow. Old snow layers consist of metamorphosed snow crystals.EAWS glossary in these regions too. In some places, avalanches can also be triggered in these deep layers.

Below approximately 1600 m, the snowpackSnow deposited on the ground in a multiplicity of layers.EAWS glossary is wet, especially in the north and west, due to the sometimes heavy rain.

In the west and north, conditions were overcast and there was heavy precipitation at times. The snowfall level in the extreme west of Lower Valais and on the northern flank of the Alps was between 1500 and 1800 m, elsewhere between 1000 and 1500 m, while there were sunny intervals in the south.

Fresh snow

From the onset of precipitation on Tuesday morning until Wednesday afternoon above 2200 m:

• northern and extreme west of Lower Valais: 50 to 70 cm
• western part of the northern flank of the Alps, rest of northern Valais and Lower Valais: 30 to 50 cm
• rest of the northern flank of the Alps, rest of Upper Valais, western Ticino: 15 to 30 cm, less elsewhere

Most of the fresh snow has fallen since Tuesday evening.

Temperature

At midday at 2000 m, between 0 °C in the north and -2 °C in the south

Wind

Moderate to strong from westerly directions in the north, light to moderate southerly in the south

In the north, conditions will be very cloudy, with heavy precipitation at times. During the night to Thursday, the snowfall level in the west and north will rise to 1800 m at times, but otherwise it will be between 1000 and 1400 m.

There will be only a small amount of precipitation in the south, with some bright spells.

Fresh snow

From midday on Wednesday to midday on Thursday above 2000 m:

• northern and extreme west of Lower Valais, Vaud Alps: 40 to 70 cm
• rest of Lower Valais, rest of northern Alpine ridge west of the Reuss: 30 to 40 cm
• rest of Valais, rest of northern flank of the Alps, rest of northern Alpine ridge, northern Prättigau: 15 to 30 cm
• elsewhere a widespread few centimetres

The peaks of western Jura will also see 15 to 30 cm of snowfall.

Temperature

At midday at 2000 m, around -2 °C

Wind

Veering from southwesterly to northwesterly

• storm-force in the west and north
• elsewhere mostly moderate to strong

Friday

From midday on Thursday into the night to Friday, heavy precipitation will continue to fall in the west and north. A further 20 to 40 cm of fresh snow is expected in Lower Valais, on the northern flank of the Alps and in northern Grisons. The snowfall level will be between 1000 and 1200 m. Precipitation will come to an end during the second half of the night and skies will clear. During the day there will be sunny intervals, with conditions being mostly sunny in the south and in Grisons. In the afternoon, cloud will move in again from the west. The night will continue to see storm-force westerly winds, which will die down during the day.

Avalanche danger will increase somewhat in some regions during the night to Friday and may reach danger level 4 (high) in further regions on the northern flank of the Alps and in northern Grisons. Very large avalanches are possible at times in Valais and on the northern flank of the Alps, these being capable of descending right down into the valleys and endangering exposed transportation routes. In Grisons, the risk relates primarily to alpine snow sports terrain. As the precipitation comes to an end, naturally occurring slab avalancheThe abrupt release of a slab of snow on a mountain slope. After crack initiation and crack propagation in a weak layer the released slab comprises 3 elements: The weak layer, the gliding horizon and the slab. If the slope angle is steep enough, the slab will glide down. If the slope is not steep enough, the slab will settle down on the failed weak layer. Possibly a whumpf sound will follow due to air pressed out of the weak layer. The fracture line will be sharp edged.EAWS glossary activity will decrease. Numerous loose snow avalanches are to be expected in areas with a lot of fresh snow.

Saturday

On Saturday, conditions will be very cloudy with a little widespread precipitation. Avalanche danger will decrease somewhat in the west and north but will not change significantly in other regions.