Western Prealps
Vallée de Joux
Avalanche Problems
Wind slab
2-
Field notes
Look for fresh snow deposits on Slope facing downwind/ wind sheltered aspect. Snow deposits can attain several times average snow depth.EAWS glossary, in Usually a steep, elongated, eroded trench; typically prone to snowdrift accumulation.EAWS glossary and Rounded or elongated concavity; typically tending to accumulate snowdrift.EAWS glossary, and behind ridgelines and abrupt changes in terrain — these are where the wind has loaded the snow into slabs that are easy to trigger. Recent avalanches, shooting cracks under your weight, and Distinctive noise (resembling a “whumpf” sound) occurring when a weak layer beneath a slab collapses. The air of the weak layer is forced outside the snowpack, with the resulting whumpf sound being heard. The sound usually indicates an unstable situation and can be accompanied by surface cracking. A whumpf is a clear avalanche alert.EAWS glossary sounds are clear confirmations of the problem. The safest move is to avoid wind-loaded terrain entirely. NB wind signs alone don't always mean an avalanche problem exists, and the age of drifted snow is hard to judge.
Snowpack & Weather
Snowpack
Tuesday’s strong to storm-force southerly winds have formed widespread snowdrift accumulations. Ridgelines have often been blown away. In the south, the fresh snow was additionally deposited on top of a weak Snow deposited on the ground in a multiplicity of layers.EAWS glossary. Fresh snow and snowdrift accumulations are often lying on an unfavourable old snow surface of Snow grains, solid, with multiple flat glassy faces and sharp edges. Developed from faceting metamorphism, usually poorly bonded to one another (fewer contact points), a critical factor in avalanches if a faceted layer is covered with bonded snow. Typical grain size: 0.5 to 3 mm See also: www.snowcrystals.itEAWS glossary or snow-covered Transparent, frequently flat crystals forming on the snowpack when moist air settles on the surface. It forms most often during cold, clear, humid nights. Once it is blanketed by fresh fallen snow, surface hoar is one of the most critical weak layer. See also: www.snowcrystals.itEAWS glossary and are easily triggered in many places.
There are pronounced Snowpack 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 in the Snow deposited on the ground in a multiplicity of layers.EAWS glossary in southern Valais, Ticino and Grisons. At the weekend, human activity triggered lots of medium and sometimes large avalanches in the weak Snow layers deposited from earlier precipitation, prior to fresh fallen snow. Old snow layers consist of metamorphosed snow crystals.EAWS glossary in these regions, often from a distance. Snowpack structure is somewhat more favourable on the northern flank of the Alps and in northern Valais, but there are weak layers deeper in the snowpack in these regions too. These may still be triggered, especially where there is little snow and at transitions from a deep to shallow snowpack.
Weather review for Tuesday
Snow fell down to low altitudes on the Main Alpine Ridge and south of there. There were sunny intervals in the west and north.
Fresh snow
From Monday evening to Tuesday afternoon:
- from Val Bregaglia to the Bernina Pass: 15 to 25 cm
- central part of the southern flank of the Alps: 10 to 20 cm
- rest of the Main Alpine Ridge: 5 to 10 cm; elsewhere: less or dry.
Temperature
At midday at 2000 m, between -2°C in the north and -5°C in the south
Wind
- Strong to storm-force southerly; strong foehn wind in the Alpine valleys of the north
- Dropping somewhat in the afternoon
Weather forecast to Wednesday
Conditions will often be cloudy on Wednesday and a little snow will fall in the south. The snowfall level will be around 1000 m. There will be brighter intervals in the afternoon in the west.
Fresh snow
Main Alpine Ridge and south of there: a few centimetres;
Temperature
At midday at 2000 m, between -1°C in the north and -5°C in the south
Wind
Light in the south, otherwise moderate from southerly directions
Outlook
Thursday
Thursday will be very sunny. Winds will be light to moderate, strengthening from the south in the evening.
Avalanche risk will decrease somewhat in all regions. In southern Valais, Ticino and Grisons, however, it will decrease only very slowly due to the distinct and persistent Snowpack 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.
Friday
Precipitation will set in from the west overnight to Friday. During the day, conditions will be very cloudy and there will be occasional precipitation, falling as snow above approximately 1200 m. Around 20 to 30 cm will fall in the extreme west of Lower Valais. The wind will increasingly become a moderate southwesterly.
Avalanche risk will increase in the west with the fresh snow, but will otherwise not change significantly.