Upper Engadine and Silvretta
Sur Tasna
Avalanche Problems
Wind slab + Persistent weak layers
2-
Field notes
Wind slab
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.
Persistent weak layers
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 are very challenging to recognise. 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 and shooting cracks are typical when present, but you can have a serious weak layer with no warning signs at the surface. Knowledge of how the Snow deposited on the ground in a multiplicity of layers.EAWS glossary has evolved is essential. SLF's travel advice is direct: travel conservatively, avoid terrain where the consequences of being caught are large, and treat the history of weather and snow as more important than what you can see today. Read the full bulletin.
Snowpack & Weather
Snowpack
In most areas, there is less snow than is usual at this time of year. Only in northern Lower Valais are the snow depths about average. On southern slopes and generally below 2400 m, the Snow deposited on the ground in a multiplicity of layers.EAWS glossary is mostly wet or has frozen solid.
On the Main Alpine Ridge and to the south of it, this week’s fresh snow overlays a thin but weak Snow layers deposited from earlier precipitation, prior to fresh fallen snow. Old snow layers consist of metamorphosed snow crystals.EAWS glossary 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 at altitude.
Particularly in the inneralpine regions of Valais and throughout Grisons, there are sometimes 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 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 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 in the Snow deposited on the ground in a multiplicity of layers.EAWS glossary, especially on northern and eastern slopes above approximately 2400 m. However, hardly any avalanches have been triggered in these layers for some days.
Further north, the structure of the Snow deposited on the ground in a multiplicity of layers.EAWS glossary is more favourable.
Despite the strong southerly wind, only small wind slabs will be encountered in some localities, as there is hardly any transportable snow left.
Weather review for Saturday
It was quite sunny in the north and in the Engadine, but often cloudy in the south and on the Main Alpine Ridge in Valais.
Fresh snow
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Temperature
At midday at 2000 m, between +3 °C in the north and 0 °C in the south
Wind
Light to moderate from southerly directions
Weather forecast to Sunday
It will be quite sunny in the mountains in the north and in the Engadine. It will be cloudy in the south. A little snow will fall on the Main Alpine Ridge in Valais and in western Ticino. The snowfall level will be around 1500 m.
Fresh snow
From Saturday evening to Sunday afternoon above approximately 1800 m:
- Main Alpine Ridge in Valais, upper Valle Maggia, Val Bedretto: 5 to 10 cm
- Elsewhere, a few centimetres locally and dry to the north of the Main Alpine Ridge
Temperature
At midday at 2000 m, between +4 °C in the north and -1 °C in the south
Wind
- Moderate to strong from the south
- A sometimes strong foehn wind in the alpine valleys of the north
Outlook
On Monday and Tuesday, it will be partly sunny and mild in the north. The southerly wind will still be moderate on Monday and mostly light on Tuesday. In the south, a little snow will fall above approximately 1300 m until Tuesday morning. On the western part of the Main Alpine Ridge and in western Ticino, 5 to 10 cm of fresh snow is possible.
The avalanche danger may increase slightly in the south with arrival of new snow. There will be little change in other regions.