Central Prealps
Stoos
Day Risk Profile
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 the south, the fresh and drifted snow is being deposited on an unfavourable Snow deposited on the ground in a multiplicity of layers.EAWS glossary. Avalanches can be triggered not only in the fresh snow, but also in deeper layers of the snowpack. Avalanches can also still be triggered deeper in the snowpack in many areas in the inneralpine regions of Valais and Grisons.
North of a line from the Rhone to the Rhine, strong southerly winds are causing snowdrift accumulations, some of which are prone to triggering. In the west, these are growing larger with the fresh snowfall. 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 deeper in the Snow layers deposited from earlier precipitation, prior to fresh fallen snow. Old snow layers consist of metamorphosed snow crystals.EAWS glossary are somewhat rarer and less prone to triggering in these regions.
Weather review for Tuesday
After a mostly clear night, clouds rapidly gathered in the west. In the east and south, conditions remained very sunny into the afternoon.
Fresh snow
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Temperature
At midday at 2000 m, between +4°C in the north and -3°C in the south
Wind
- Moderate to strong southwesterly in the north overnight, elsewhere light to moderate
- During the day, widespread moderate to strong southerly at high altitudes; moderate foehn winds at times in the valleys of the north
Weather forecast to Wednesday
Snow will fall on the Main Alpine Ridge and south of there. The snowfall level will drop briefly to 500 m and will be at between 800 and 1000 m during the day. The north will see intermittent light precipitation, which will be more persistent in western Jura. Regions exposed to the foehn wind in central and eastern Switzerland will remain mostly dry. In the north the snowfall level will rise briefly to 1500 m in the Jura and will be around 800 m in all regions during the day.
Fresh snow
From Tuesday evening to Wednesday afternoon, above approximately 1400 m:
- Main Alpine ridge from Val Bregaglia to the Bernina Pass, Sotto Ceneri: 15 to 30 cm
- western Jura, rest of the Main Alpine Ridge from the Great St Bernard Pass to Upper Engadine, rest of Ticino: 10 to 20 cm
- otherwise a widespread 5 to 10 cm, and mostly dry from the Bernese Oberland to Liechtenstein and in Prättigau
Temperature
At midday at 2000 m, between -3°C in the west and south and 0°C in the east
Wind
- Strong southerly at night, with a strong foehn wind in the Alpine valleys in the north
- Dying down during the day, becoming mostly light to moderate southerly
Outlook
Thursday
Overnight to Thursday, 5 to 15 cm of snow will fall down to low altitudes in the north and in Grisons. During the day conditions will quickly brighten in the west and south, but will remain cloudy for longer in the east with light snowfall. The wind will be a light to moderate westerly to northerly. Avalanche risk will increase somewhat in the north.
Friday
Around 10 cm of snow will fall in the west. In the east conditions will become increasingly cloudy and there will be light snowfall at times. The snowfall level will be around 1000 m. In Grisons and Ticino, conditions will remain sunny for longer and dry into the evening. The wind will be a light to moderate westerly to northwesterly. There will be hardly any change in avalanche risk.