Upper Engadine and Silvretta
Sur Tasna
Avalanche Problems
Persistent weak layers
1
Field notes
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 westernmost and northern Lower Valais, there is somewhat more snow at high altitudes than is usual at this time of year. Elsewhere, the snow depths are below average and in the south they are well below average. On south-facing slopes and generally below 2400 m, the Snow deposited on the ground in a multiplicity of layers.EAWS glossary is mostly wet or crusted.
On the Main Alpine Ridge and to the south of it, the fresh snow and wind slabs of the past two days are lying on a thin but heavily faceted Snow layers deposited from earlier precipitation, prior to fresh fallen snow. Old snow layers consist of metamorphosed snow crystals.EAWS glossary at high altitudes.
North of the Main Alpine Ridge, there was little transportable snow, meaning that despite strong southerly winds, only small wind slabs were created in some localities. Primarily on northern and eastern slopes above approximately 2400 m, the Snow layers deposited from earlier precipitation, prior to fresh fallen snow. Old snow layers consist of metamorphosed snow crystals.EAWS glossary contains some 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, particularly in the inneralpine regions of Valais and throughout Grisons, but hardly any avalanches have been reported for days.
Weather review for Wednesday
In the north, a few flakes of snow fell locally during the night. As the day progressed, it became partly sunny from the west. In the south, it was very cloudy and it snowed, especially during the night. The snowfall level was mostly between 1200 and 1500 m.
Fresh snow
From Tuesday morning to Wednesday morning, the following amounts fell above approximately 2000 m:
- Ticino and Moesano: 15 to 20 cm, with up to 30 cm in the Valle Maggia
- Rest of the Main Alpine Ridge: 5 to 15 cm
- Elsewhere mainly dry
Temperature
At midday at 2000 m, around 0 °C
Wind
- Moderate southerly wind in the north during the night
- Otherwise light
Weather forecast to Thursday
It will be very cloudy in the south and a little snow will fall above 1600 m overnight to Thursday. It will be sunny in all other regions.
Fresh snow
Up to 5 centimetres in the south
Temperature
At midday at 2000 m, between +3 °C in the north and -1 °C in the south
Wind
- Light, becoming a moderate southwesterly wind in the west and in the Areas above approximately 3000 m (particularly glaciated areas).EAWS glossary as the day progresses
- A foehn wind will develop in the north during the course of the day
Outlook until Saturday
On Friday conditions will be mostly sunny in the mountains. There will be moderate and locally strong southwesterly winds in the west and foehn winds in the valleys of the north. On Saturday it will be mostly sunny at first, then clouds will gather from the west. There will be light winds.
The avalanche danger will decrease, but only slowly in Valais, Grisons and the south due to the weak structure of the Snow deposited on the ground in a multiplicity of layers.EAWS glossary.