Upper Engadine and Silvretta
Sur Tasna
Day Risk Profile
Avalanche Problems
Persistent weak layers
3=
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 the south, the fresh and drifted snow from the past few days has been deposited on a weak Snow deposited on the ground in a multiplicity of layers.EAWS glossary. The northerly wind has transported the new snow. Avalanches can be triggered very easily in the Snow layers deposited from earlier precipitation, prior to fresh fallen snow. Old snow layers consist of metamorphosed snow crystals.EAWS glossary over a wide area. Remote triggering and naturally triggered avalanches are possible. Avalanches are also to be expected deeper in the snowpack in the inneralpine regions of Valais and Grisons, especially on northern and eastern slopes.
North of a line from the Rhône to the Rhine, strong southerly winds on Tuesday produced snowdrift accumulations, some of which are prone to triggering. Some fresh snow will cover these avalanche-prone locations and the westerly winds will lead to more snowdrift accumulations at higher altitudes. 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 deep in the Snow deposited on the ground in a multiplicity of layers.EAWS glossary are less prone to triggering in these regions and avalanche-prone locations are rarer than in other regions.
Weather review for Wednesday
In the north, it was mostly very cloudy, with precipitation falling mainly in the Jura. The snowfall level dropped from 1500 m to around 800 m. It was overcast in the south. There was persistent precipitation along the Main Alpine Ridge and to the south of it. The snowfall level rose from 800 m to around 1200 m.
Fresh snow
From the beginning of the precipitation on Tuesday afternoon until Wednesday afternoon, the following amounts fell above approximately 1400 m:
- Main Alpine Ridge from Val Bregaglia to Bernina Pass, central and southern Ticino, Moesano: 15 to 30 cm
- Jura, rest of the Main Alpine Ridge from the Great St Bernard Pass to the Gotthard region, northern Ticino: 10 to 20 cm
- Elsewhere a widespread 5 to 10 cm
- Largely dry from the Bernese Oberland to Liechtenstein, northern Grisons, Lower Engadine
Temperature
At midday at 2000 m, between -4 °C in the west and south and -1 °C in the east.
Wind
- Strong southerly winds overnight to Wednesday, at times strong to storm force on the Northern Alpine Ridge
- The wind died down during the day, becoming a mostly light to moderate southerly
Weather forecast to Thursday
There will be widespread precipitation during Wednesday night. In the south, the precipitation will end during the second half of the night, while in the north it will continue until the late morning. As the day progresses, it will brighten rapidly in the west and south. In the east, it will remain cloudy with light snowfall. The snowfall level will be around 1000 m in the south and will drop to around 600 m in the north.
Fresh snow
From Wednesday evening to Thursday afternoon, the following amounts will fall above 1000 m:
- Northern flank of the Alps, northern Grisons: 10 to 20 cm
- Elsewhere a widespread 5 to 10 cm, less on the southern flank of the Alps
Temperature
At midday at 2000 m, around -6 °C in the north and -3 °C in the south
Wind
- In the north, moderate to strong westerly winds at times during the night, with light to moderate westerly to northwesterly winds during the day
- Light to moderate northerly wind in the south, with a brief but strong northerly foehn wind
Outlook to Saturday
In the west and north, it will be variably cloudy and some precipitation will fall at times. The snowfall level will be around 800 m. Between 5 and 15 cm of snow is expected to fall by Saturday afternoon, with 20 cm in westernmost Lower Valais on the border with France and in western Jura. Mostly sunny in the south and south-east but only partly sunny in Ticino on Saturday. There will be a moderate and occasionally strong westerly wind in the north on Friday, but elsewhere the wind will be mostly light to moderate.
The avalanche danger may increase slightly, especially in the west. Elsewhere it will hardly change. Even in the south, the avalanche danger is only slowly decreasing due to the weak Snow layers deposited from earlier precipitation, prior to fresh fallen snow. Old snow layers consist of metamorphosed snow crystals.EAWS glossary and the situation for off-piste snow sports remains critical.