Bernese Oberland
Lenk
Avalanche Problems
New snow + Persistent weak layers
3+
Field notes
New snow
Easy to spot - the snow is fresh. Look at recent avalanche activity in similar terrain, and pay attention to how much new snow has fallen in the last three days. Critical loading depends on temperature, wind, and what was on the surface before the storm. SLF advises waiting until the Snow deposited on the ground in a multiplicity of layers.EAWS glossary has had time to bond before committing to steep terrain in fresh snow.
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
Since Sunday evening there has been widespread precipitation, heavy in the Lower Valais and on the western part of the northern flank of the Alps. The centre of precipitation will shift to the central and eastern parts of the northern flank of the Alps on Wednesday. This week’s new and drifted snow was deposited above approximately 1800 m on a sometimes unfavourable old snow surface and is easily triggered. With the northerly winds, more wind slabs will develop on Wednesday, especially at high altitudes and in the south.
Deeper in the Snow deposited on the ground in a multiplicity of layers.EAWS glossary, there are faceted 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 that are prone to triggering, especially on shady slopes above approximately 2400 m and generally in the Areas above approximately 3000 m (particularly glaciated areas).EAWS glossary. Some avalanches may also be triggered in these deeper layers.
Weather review for Tuesday
It was very cloudy on the Main Alpine Ridge and north of it, with heavy precipitation in places. The snowfall level dropped to around 1000 m. South of the Main Alpine Ridge, only a little snow fell above around 800 m overnight to Tuesday, and during the day it was quite sunny with northerly winds.
Fresh snow
From Monday evening to Tuesday afternoon, the following amounts of fresh snow fell above approximately 1400 m:
- Extreme west and northern Lower Valais, Vaud Alps: 30 to 50 cm
- Other western parts and the central part of the northern flank of the Alps, rest of Lower Valais: 20 to 30 cm
- Elsewhere: a widespread 10 to 20 cm, less in the south.
In total, the following amounts of new snow fell above approximately 2000 m from Sunday evening to Tuesday afternoon:
- Extreme west and northern Lower Valais: 60 to 100 cm
- Neighbouring extreme west of Lower Valais, Vaud Alps, western Bernese Oberland: 40 to 60 cm
- Other regions on the western part of the northern flank of the Alps: 20 to 40 cm
- Jura, remaining northern flank of the Alps, northern Upper Valais and widespread areas of Grisons: 10 to 20 cm, locally up to 30 cm
- Elsewhere: a few centimetres.
Temperature
In the middle of the day at 2000 m, between -6 °C in the west and north and -3 °C in the south
Wind
- Mostly moderate from westerly directions
- Moderate northerly wind in the south during the day
Weather forecast to Wednesday
In the north, it will be very cloudy and snow will fall up to around 500 m. Bright spells are possible in the inneralpine regions. On the southern flank of the Alps, it will be mostly sunny, with moderate to strong northerly winds extending down to the valleys.
Fresh snow
Until Wednesday afternoon, the following amounts will fall above around 800 m:
- Northern flank of the Alps, northern Lower Valais: 20 to 30 cm, and up to 40 cm on the central and eastern parts of the northern flank of the Alps
- Extreme west of Lower Valais, northern Grisons: 10 to 20 cm
- Elsewhere 5 to 10 cm, dry in the south
Temperature
At midday at 2000 m, between -8 °C in the north and -4 °C in the south
Wind
- Fresh and moderate to strong in the Areas above approximately 3000 m (particularly glaciated areas).EAWS glossary, elsewhere light to moderate from the north
- Moderate to strong northerly wind on the southern flank of the Alps and a sometimes strong foehn wind from the north extending into the valleys
Outlook
During Wednesday night into Thursday, a little snow will continue to fall in the northeast down to low altitudes. On Thursday and Friday, it will be mostly sunny in the mountains during the day. There will be a moderate northeasterly wind on Thursday. On Friday, the wind will ease and the zero-degree level will rise to around 2000 m.
The danger of dry avalanches will decrease rapidly in the regions exposed to heavier precipitation and gradually in other regions. As a result of solar radiation, more loose snow avalanches arising from the new snow are to be expected on Thursday.