Prättigau and Davos
Südliches Prättigau
Avalanche Problems
Wind slab + Persistent weak layers
3-
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
On Monday with milder temperatures, new snow and drifted snow were deposited above approximately 1800 m on the still cold and often loose snow that fell last week. Along with often strong westerly winds, more snow will fall on Tuesday, especially in the west and north. During this precipitation spell, the fresh and drifted snow is likely to be easily triggered.
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 release in these deeper layers.
Weather review for Monday
Conditions were mostly very cloudy. On Sunday evening, precipitation set in in the west and north. This was most intense in the extreme west of Lower Valais and along the Prealps, but the volumes were mostly below those forecast.
Fresh snow
The snowfall level was between 1600 and 1800 m in the west, 1200 to 1400 m in the east and below 1000 m in the south. The following amounts of snow fell above 2000 m:
- Extreme west of Lower Valais from Val d'Iliez to Val Ferret: 30 to 50 cm, and locally more;
- Neighbouring western and northern Lower Valais, northern Upper Valais into the Aletsch region, western Bernese Oberland: 15 to 30 cm
- Remaining northern flank of the Alps, northern Prättigau, central part of the main Alpine ridge: 5 to 15 cm
- Elsewhere less or dry
Temperature
At midday at 2000 m, between 0 °C in the north and -5 °C in the south
Wind
Moderate to strong southwesterly wind at high altitudes
Weather forecast to Tuesday
On the Main Alpine Ridge and north of it, it will be mostly very cloudy and further snow will fall, sometimes persistently so in the extreme west of Lower Valais and on the northern flank of the Alps. The snowfall level will drop to around 500 m.
South of the Main Alpine Ridge there will continue to be precipitation during the night, but during the day it will clear up with northerly winds.
Fresh snow
Until Tuesday afternoon, the following amounts will fall above approximately 1000 m:
- Westernmost Lower Valais, Vaud Alps: 30 to 50 cm
- Remaining western and central parts of the northern flank of the Alps, Jura: 20 to 40 cm
- Eastern part of the northern flank of the Alps, the rest of Lower Valais, Prättigau, the Main Alpine Ridge from Val Bregaglia to Val Poschiavo: 15 to 30 cm
- Less elsewhere
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
- Moderate to strong southwesterly wind on the northern flank of the Alps
- Elsewhere, moderate to strong wind at high altitudes, shifting from southwest to northwest
- On the southern flank of the Alps, increasingly moderate to strong foehn wind from the north extending into the valleys
Outlook
Wednesday
In the north, snow will continue to fall in showers to below 1000 m, mostly from the eastern Bernese Oberland to the Alpstein region, where 15 to 30 cm is expected. Bright spells are possible, especially in inneralpine regions. On the southern flank of the Alps, it will be mostly sunny, with sometimes strong northerly winds extending down to the valleys.
The avalanche danger will decrease gradually in the west, but will increase slightly in the north and east. It will not change significantly in the south.
Thursday
The precipitation will end during the night into Thursday. During the day it will be quite sunny in the mountains in the north and mostly sunny in the south. The northerly wind will decrease appreciably.
The avalanche danger will decrease slowly.