Prättigau and Davos
Schanfigg
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
After this week's snowfall, the snow depths in the north and west are well above average for the time of year. The fresh and drifted snow has been deposited on a sometimes unfavourable old snow surface. In addition, 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. In the regions exposed to heavier precipitation in the west, these layers have in some cases been thickly overlaid and in these places the snowpack is becoming increasingly stabilized. In regions which have seen less fresh snowfall, avalanches in these weak layers can still be triggered by human activity.
With northerly winds, Wednesday saw the formation of snowdrift accumulations which are prone to triggering, especially on the Main Alpine Ridge and in the south.
Weather review for Wednesday
In the north, conditions were very cloudy with showers. The snowfall level was between 500 and 700 m, there were clear intervals in the inneralpine regions and the southern flanks of the Alps were sunny with northerly winds.
Fresh snow
From Tuesday evening to Wednesday afternoon, above approximately 1000 m:
- northern flank of the Alps, northern Lower Valais: 20 to 40 cm
- eastern Jura, rest of Lower Valais, northern and central Grisons: 10 to 20 cm
- elsewhere a few centimetres, dry in the south.
In total, the following amounts of fresh snow fell above approximately 2000 m between Sunday evening and Tuesday afternoon:
- extreme west and northern Lower Valais: 90 to 120 cm, with up to 150 cm on the border with France
- neighbouring western Lower Valais, Vaud Alps, northern flank of the Alps from the western Bernese Alps to Titlis: 60 to 90 cm
- Jura, rest of Lower Valais, northern Upper Valais, northern flank of the Alps east of Titlis: 40 to 60 cm
- southern Upper Valais without southern Visp valleys and without southern Simplon area, and over a widespread area of Grisons: 20 to 40 cm
- southern Visp valleys, southern flank of the Alps: 5 to 20 cm
Temperature
At midday at 2000 m, between -7 °C in the north and -4 °C in the south
Wind
- On the main Alpine ridge and in the south during the night to Wednesday moderate to strong, during the day moderate northwesterly to northerly
- Otherwise light to moderate from the north
Weather forecast to Thursday
During the night to Thursday, there will be some further snow showers on the northern flank of the Alps and in Grisons down to low altitudes. Some clouds will remain in the east in the morning, otherwise it will be sunny in the mountains during the day.
Fresh snow
Northern flank of the Alps, Grisons: 1 to 10 cm
Temperature
At midday at 2000 m, around -5 °C in the north and -2 °C in the south
Wind
- On the main Alpine ridge and south of there during the night to Thursday, sometimes moderate to strong, otherwise light to moderate north-northeasterly
- Moderate Bise wind in the western Jura
Outlook
On Friday conditions will be mostly sunny in the mountains. In the afternoon, cloud will gather from the north-west. The zero-degree level will rise to 2000 m, and the wind will be light to moderate from the north-east. During the night to Saturday, there will be a little precipitation in the north, falling as snow above approximately 1500 m. On Saturday there will be variable cloud, and conditions will be sunny in the south. Winds will be light to moderate west-northwesterly.
The risk of dry avalanches will continue to decrease, while the possibility of gliding avalanches will increase in the west and north on steep sunny slopes.