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
Especially on wind-protected shady slopes, fresh and drifted snow is lying on an old snow surface that is faceted in many places, and sometimes also on Transparent, frequently flat crystals forming on the snowpack when moist air settles on the surface. It forms most often during cold, clear, humid nights. Once it is blanketed by fresh fallen snow, surface hoar is one of the most critical weak layer. See also: www.snowcrystals.itEAWS glossary. The connection to the old surface of the Snow deposited on the ground in a multiplicity of layers.EAWS glossary is poor in many places. Deeper layers of the snowpack are relatively well consolidated in the extreme west of Lower Valais and on the northern flank of the Alps. South of a line from the Rhône to the Rhine, the entire snowpack is often faceted and loose. Where fresh and drifted snow is deposited on this weak snowpack, avalanches can start deeper in the snowpack.
Weather review for Friday
There were clear spells, especially in Grisons, otherwise conditions were mainly heavily overcast. There was sometimes heavy snowfall during the night in the west and north. There were snow showers but also some breaks in the precipitation during the day.
Fresh snow
From Thursday afternoon to Friday afternoon, 20 to 40 cm fell in Valais and on the northern Alpine ridge, and up to 60 cm in the extreme west of Lower Valais. This means that the following amounts of snow have fallen above around 1500 m since precipitation began on Wednesday evening:
- northern Alpine ridge from Les Diablerets to the Aletsch region, extreme west of Lower Valais, Main Alpine Ridge of the Valais from Val Ferret to Monte Rosa: 40 to 60 cm, locally up to 80 cm
- rest of northern flank of the Alps apart from the Prealps, rest of Valais, Gotthard region, northern Grisons, Samnaun: 20 to 40 cm
- Jura, Prealps, central Grisons, Engadine apart from Samnaun: 10 to 20 cm
- elsewhere less or dry
Temperature
At midday at 2000 m, around -5°C
Wind
Westerly:
- moderate to strong in the west and north
- mainly light to moderate in the south and in Grisons
Weather forecast to Saturday
Conditions will be mainly heavily overcast with snowfall down to low altitudes in the north while in the south there will be variable cloud cover with bright spells over the course of the day.
Fresh snow
From Friday afternoon to Saturday afternoon above approximately 1200 m:
- western Jura, extreme west of Lower Valais, northern Alpine ridge: 20 to 40 cm
- eastern Jura, rest of Valais west of the Mattertal, other regions of the northern flank of the Alps, northern Prättigau: 15 to 30 cm
- Visp valleys, Simplon region, southern Goms, southern Gotthard region, rest of northern Grisons, northern Lower Engadine: 5 to 15 cm
- further south a few centimetres or dry
Temperature
At midday at 2000 m, around -10°C in the north and -7°C in the south
Wind
Strong westerly to northwesterly, especially in the west and north
Outlook to Monday
Snowfall will pick up again in the north overnight to Sunday. Between Saturday afternoon and Sunday morning, another 20 to 40 cm of snow may fall on the northern flank of the Alps. On Sunday, there will be sunny spells in the north while conditions will be mainly sunny in the south. On Monday, conditions will mainly be overcast in the north, but with only little snowfall. There will be sunny spells in the far south on Monday.
Avalanche danger will increase overnight to Sunday on the northern flank of the Alps where naturally triggered avalanches are to be expected. Avalanche risk will slowly decrease in the north on Sunday and Monday. Especially in those regions to the south of a line from the Rhône to the Rhine, the weak Snow layers deposited from earlier precipitation, prior to fresh fallen snow. Old snow layers consist of metamorphosed snow crystals.EAWS glossary means that this decrease will be only very slow.