Upper Engadine and Silvretta
Zuoz
Avalanche Problems
Wind slab + Persistent weak layers
2+
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
In particular on wind-protected shady slopes, fresh snow is falling on a snow surface which 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. Deeper layers of the Snow deposited on the ground in a multiplicity of layers.EAWS glossary 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.
Given ongoing snowfall, even relatively large avalanches are increasingly to be expected in the west. These can entrain fresh snow in their track. However, as there is still little lying snow overall, avalanches will remain within known avalanche tracks.
Weather review for Thursday
Conditions were overcast and there was snowfall in the west and north. The snowfall level was at low altitudes in the morning and slowly rose during the day.
Fresh snow
From Wednesday evening to Thursday afternoon above approximately 1200 m:
- northern Alpine ridge from Les Diablerets to the Gotthard region and southern Valais between Val Hérémence and Mattertal: 15 to 30 cm
- rest of the northern flank of the Alps, rest of Valais, northern Grisons, Jura: 5 to 15 cm
- elsewhere less, mostly dry in the south
Temperature
At midday at 2000 m, between -3°C in the northwest and -7°C in the southeast.
Wind
Moderate to strong south to southwesterly
Weather forecast to Friday
In the west and north, conditions will be mostly overcast with further snowfall. Overnight in the north, the snowfall level will temporarily rise to between 1400 and 1600 m before falling back down to around 800 m during the day. In Ticino, central and southern Grisons there will be variable cloud cover with occasional general precipitation from the north.
Fresh snow
From Thursday afternoon to Friday afternoon above approximately 1600 m:
- extreme west and northern Lower Valais: 50 to 80 cm
- northern flank of the Alps west of the Reuss, rest of Valais excluding Visp valleys and Simplon region, western Jura: 30 to 50 cm
- northern flank of the Alps east of the Reuss, Visp valleys, Simplon region, regions north of the Anterior Rhine, northern Prättigau, central and eastern Jura: 20 to 40 cm
- elsewhere a widespread 10 to 20 cm, less or dry in the far south
Temperature
At midday at 2000 m, around -6°C in the north and -4°C in the south
Wind
- Valais and northern flank of the Alps: strong to stormy southwesterly to westerly
- Ticino, Grisons: moderate, sometimes strong westerly in Areas above approximately 3000 m (particularly glaciated areas).EAWS glossary
Outlook to Sunday
High-altitude winds will slowly veer to the northwest. A barrier effect will develop on the northern flank of the Alps. In the west and north, there will be snowfall down to low altitudes, with the heaviest falls overnight from Saturday to Sunday. Over the two days, a total of 30 to 50 cm of fresh snow is again expected in Lower Valais and on the northern flank of the Alps, and 15 to 30 cm in Upper Valais and northern Grisons. Winds will be strong to storm force from westerly to northwesterly directions.
Avalanche risk will continue to increase somewhat in many places. In those regions exposed to heavier precipitation on the northern flank of the Alps and in Valais, naturally triggered avalanches, some very large, are to be expected. There will be scarcely any change in avalanche risk on the southern flank of the Alps.