Valais Chablais
Monthey-Val d'Illiez
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
After a long spell of fine weather, there was widespread snowfall last weekend. This was deposited on bare ground on southern slopes below approximately 2800 m and on northern slopes below approximately 2000 m. On shady slopes above approximately 2400 m and generally in the Areas above approximately 3000 m (particularly glaciated areas).EAWS glossary, however, the fresh snow often fell on 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. These are still prone to triggering.
In addition, with new fallen snow and weak to moderate westerly winds, mostly small snowdrift accumulations will form that are prone to triggering, especially in the north and at high altitudes.
Outlook
On Friday, except on the southern flank of the Alps, some snow will fall across a wide area down to low altitudes. It will often be cloudy in the south as well. On Saturday, the snowfall will end in the northeast too, but it will remain cloudy there. It will be mostly sunny in the west and south. On both days it will be cold with around -10 °C at 2000 m and moderate northeasterly winds. In total, another 10 to 20 cm of snow will fall in the north from Thursday evening to Saturday morning, and up to 30 cm along the central Prealps. 5 to 15 cm of snow is expected in Lower Valais and Grisons, otherwise less.
Avalanche risk will increase somewhat, especially over much of the northern flank of the Alps. There will be little change in other regions. Isolated avalanches may be triggered not only in fresh snowdrift accumulations but also in the Snow layers deposited from earlier precipitation, prior to fresh fallen snow. Old snow layers consist of metamorphosed snow crystals.EAWS glossary, especially on northern slopes above 2400 m and in Areas above approximately 3000 m (particularly glaciated areas).EAWS glossary.