Valais Chablais
Monthey-Val d'Illiez
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
Fresh and drifted snow from the last week is lying in many places on a faceted old snow surface or 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, especially on wind-protected shady slopes. The connection to the old surface of the Snow deposited on the ground in a multiplicity of layers.EAWS glossary is still poor. South of a line from the Rhône to the Rhine, the entire snowpack is often faceted and loose. Here, avalanches may penetrate near-ground layers. On the northern flank of the Alps, in Valais and in northern Grisons, avalanches may still become dangerously large. Many 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 remote avalanche triggering over larger distances are still being reported from Valais and northern Grisons. The probability of The abrupt release of a slab of snow on a mountain slope. After crack initiation and crack propagation in a weak layer the released slab comprises 3 elements: The weak layer, the gliding horizon and the slab. If the slope angle is steep enough, the slab will glide down. If the slope is not steep enough, the slab will settle down on the failed weak layer. Possibly a whumpf sound will follow due to air pressed out of the weak layer. The fracture line will be sharp edged.EAWS glossary being triggered remains high here and the situation requires patience.
In the regions exposed to the foehn wind in the north, snowdrift accumulations that are prone to triggering will form at high altitudes during the night to Friday. The snow is wet at low and intermediate altitudes. Isolated gliding avalanches are possible on steep sunny slopes, especially on the northern flank of the Alps.
Weather review for Thursday
There was a little precipitation in the Jura in the early morning, otherwise it was quite sunny in the north. It was cloudy and mainly dry in the south.
Fresh snow
A few centimetres on the highest Jura peaks
Temperature
At midday at 2000 m, between +1 °C in the north and -2 °C in the south
Wind
- Moderate and locally strong from the southwest
- An increasing foehn wind in the north during the course of the day
Weather forecast to Friday
In the north it will be mostly sunny with foehn winds. In the south it will be very cloudy with a little snow falling above 1300 m.
Fresh snow
Ticino and Moesano: up to 5 cm
Temperature
At midday at 2000 m, between +1 °C in the north and -3 °C in the south
Wind
Southerly winds
- Strong during the night to Friday in the regions exposed to the foehn wind in the north
- Otherwise mostly moderate
Outlook to Sunday
There will be mostly moderate southerly winds and a foehn wind in the valleys of the north. There will be clear spells in the north on Saturday. On Sunday it will be mostly sunny in the mountains. It will be very cloudy in the south on both days and snow will fall above approximately 1300 m. Around 5 to 15 cm will fall on Saturday, while less is expected on Sunday.
In the south and on the Main Alpine Ridge in Valais, the avalanche risk will increase slightly on Saturday with the fresh snow. Otherwise it will not change significantly. 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 in the Snow layers deposited from earlier precipitation, prior to fresh fallen snow. Old snow layers consist of metamorphosed snow crystals.EAWS glossary remain prone to triggering in many areas.