Lower Valais
Val d'Hérens
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
On the northern flank of the Alps, the new and drifted snow of the last few days cover, at higher altitudes on shaded, wind-sheltered slopes an often faceted and therefore weak old snow surface. Elsewhere, the snow surface has often been shaped by the wind and is frequently frozen solid below 2400 m.
In western and northern Lower Valais, the Snow deposited on the ground in a multiplicity of layers.EAWS glossary structure is relatively favourable and the danger comes mainly down to the fresh drifted snow.
In other regions, the Snow deposited on the ground in a multiplicity of layers.EAWS glossary is thin and weak, and mostly consists of Snow grains, solid, with multiple flat glassy faces and sharp edges. Developed from faceting metamorphism, usually poorly bonded to one another (fewer contact points), a critical factor in avalanches if a faceted layer is covered with bonded snow. Typical grain size: 0.5 to 3 mm See also: www.snowcrystals.itEAWS glossary with embedded crusts. Snowdrift accumulations deposited on top of this snowpack are in some cases easily triggerable. In addition, individual avalanches may still be triggered in the near-ground Snow layers deposited from earlier precipitation, prior to fresh fallen snow. Old snow layers consist of metamorphosed snow crystals.EAWS glossary, especially on the Main Alpine Ridge in Upper Valais and on the central part of the southern flank of the Alps.
Weather review for Monday
It was sunny but cold in the mountains.
Fresh snow
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Temperature
At midday at 2000 m, around -10°C
Wind
Light to moderate from westerly directions
Weather forecast to Tuesday
In the mountains conditions will be sunny but cold.
Fresh snow
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Temperature
At midday at 2000 m, around -13 °C in the north and -9 °C in the south
Wind
Light to moderate from northerly directions
Outlook to Thursday
Wednesday
It will be sunny and cold. In the afternoon, there will be a moderate northwesterly wind at high altitudes.
The avalanche danger will decrease, but only slowly due to the often unfavourable Snow deposited on the ground in a multiplicity of layers.EAWS glossary structure.
Thursday
With strong to storm-force northwesterly winds, it will be partly sunny in the south, while elsewhere it will be very cloudy with snowfall down to the lowlands. In the north and west, 10 to 20 cm of snow is expected to fall, but the amounts are still uncertain.
New snow and drifted snow will be deposited on an often weak Snow layers deposited from earlier precipitation, prior to fresh fallen snow. Old snow layers consist of metamorphosed snow crystals.EAWS glossary. The avalanche danger will increase, especially in the west and north.