Lower Valais
Val d'Hérens
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
After this week's snowfall, the snow depths in the north and west are well above average for the time of year. Deeper in the Snow deposited on the ground in a multiplicity of layers.EAWS glossary, especially on shady slopes above approximately 2400 m and in the Areas above approximately 3000 m (particularly glaciated areas).EAWS glossary in general, there are 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 in which avalanches, some large, have been triggered in recent days. In the regions exposed to heavier precipitation in the west and north, these weak layers are now thickly overlaid and are therefore no longer so easily triggered by winter sports participants. However, any avalanches may still become large. In areas with less fresh snow, avalanches are easier to trigger in these weak layers, but will not be quite as large.
With the north to northeasterly wind, snowdrift accumulations formed at high altitudes on Thursday.
Weather review for Thursday
A four-day period of precipitation, which brought a lot of fresh snow to the north and most especially the west, ended during the night to Thursday, with Thursday being mostly sunny in the mountains.
Fresh snow
Since Wednesday afternoon, 5 to 10 cm of snow has fallen in the north down to low altitudes. From Sunday evening until during the night to Thursday, the following amounts of fresh snow had thus fallen above approximately 1800 m:
- northern flank of the Alps, northern and extreme west of Lower Valais: 70 to 110 cm, with up to 150 cm in the far west
- rest of Lower Valais, northern Upper Valais, northern and central Grisons, Jura: 30 to 60 cm
- elsewhere: 10 to 30 cm
Temperature
At midday at 2000 m, around -5 °C in the north and -2 °C in the south
Wind
Moderate, locally strong north-northeasterly
Weather forecast to Friday
Conditions will be mostly sunny in the mountains.
Fresh snow
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Temperature
At midday at 2000 m, around 0 °C
Wind
The northeasterly wind will die down during the night and will be mostly light during the day.
Outlook
Saturday
In the north, a few centimetres of snow will fall above around 1500 m during the night to Saturday. Saturday will be dry with relatively long bright intervals. Conditions will be very sunny in the south.
Avalanche risk will continue to decrease in the west and north. There will be no significant change in the south.
Sunday
Conditions will be very cloudy and some snow will fall in the west and north above around 1200 m, with 10 to 20 cm mostly in the far west. Winds will be moderate from the southwest, then from the west.
The risk of dry avalanches will increase slightly in the far west, otherwise it will not change significantly.