Central Grisons and Anterior Rhine
Domleschg - Lenzerheide
Avalanche Problems
Persistent weak layers
1
Field notes
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 ae deposited on the northern flank of the Alps on a snow surface which is often transformed and loose on wind-protected shady slopes, especially at high altitudes. Elsewhere, the snow surface has often been shaped by the wind and is frequently frozen solid below 2400 m.
On the Main Alpine Ridge in Valais and on the central part of the southern flank of the Alps, the fresh snow from Christmas and snowdrift accumulations are lying on a thin but weak Snow layers deposited from earlier precipitation, prior to fresh fallen snow. Old snow layers consist of metamorphosed snow crystals.EAWS glossary 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. Isolated avalanches may still be triggered in the old snowpack in these regions.
The Snow deposited on the ground in a multiplicity of layers.EAWS glossary in central Valais, northern Upper Valais and throughout Grisons 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 and embedded crusts. There is a thin, hard layer of drifted snow on the surface in places and a Layer of hard-compacted snow resulting from a melt-freeze process; increases firmness.EAWS glossary on steep south-facing slopes.
Weather review for Friday
There was broken to heavy cloud cover to the north of the Main Alpine Ridge and a few centimetres of snow fell in some locations on the northern flank of the Alps. Further south, conditions were mostly sunny.
Fresh snow
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Temperature
At midday at 2000 m, between -8°C in the north and -4°C in the south
Wind
Mostly moderate from the west, rising to strong especially on the northern flank of the Alps and in Valais at high altitudes
Weather forecast to Saturday
Overnight to Saturday, some snow will fall on the northern flank of the Alps and in northern Grisons, while conditions will remain mainly dry further south. Some clouds will remain during the day, especially along the Prealps. Conditions will be mostly sunny in other regions and later also in the Prealps.
Fresh snow
By Saturday morning, the following amounts of snow will fall down to low altitudes:
- northern Alpine ridge from the Hasli valley to the Alpstein region: 10 to 20 cm
- rest of the northern flank of the Alps, northern Grisons: 5 to 10 cm
- further south: less or dry
Temperature
At midday at 2000 m, between -10°C in the north and -6°C in the south
Wind
From the west
- stormy at times during the night, especially in the Bernese Oberland and Valais at high altitudes
- elsewhere and during the day moderate to strong
Outlook to Monday
Both days will be sunny but cold in the mountains. The northerly wind will be mostly light.
The unfavourable Snow deposited on the ground in a multiplicity of layers.EAWS glossary structure means that avalanche risk will decrease only very slowly.