Central Prealps
Engelberg
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
Especially on shady slopes above approximately 2400 m and generally in Areas above approximately 3000 m (particularly glaciated areas).EAWS glossary, there are faceted, long-lasting 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 deep in the Snow layers deposited from earlier precipitation, prior to fresh fallen snow. Old snow layers consist of metamorphosed snow crystals.EAWS glossary. In many places in western regions with heavy snow, these layers are overlaid with deep fresh snow and so can only be triggered by winter sports participants in isolated cases. However, if avalanches are triggered in these deep layers, they can become large. In other regions, the weak old snowpack layers are closer to the surface. Here the Snow deposited on the ground in a multiplicity of layers.EAWS glossary, which is mostly still thin, is becoming completely faceted and loose, meaning that here too avalanches can only be triggered in places. This week's mostly small snowdrift accumulations are in some cases still prone to triggering.
On wind-protected shady slopes, the surface of the Snow deposited on the ground in a multiplicity of layers.EAWS glossary often consists of loose, partly faceted snow and partly 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.
Weather review for Thursday
In the north, conditions were quite sunny in the mountains with broken cloud. There was heavy cloud cover in the south. Above approximately 1300 m, a little snow fell in some localities in the west and south.
Fresh snow
Western Lower Valais, western part of the main Alpine ridge, Ticino: 1 to 5 cm, locally up to 10 cm
Temperature
At midday at 2000 m, between -1°C in the north and -3°C in the south
Wind
Light to moderate from southerly directions, occasionally strong in regions exposed to the foehn wind in the north
Weather forecast to Friday
In the north, conditions will be cloudy at first and a few centimetres of snow will fall above approximately 1000 m by midday. In the south, it will be cloudy during the night and a little snow will fall locally above approximately 1300 m. Conditions will become increasingly sunny from the morning onwards in the south and generally in the Areas above approximately 3000 m (particularly glaciated areas).EAWS glossary.
Fresh snow
A few centimetres in widespread areas, locally up to 10 cm in northern and central Grisons
Temperature
At midday at 2000 m, around -4°C in the north and -2°C in the south
Wind
Light, becoming northerly
Outlook
Saturday
In the north and Valais, conditions will be mostly cloudy with light precipitation. The snowfall level will rise to between 1200 m and 1600 m. Up to 10 cm of snow will fall in the far west, only a few centimetres elsewhere. Conditions will be fairly sunny and dry in Ticino and Grisons. The wind will freshen, becoming a moderate to strong southwesterly to westerly in the north and generally at high altitudes. Snowdrift accumulations, which are likely to be small but easily triggered, will form and there will be a slight increase in avalanche risk.
Sunday
There will be heavy cloud cover and widespread precipitation. The snowfall level will rise from 1500 m to 2000 m, in the west to 2200 m. In Valais and on the western part of the northern flank of the Alps, 15 to 30 cm of snow falls at high altitudes, less elsewhere. However, the amount of precipitation is still uncertain. Conditions will remain dry in central and southern Ticino and there be sunny intervals during the day. During the night, there will be a moderate to strong westerly wind in the west and north which will elsewhere be light to moderate.
At high altitudes, the risk of dry avalanches will increase in many places, significantly so in the west. In addition, rainy conditions will mean that the risk of gliding avalanches and moist snow slides will increase below approximately 2200 m.