Prättigau and Davos
Westliche Silvretta
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 previous 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. Many avalanches triggered by human activity, including those from the end of this week, show that the connection to the old surface of the Snow deposited on the ground in a multiplicity of layers.EAWS glossary is still weak. 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. 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 triggering over larger distances have continued to be reported from Valais and northern Grisons in recent days. 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 will decrease only slowly. The situation requires patience and caution.
With strong southerly winds at times, snowdrift accumulations that are prone to triggering have also formed in recent days.
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 Saturday
It was mostly cloudy in the north and partly sunny in central Valais and the east. It was very cloudy in the south with precipitation at times. The snowfall level was around 1300 m.
Fresh snow
From Friday afternoon to Saturday afternoon, above approximately 1500 m:
- Western Ticino, southern Simplon region: 10 to 25 cm
- Rest of the Main Alpine Ridge from the Great St. Bernard Pass to Val Bregaglia and south of there: 5 to 15 cm
Temperature
At midday at 2000 m, between +3 °C in the north and -2 °C in the south
Wind
From south-east to south
- Moderate to strong in the north, strong foehn wind at times in the regions exposed to the foehn wind in the north
- Sometimes moderate to strong south of the Main Alpine Ridge during the night to Saturday, weak to moderate during the day
Weather forecast to Sunday
In the north, it will be mostly sunny in the mountains with high cloud cover and a continuing foehn wind. It will be very cloudy in the south and a few centimetres of snow will fall above approximately 1300 m.
Fresh snow
Western part of the Main Alpine Ridge on the border with Italy, southern flank of the Alps: up to 5 cm
Temperature
At midday at 2000 m, around +3 °C in the north and -2 °C in the south
Wind
From south to southwest
- Moderate to strong in the north, strong foehn wind in the regions exposed to the foehn wind in the north
- Weak to moderate south of the Main Alpine Ridge
Outlook to Tuesday
In the north, it will be mostly sunny in the mountains on both days with high cloud cover. The zero-degree level will be between 2000 and 2200 m. The southerly wind will be moderate. On Monday, it will still be strong at times in the regions exposed to the foehn wind in the north. The foehn wind will ease somewhat on Tuesday.
It will be very cloudy in the south on Monday. On the Main Alpine Ridge and south of it, 5 to 10 cm of snow is expected to fall above approximately 1500 m. On Tuesday, it will be increasingly sunny in the south. There will be a weak to moderate southerly wind.
The avalanche risk will not change significantly in the south. It will decrease in the north, but only slowly in the inneralpine regions. 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 will remain prone to triggering.