Central Prealps
Engelberg
Hard-to-read day. Considerable, with wind slab and persistent weak layers in play — you can read the surface problem; the buried one sets the consequence.
Day Risk Profile
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
New and drifted snow are overlaid on an Snow layers deposited from earlier precipitation, prior to fresh fallen snow. Old snow layers consist of metamorphosed snow crystals.EAWS glossary, the top section of which contains coarse-grained 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, primarily on north- and east-facing slopes at high altitudes.
Weather review for Ascension Day
Conditions were mostly very cloudy. Snow fell at times above approximately 1500 m. There were brief sunny spells in between, with longer sunny periods in Valais.
Fresh snow
Up until Ascension Day afternoon, the following amounts of snow fell above approximately 2000 m:
- Extreme west of Lower Valais, Val Bregaglia to Bernina region: 15 to 30 cm
- Elsewhere a widespread 5 to 15 cm, less in the Upper Valais and Ticino
Temperature
At midday at 2000 m, between -2 °C in the north and +2 °C in the south
Wind
Moderate, sometimes strong from the west along the Prealps
Weather forecast to Friday
It will be mostly cloudy, with snow falling above approximately 1300 m.
Fresh snow
From Ascension Day to Friday afternoon, the following amounts of snow will fall above approximately 1800 m:
- Northern flank of the Alps, Lower Valais and Grisons: 5 to 15 cm
- Upper Valais and Ticino: less
Temperature
At midday at 2000 m, around -2 °C
Wind
Mainly light
Outlook
Saturday
In the north, it will be very cloudy, with snow falling above approximately 1300 m. The heaviest snowfall will occur along the northern Alpine ridge from the Bernese Oberland to the Säntis, with approximately 20 cm expected.
In the south it will be sunny with northerly winds.
There will be a widespread increase in avalanche risk. In Upper Valais and Ticino, it will not change significantly.
Sunday
After a mostly clear night, it will be sunny. Winds will be light. The zero-degree level will rise to 2100 m in the north and 2500 m in the south.
The danger of dry avalanches will decrease. As a consequence of solar radiation, near-surface wet avalanches are to be expected as the day progresses.