Central Prealps
Stoos
Avalanche Problems
New snow + Persistent weak layers
3+
Field notes
New snow
Easy to spot - the snow is fresh. Look at recent avalanche activity in similar terrain, and pay attention to how much new snow has fallen in the last three days. Critical loading depends on temperature, wind, and what was on the surface before the storm. SLF advises waiting until the Snow deposited on the ground in a multiplicity of layers.EAWS glossary has had time to bond before committing to steep terrain in fresh snow.
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 last four days has been deposited on an old snow surface which is faceted in many places 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. The connection to the old surface of the Snow deposited on the ground in a multiplicity of layers.EAWS glossary is therefore still unfavourable in many places. Deeper layers of the snowpack are relatively well consolidated in the extreme west of Lower Valais and on the northern flank of the Alps. South of a line from the Rhône to the Rhine, the entire snowpack is often faceted and loose. In these regions, avalanches can start deeper in the snowpack.
Numerous natural avalanches, as well as various avalanches triggered by human activity, some remotely over relatively large distances, are evidence of the current widespread instability of the Snow deposited on the ground in a multiplicity of layers.EAWS glossary. Dangerously large avalanches can still be easily triggered by human activity, especially on the northern flank of the Alps, in Valais and in northern Grisons.
Weather review for Sunday
Snow stopped falling in the north overnight to Sunday. During the day, it was sunny in Valais and on the southern flank of the Alps. In the north and east, it also became increasingly sunny as the day progressed.
Fresh snow
Overnight to Sunday, some more snow fell in the north, most of it in the Glarus Alps, with amounts of between 20 and 40 cm. This means that the following total amounts of snow have fallen above 1400 m from the start of the precipitation on Wednesday evening until Sunday morning:
- Northern Alpine Ridge from the Diablerets to the Aletsch region, extreme west of Lower Valais, Glarus Alps: 80 to 120 cm
- Rest of the northern flank of the Alps apart from the Prealps, southern Valais, Gotthard region, northern Grisons: 40 to 70 cm
- Jura, Prealps: 30 to 50 cm
- Lower Engadine north of the Inn: 20 to 40 cm
- Rest of northern Ticino, Lower Engadine south of the Inn, central Grisons, Upper Engadine: 15 to 30 cm
- Further south: less or dry
Temperature
At midday at 2000 m, around -9°C
Wind
- On the southern flank of the Alps and generally in the Areas above approximately 3000 m (particularly glaciated areas).EAWS glossary: moderate to strong from west to north, otherwise mainly light
- Subsiding over the course of the day
Weather forecast to Monday
There will be some snowfall in the north overnight to Monday and during the course of the day. On Monday, conditions will be mainly cloudy in the north and very sunny in the south.
Fresh snow
Between Sunday evening and Monday afternoon above 1000 m:
- Northern Alpine Ridge, Valais, northern Prättigau, Silvretta: 5 to 15 cm
- Elsewhere a few cm, dry on the southern flank of the Alps
Temperature
At noon at 2000 m, between +2 °C in the west and -2 °C in the east and south
Wind
- Jura, Prealps, and generally at high altitudes: moderate with occasionally strong westerly winds in exposed locations
- Elsewhere mostly light
Outlook to Wednesday
On Tuesday, it will be quite sunny in the north and often cloudy in the south. On Wednesday it will be brighter in the inneralpine regions, while elsewhere it will be mostly cloudy. The wind will be mostly moderate at high altitudes on Tuesday and strong at times on Wednesday. The zero-degree level on Tuesday will be around 2600 m in the west and north and around 2000 m elsewhere.
The danger of dry avalanches will continue to decrease. However, especially in those regions to the south of a line from the Rhône to the Rhine, the weak Snow layers deposited from earlier precipitation, prior to fresh fallen snow. Old snow layers consist of metamorphosed snow crystals.EAWS glossary means that this decrease will be only very slow. Given the rising temperatures and sunny conditions, moist snow slides and avalanches are to be expected on very steep slopes. There will be scarcely any change in avalanche danger on the southern flank of the Alps.