Mittelland and Jura
Val de Ruz - Colombier
Avalanche Problems
Wind slab
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Field notes
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.
Snowpack & Weather
Snowpack
The Snow layers deposited from earlier precipitation, prior to fresh fallen snow. Old snow layers consist of metamorphosed snow crystals.EAWS glossary 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 in many places. North of a line from the Rhône to the Rhine, these are thickly covered and are only triggerable in isolated cases. The main danger here stems from the fresh and drifted snow which, on northern slopes, has in places been deposited 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. South of a line from the Rhône to the Rhine, breaks in the deeper weak layers are again expected with the sometimes large amounts of fresh snow over the weekend, and, as a result, avalanches in these areas may sometimes become very large.
Weather review for Saturday
Saturday saw mostly heavy cloud and widespread precipitation, which was heavy in the south during the day. The snowfall level dropped from 1300 m to low altitudes in the north and from 1600 to 1300 m in the south.
Fresh snow
Up until Saturday afternoon, above 1800 m:
- main Alpine ridge from the Simplon region to the San Bernardino Pass and south of there: 20 to 40 cm, in western Ticino up to 50 cm
- western and central parts of the northern flank of the Alps, Lower Valais, Val Bregaglia: 10 to 25 cm
- elsewhere less, dry in northern Grisons and Lower Engadine
Temperature
At midday at 2000 m, between -6 °C in the northwest and -1 °C in the southeast
Wind
- Often strong from southerly directions, light to moderate northwesterly during the day in the west and on the northern flank of the Alps
- In the Jura, strong to storm-force southwesterly overnight and northwesterly during the day
Weather forecast to Sunday
There will be widespread precipitation during the night, which will be very heavy in the south, and will also spread northwards well over the main Alpine ridge due to the countercurrent situation. Precipitation will continue during the day in the south, but will be less heavy. In other regions there will be sunny intervals. The north will see snowfall down to low altitudes, while in the south snow will fall above 800 to 1200 m.
Fresh snow
From Saturday afternoon until precipitation ends on Sunday, above 1000 m in the north and 1500 m in the south:
- main Alpine ridge from the Monte Rosa region to the San Bernardino Pass and south of there, Val Bregaglia to Bernina Pass: 50 to 70 cm, in western Ticino up to 80 cm
- main Alpine ridge in Lower Valais, rest of Upper Valais, northern flank of the Alps from the eastern Bernese Oberland to the Glarus Alps excluding the Prealps, central Grisons, rest of Upper Engadine and Val Poschiavo: 30 to 50 cm
- elsewhere a widespread 20 to 30 cm, less in the Jura, the Vaud and Fribourg Alps and Lower Engadine
Temperature
At midday at 2000 m, between -6 °C in the north and -2 °C in the south
Wind
- Moderate easterly overnight, sometimes strong in the south
- Moderate and locally strong southerly during the day at high altitudes
Outlook to Tuesday
In the north, Monday morning will see sunny intervals before becoming very cloudy. Between midday on Monday and Tuesday morning, 5 to 15 cm of snow will fall on the northern flank of the Alps above 800 m. Tuesday will see residual clouds in the east at the beginning, while otherwise conditions will be sunny. Both days will be sunny in the south. During the day on Monday the wind will be a moderate to strong westerly, while overnight to Tuesday the wind will be a strong northerly at high altitudes, occasionally storm-force in the south, transporting the often loose fresh snow from the weekend.
Avalanche danger will generally decrease. With the strong wind, however, snowdrift accumulations that are prone to triggering will develop, on Monday in the north and on Tuesday in the south. Where there is sunshine, the new snow will also make moist avalanches likely, with these becoming numerous and occasionally large on Monday in the south.