Sur Tasna

Fresh and drifted snow from the last week is lying in many places on a faceted old snow surface or on surface hoarTransparent, 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. Avalanches triggered by human activity show that the connection to the old surface of the snowpackSnow 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. Many whumpfingDistinctive 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 are still being reported from Valais and northern Grisons. The probability of slab avalanchesThe 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 remains elevated here and the situation requires patience.

In the regions exposed to the foehn wind in the north, snowdrift accumulations that are prone to triggering developed on Friday. They will grow a little more on Saturday.

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.

In the north, it was mostly sunny in the mountains, with patches of cloud in the east. Conditions were very cloudy in the south with a little snowfall above approximately 1200 m.

Fresh snow

Southern Simplon region, Ticino and Moesano: up to 5 cm

Temperature

At midday at 2000 m, between +1 °C in the north and -2 °C in the south

Wind

From southerly directions:

• Sometimes strong foehn wind during the night to Friday in the regions exposed to the foehn wind in the north, otherwise mostly moderate in the north.
• Weak to moderate in the south.

In the north, it will be partly sunny in the mountains with patches of cloud. In the south, it will be very cloudy with precipitation, which will fall as snow above approximately 1200 m.

Fresh snow

Until Saturday afternoon, above approximately 1500 m:

• Main Alpine Ridge in Upper Valais on the border with Italy, northern Ticino: 10 to 20 cm
• Rest of the Main Alpine Ridge from the Great St. Bernard Pass to Val Bregaglia: 5 to 10 cm

Temperature

At midday at 2000 m, around +3 °C in the north and -2 °C in the south

Wind

Southerly winds

• Moderate to strong in the north, strong foehn wind at times in the regions exposed to the foehn wind in the north
• Weak to moderate south of the Main Alpine Ridge

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 2200 and 2400 m. The southerly wind will be moderate on Sunday and strong on Monday.

Conditions will be very cloudy in the south on both days. Some snow will fall on the Main Alpine Ridge and to the south of it. With 15 to 25 cm, on Monday most of the snow will fall on the Main Alpine Ridge in Upper Valais and in Ticino. However, the forecast for Monday is still uncertain. The snowfall level will be between 1000 and 1200 m. The southerly wind will be weak to moderate on Sunday and moderate to strong at high altitudes on Monday.

In the south and on the Main Alpine Ridge in Valais, the avalanche risk will increase on Monday with fresh snow and wind. Otherwise it will not change significantly. Weak layersSnowpack 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 old snowpackSnow layers deposited from earlier precipitation, prior to fresh fallen snow. Old snow layers consist of metamorphosed snow crystals.EAWS glossary and the fresh drift snow are prone to triggering in many areas.