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Avalanche bulletin through Friday, 27 May 2016

Favourable morning avalanche scenario by and large, wet-snow avalanches as day unfolds

Edition: 25.5., 17:00 Next update: 27.5., 18:00 Validity

Favourable morning avalanche scenario by and large, wet-snow avalanches as day unfolds
Edition: 25.5.2016, 17:00 / Next update: 27.5.2016, 18:00
Avalanche danger
Dry-snow avalanches
Both the fresh fallen snow and the freshly formed snowdrift accumulations from the beginning of the week can from place to place be triggered by skiers and freeriders. The avalanche prone locations are found on very steep north-facing slopes in high alpine regions more than anywhere else. A prudent route selection is recommended.
Wet-snow avalanches
As a consequence of solar radiation and the daytime warming curve, wet-snow avalanches can be expected during the course of each day. This is particularly the case on north-facing slopes up as far as approximately 3000 m, in the other aspects up as far as approximately 4000 m. On north-facing slopes between 2400 and 3000 m more than anywhere else, wet-snow avalanches can fracture down to more deeply embedded layers inside the snowpack, sweep the entire snow cover away and thereby grow to medium size. High altitude, exposed transportation routes can in isolated cases be placed at risk. Backcountry skiing and freeriding tours in outlying terrain should be brought to a close early in the day. Also, hikes along exposed trails at high altitude need to take avalanche hazards into consideration.
Snow and weather
Snowpack
The layers of fresh fallen snow and freshly formed snowdrift accumulations from the beginning of the week, which are quite deep in some places, are stabilising to an increasing degree. More deeply embedded layers inside the snow cover are thoroughly wet up as far as approximately 2800 m on north-facing slopes, and up as far as high alpine regions on steep, south-facing slopes. In the wake of nights when skies are clear, the snowpack surface is generally frozen with a crust which is capable of bearing loads. Only on very steep north-facing slopes in high alpine regions is the snowpack surface still loosely-packed in some places.
At 2500 m in the Valais, on the northern flank of the Alps, in northern Ticino and in northern Grisons, the snowpack is still 100 to 200 cm deep; in northern regions, it is up to 300 cm deep from area to area. In the remaining regions of Switzerland, the snowpack is approximately 100 cm deep.
Weather review to Sunday, 22 May:
Between Sunday evening and Tuesday morning, the following amounts of fresh fallen snow were registered above approximately 2500 m:
  • northern flank of the Alps from Chablais to the Glarus Alps, Valais, Upper Engadine, Val Bregaglia, Val Poschiavo: 40 to 60 cm;
  • remaining regions of Switzerland: 20 to 40 cm.
The new fallen snow was transported by the northerly winds which were intermittently blowing at strong velocity. On Monday, the snowfall level descended for a brief interim down towards 1200 m.
On Tuesday in western and southern regions, it was predominantly sunny. In northern regions, the skies brightened during the course of the day. The northerly winds slackened off considerably.
On Wednesday it was sunny for the most part, accompanied by light winds. The zero-degree level ascended to approximately 3000 m.
Weather forecast through Friday, 27 May:
Skies on Wednesday night are expected to be predominantly clear. During the day on Thursday, convective cloud build-up is expected during the course of the day amidst sunny weather conditions. On Thursday night, skies will be only partially clear. During the daytime on Friday, it will be quite sunny. Over the course of the afternoon, isolated showers and thunderstorms are possible.
On both days, winds will be southwesterly, blowing at moderate strength at high altitudes. The zero-degree level is expected to ascend to 3200 m on Thursday, then towards 3800 m on Friday.
Outlook
On Saturday, it will be sunny to begin with. Swift convective cloud build-up is anticipated, accompanied by shower activity. On Sunday in southern and western regions, precipitation is expected which in some places will be quite intensive. The snowfall level will be above 2500 m. The hazards of dry-snow avalanches will diminish further on Saturday, subsequently increase in western and southern regions at high alpine altitudes on Sunday. The hazards of wet-snow avalanches are expected to increase during the course of the day on Saturday. On Sunday, as a result of rainfall, wet-snow avalanches are expected to trigger on high altitude north-facing slopes more than anywhere else.

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