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Avalanche bulletin through Monday, 23 October 2017

Onset of winter in northern regions brings heightened avalanche danger

Edition: 21.10., 17:00 Next update: 23.10., 18:00 Validity

Onset of winter in northern regions brings heightened avalanche danger
Edition: 21.10.2017, 17:00 / Next update: 23.10.2017, 18:00
Avalanche danger
Northern Alpine Ridge, northern and central Grisons, Lower Engadine north of the Inn
Fresh snow and freshly formed snowdrifts can be triggered as slab avalanches in all aspects of high alpine regions and above approximately 2500 m particularly on shady slopes. Starting on Monday, also medium-sized naturally triggered avalanches can be expected. General conditions for backcountry tours are perilous. In the remaining aspects and altitudes, the likelihood of avalanches being triggered is significantly lower. The major danger in those places stems from naturally triggered wet-snow or gliding avalanches more than anything else. Most of all, the hazards of being swept along and forced to take a fall need to be given due consideration.
Exposed transportation routes at high altitude can be placed at risk by slides emanating from hillsides flanking the roads and steep meadows or rock plates.
Remaining regions of the Swiss Alps
Fresh snow and freshly formed snowdrifts, particularly in high alpine regions, can be triggered as slab avalanches by persons. As of Monday, naturally triggered avalanches are possible in isolated cases. At intermediate altitudes, naturally triggered wet-snow or gliding snowslides are also possible. Tours in outlying terrain make ample experience in the on-site assessment of avalanche dangers imperative.
Snow and weather
Prior to the snowfall which is currently forecast, a cohesive area-wide snowpack exists over widespread areas in glaciated high alpine terrain as well as in northern and eastern regions on shady slopes above approximately 2500 m in particular. In those areas the snow cover is faceted or covered with surface hoar, particularly on shady slopes. South-facing slopes are bare of snow over widespread areas up to high alpine altitudes.
Weather review
The past 10 days have been predominantly sunny, dry and mild, the zero-degree level being between 3000 and 4000 m. The temperature disparities between sunny and shady zones are stark for this juncture of the season. As a result, the snow which fell in September and at the beginning of October has remained on the ground on shady slopes above approximately 2500 m in particular, whereas it melted away on the sunny slopes.
Weather outlook through Monday, 23 October
On Saturday night, a cold front will reach the Alps. On Monday, a northwesterly barrier cloud effect will set in. The most intensive precipitation is anticipated on Monday. The snowfall level is expected to descend from over 2000 m on Sunday down to 1000 m. On Monday, it will again ascend towards 1600 m on the northern flank of the Alps in particular. On Sunday the wind will shift from westerly to northwesterly and be blowing at strong to storm velocity.
By Monday evening, the following amounts of fresh fallen snow are anticipated above 2000 m:
  • northern Alpine Ridge from the Blüemlisalp as far as Tödi: 70 to 100 cm;
  • remaining parts of the northern Alpine Ridge, Main Alpine Ridge in the Lower Valais, northern Grisons, the Lower Engadine north of the Inn: 40 to 70 cm;
  • southern Gotthard region, central Grisons, the Upper Engadine north of the Inn: 20 to 40 cm;
  • remaining regions of Switzerland: 10 to 20 cm; in the central sector of the southern flank of the Alps it will for the most part remain dry.
At intermediate altitudes, approximately half the above prognosticated amounts of snowfall can be expected.
On Monday night, the precipitation is expected to come to an end and during the daytime on Tuesday it will turn increasingly sunny from the west. A strong-velocity northerly wind will be blowing. On Wednesday it will become quite sunny. The wind will incrementally taper off and it will get warmer. The danger of dry-snow avalanches will gradually diminish. In high alpine regions, however, the snowdrift accumulations in particular will remain prone to triggering. On sunny slopes, wet-snow and gliding snowslides can be expected. At intermediate altitudes, gliding avalanches are also possible on north-facing slopes.

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