---------------------------------------------------------------- WSL Institute for Snow and Avalanche Research SLF NATIONAL AVALANCHE BULLETIN NO. 16 for Sunday, 11 December 2011 issue date 10.12.2011, 18:30 hours ---------------------------------------------------------------- CONSIDERABLE AVALANCHE DANGER IN SOME REGIONS ---------------------------------------------------------------- CURRENT CONDITIONS On Saturday it was overcast on the northern flank of the Alps, accompanied by bright intervals in western regions. In the Valais, in Ticino and in Grisons it was predominantly sunny. The midday temperature at 2000 m was zero degrees. The westerly to southwesterly wind was blowing at moderate to strong velocity to begin with, but subsequently slackened off. Above the treeline, the distribution of snow has been heavily influenced by winds: broad ridges are often bare of snow, whereas bowls are filled to the brim. On east facing slopes in particular, snowdrift accumulations from Friday are evident, and are still capable of being triggered in some places. On shady slopes above approximately 2400 m, a thin layer of old snow is spread far and wide. On the Main Alpine Ridge from Zermatt into the Gotthard region, this layer is thicker and weaker. SHORT-TERM DEVELOPMENT On Sunday, it will be generally sunny. During the afternoon in southern regions, clouds will move in. The midday temperature at 2000 m will be minus 1 degree. The southwesterly wind will be blowing at light to moderate velocity. AVALANCHE DANGER FORECAST FOR SUNDAY Southern Valais; Gotthard region: Considerable avalanche danger (Level 3) The avalanche prone locations are to be found on steep slopes of northwestern to northern to southeastern exposition above approximately 2400 m in particular. A single backcountry skier or freerider can trigger avalanches in the weak old snowpack. Skiing and freeriding in outlying terrain away from secured ski runs necessitates experience in evaluating avalanche dangers. Regions north of the Rhine-Rhone line not including Gotthard region; in addition, Grisons not including Gotthard region; in addition, central and northern Ticino not including Gotthard region: Moderate avalanche danger (Level 2) In the indicated regions north of the Rhine-Rhone line, as well as from northern Praettigau over the Silvretta into the Samnaun, the avalanche prone locations are to be found on steep slopes, particularly of northwestern to northern to southeastern exposition, above approximately 2000 m. More recently formed, predominantly small sized snowdrift accumulations can still be triggered in some places. In addition, the old snowpack can also be triggered in isolated cases, generally through large additional loading. These avalanche prone locations occur increasingly frequently with ascending altitude. Particularly the transition areas from shallow to deep snow demand great caution. In the remaining regions of this danger level, the avalanche prone locations are to be found primarily on wind loaded slopes of northwestern to northern to southeastern exposition above approximately 2400 m. More recently formed and, particularly on north facing slopes, even older snowdrift accumulations are ordinarily small sized, but can still be triggered in places. Both frequency and size of the avalanche prone locations increase with ascending altitude. In all regions of the Swiss Alps below about 2500 m, particularly on steep, south facing slopes, full depth snowslides can be expected. The zones below glide cracks should be avoided whenever possible. TREND FOR MONDAY AND TUESDAY On Monday it will be heavily overcast and precipitation is expected to move in from the west. Above approximately 1400 m along the northern flank of the Alps, 10 to 20 cm of snowfall is anticipated, elsewhere less. The wind will shift from southwesterly to northwesterly. On Tuesday there will be an increased tendency towards foehn winds; it will be partly sunny and, in northern regions, mild. The avalanche danger is expected to increase slightly from region to region on Monday.