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      WSL Institute for Snow and Avalanche Research SLF
               NATIONAL AVALANCHE BULLETIN NO. 131
                for Saturday, 19 March 2011
             issue date 18.3.2011, 18:30 hours
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  CONSIDERABLE AVALANCHE DANGER, A TREACHEROUS AVALANCHE SITUATION IN OUTLYING TERRAIN
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CURRENT CONDITIONS

On Friday in the Valais and in southern regions it was predominantly sunny, in the western part of 
the northern flank of the Alps increasingly so. In the remaining regions, it was overcast, 
accompanied by isolated bright intervals. Between Wednesday and Friday morning from the eastern 
Bernese Alps to Liechtenstein, in northern Grisons, in the northern Lower Engadine, in the Upper 
Engadine and on the Main Alpine Ridge from the Great St. Bernard into the Gotthard region, there 
was 20 to 40 cm of snowfall, elsewhere 10 to 20 cm of new fallen snow. The midday temperatures at 
2000 m on Friday were zero degrees in western and southern regions and minus 2 degrees in eastern 
regions. The wind in western regions was blowing at light to moderate strength and in southern and 
eastern regions at moderate to strong velocity from northerly directions.

The old snowpack is thoroughly wet in all expositions below about 2200 m. At high altitudes, the new 
fallen snow and the snowdrift from the last few days, more than anything else, can be easily 
triggered.


SHORT-TERM DEVELOPMENT

On Friday night, snowfall is expected to set in. During the day on Saturday, it will be heavily 
overcast for the most part. South of the Main Alpine Ridge, the skies will brighten up increasingly 
over the course of the day. By Saturday evening above approximately 1500 m on the northern flank 
of the Alps, 10 to 20 cm of snowfall is anticipated; in the Valais and in Grisons, 5 to 10 cm of 
fresh fallen snow. The midday temperatures at 2000 m will be minus 4 degrees in northern regions, 
minus 1 degree in southern regions. The northerly wind will be blowing at light to moderate velocity 
and become increasingly brisk in strength during the afternoon.

The new fallen snow blankets over the hazardous zones. Particularly in areas adjacent to ridge lines 
and in pass areas, new snowdrift accumulations, generally of small size, will form.


AVALANCHE DANGER FORECAST FOR SATURDAY

Entire region of the Swiss Alps:
  Considerable avalanche danger (Level 3)
  
On the northern flank of the Alps from the eastern Bernese Oberland to Liechtenstein, in the Upper 
Valais, in Ticino and in Grisons, the avalanche prone locations are to be found on steep slopes in 
all expositions above approximately 2000 m. New fallen snow and snowdrift layers from the last few 
days can be triggered even by a single backcountry skier or freerider. Remote triggering from well 
outside the starting zone is possible. Avalanches can attain medium size.

On the northern flank of the Alps west of the Wildstrubel and in the Lower Valais, the avalanche 
prone locations are to be found primarily on steep slopes of western to northern to eastern 
exposition, and in areas adjacent to ridge lines and pass areas in all expositions, above 
approximately 2200 m. Somewhat less recent as well as freshly formed snowdrift accumulations can 
be triggered by a single backcountry skier or freerider in some places.

For skiing and freeriding tours in outlying terrain in all regions of the Swiss Alps, both 
experience in the evaluation of avalanche hazards and caution are imperative.

In all regions below about 2200 m, moist and wet avalanches can be naturally triggered. This is 
particularly the case on steep slopes which have not yet discharged. Backcountry skiers and 
freeriders in outlying terrain away from secure ski runs can trigger avalanches in the thoroughly 
wet old snowpack particularly between 1800 and 2200 m. This is especially the case on north facing 
slopes.


TREND FOR SUNDAY AND MONDAY

On Saturday night, the snowfall will come to an end in northern regions. On both days, it is 
expected to be predominantly sunny. The avalanche danger will subside incrementally. The danger 
of wet snow avalanches will be increasingly subject to the daytime warming cycle.