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      WSL Institute for Snow and Avalanche Research SLF
               NATIONAL AVALANCHE BULLETIN NO. 138
                for Saturday, 26 March 2011
             issue date 25.3.2011, 18:30 hours
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  AVALANCHE DANGER BOTH CONSIDERABLE AND MODERATE, CAUTION URGED ON NORTH FACING SLOPES
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CURRENT CONDITIONS

On Friday it was sunny. During the afternoon, isolated convective cloud built up. The midday 
temperature at 2000 m was plus 4 degrees. The wind was blowing at light to moderate velocity from 
southwesterly directions.

On north facing slopes more than anywhere else, layers of bonded snow lie on top of a soft, faceted 
old snowpack. These bonded layers are prone to triggering from place to place, particularly in the 
southern regions of the Valais and in the inneralpine regions of Grisons. During the morning hours, 
south facing slopes are capable of bearing loads, but subsequently soften up over the course of the 
day. On east and west facing slopes, the surface crust of the snowpack is capable of bearing loads 
only to a certain extent, and in some places.

SHORT-TERM DEVELOPMENT

On Friday night, the skies are expected to be predominantly clear. On Saturday morning, it will be 
partly sunny. During the afternoon, clouds will move in from the north and scattered showers are 
possible. The midday temperature at 2000 m will be plus 3 degrees. The wind will be blowing at light 
to moderate strength from westerly to northwesterly directions.

AVALANCHE DANGER FORECAST FOR SATURDAY

Main Alpine Ridge from Great St. Bernard to the Monte Rosa region; Val d'Anniviers; Turtmanntal; 
lower valleys of Visp; northern Simplon region; southern Goms:
  Considerable avalanche danger (Level 3)

The avalanche prone locations are to be found primarily on steep slopes in western to northern to 
eastern exposition. In the indicated regions of Lower Valais, the hazardous zones are found above 
approximately 2400 m; in the indicated regions of Upper Valais, they are found above approximately 
2200 m.

In isolated cases, avalanches attaining medium size can be triggered even by a single backcountry 
skier or freerider. On north facing slopes, such avalanches can fracture down more deeply inside 
the old snowpack. The avalanche prone locations are most widespread in outlying touring terrain 
which is not as heavily frequented. Alarm signals such as whumpfing sounds are indicators of the 
imminent danger. For backcountry skiing and freeriding tours in outlying terrain away from secured 
ski runs, experience in the assessment of avalanche danger is essential.

Northern flank of the Alps; northern Valais; remaining southern Valais; Ticino; Grisons:
  Moderate avalanche danger (Level 2)

The avalanche prone locations are to be found primarily on steep slopes in western to northern to 
eastern exposition. On the northern flank of the Alps, the hazardous zones are found above 
approximately 2000 m; in the indicated regions of the Valais, Ticino and Grisons, the perilous 
zones are found above approximately 2200 m. Avalanches can be triggered primarily by large additional 
loading, but in some places, particularly in the inneralpine regions of Grisons, can even be 
unleashed by a single backcountry skier or freerider. Avalanches are capable of attaining a dangerous 
size. In relatively unfrequented terrain, the avalanche prone locations are more widespread than in 
heavily frequented terrain. A defensive route selection is sensible.

In all regions of the Swiss Alps, the danger of wet snow avalanches will escalate slightly over the 
course of the day. Particularly on steep, east, south and west facing slopes below about 2400 m, 
naturally triggered wet snowslides and wet snow avalanches are possible. Backcountry skiing and 
freeriding tours in outlying terrain should be brought to an end early in the day.


TREND FOR SUNDAY AND MONDAY

On Sunday and Monday, it is expected to be predominantly overcast. Above approximately 1500 to 1800 m, 
a small amount of snowfall is anticipated. The danger of wet snow avalanches will subside. The danger 
of dry avalanches may increase somewhat from region to region on Monday.