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      WSL Institute for Snow and Avalanche Research SLF
               NATIONAL AVALANCHE BULLETIN NO. 49
                for Monday, 27 December 2010
             issue date 26.12.2010, 18:30 hours
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  THE MAJOR PERIL STEMS FROM SNOWDRIFT ACCUMULATIONS
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CURRENT CONDITIONS

On Boxing Day the skies cleared up from northwestern to southeastern regions. The midday 
temperatures at 2000 m were minus 13 degrees in northern regions and minus 9 degrees in southern 
regions. At high altitudes in the western part of the northern flank of the Alps, in Goms, in 
northern Ticino and in the Upper Engadine, the northeasterly wind was blowing at moderate to 
strong velocity, elsewhere winds were of light to moderate strength.


SHORT-TERM DEVELOPMENT

On Monday it is expected to be partially sunny. The northerly wind will be blowing at moderate to 
strong velocity Sunday night, causing old snow to be transported. During the morning hours of Monday, 
it will slacken off significantly, except in high alpine regions. The midday temperatures at 2000 m 
will be minus 5 degrees in western regions and minus 12 degrees in eastern and southern regions.


AVALANCHE DANGER FORECAST FOR MONDAY

Western and central parts of the northern flank of the Alps not including Prealps; Valais; Ticino; 
central Grisons; Upper Engadine; Val Calanca; Val Moesa; Bergell; Puschlav:
  Considerable avalanche danger (Level 3)
  
In central Valais as well as on the Main Alpine Ridge from the Great St. Bernard into the valleys 
of Visp, the avalanche prone locations are to be found on steep slopes in all expositions above 
approximately 2000 m. Both freshly formed snowdrift accumulations and layers of snow more deeply 
embedded in the snowpack can be easily triggered even by a single backcountry skier or freerider.
In the remaining regions of this danger level, the avalanche prone locations are to be found 
primarily on wind loaded slopes in eastern to southern to northwestern exposition. In the indicated 
regions of the western part of the northern flank of the Alps, the danger zones are above 
approximately 1800 m. In northern Valais, in Champex, in the Simplon region, in Goms, in the 
indicated regions of the central part of the northern flank of the Alps, in Ticino, in central 
Grisons, in the Upper Engadine, in Val Calanca, in Val Moesa, in Bergell and in Puschlav, the 
hazardous zones are above approximately 2000 m. Freshly formed snowdrift accumulations can be easily 
triggered even by a single backcountry skier or freerider. They should be avoided.

In all regions of this danger level, extensive experience in the evaluation of avalanche dangers is 
essential for backcountry skiing and freeriding tours.

Prealps; eastern part of the northern flank of the Alps; northern Grisons; Lower Engadine; 
Muenstertal:

  Moderate avalanche danger (Level 2)

The avalanche prone locations are to be found on steep slopes in all expositions. In the Prealps 
and in the eastern part of the northern flank of the Alps, the hazardous zones are above 
approximately 1600 m; in the remaining regions of this danger level, the perilous zones are above 
approximately 2000 m. Fresh snowdrift should be examined and assessed with great care, particularly 
in the western parts of the Prealps. These snowdrift accumulations can generally be easily 
recognized; they should be avoided.

In northern Grisons, in the Lower Engadine and in Muenstertal, more than anywhere else, avalanches 
can be triggered even down in lower lying layers of the snowpack, particularly through large 
additional loading. This is particularly the case in places with relatively shallow snow at the 
boundary areas of gullies, bowls or crested rims. A prudent route selection is of great importance.


TREND FOR TUESDAY AND WEDNESDAY

On Tuesday it will be heavily overcast in general, possibly accompanied by a bit of snowfall in 
northern regions. On Wednesday, it will be very sunny. Temperatures are expected to rise 
appreciably. The avalanche danger will subside.