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      WSL Institute for Snow and Avalanche Research SLF
               NATIONAL AVALANCHE BULLETIN NO. 89
                for Saturday, 5 February 2011
             issue date 4.2.2011, 18:30 hours
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  STRONG NORTHWESTERLY WINDS CAUSE FRESH SNOWDRIFT ACCUMULATIONS AT HIGH ALTITUDES
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CURRENT CONDITIONS

On Friday it was predominantly sunny in the Swiss Alps. The midday temperatures at 2000 m were zero 
degrees in northern regions and plus 3 degrees in southern regions. A westerly wind of moderate 
velocity was blowing at high altitudes.

The snow depths are below average for this juncture of the season. At 2000 m widespread, there is 
50 to 120 cm of snow on flat surfaced terrain. In the inneralpine regions of the Valais and Grisons, 
there is even less in some places.

The uppermost surface of the snowpack on steep slopes adjacent to ridge lines often consists of 
older or freshly formed snowdrift accumulations. Further distant from ridge lines, the snow 
layering is highly varied, extending from hard-compacted places to loosely packed, large-grained 
snow crystals. Increasingly frequently, steep slopes of southern exposition have crusts on the 
surface up to high altitudes, which in some places are not yet capable of bearing loads. In the 
inneralpine regions of the Valais and in Grisons, more than anywhere else, there are weak layers 
embedded inside the snowpack. In general, however, the old snowpack evidences little tendency 
towards fracture propagation, and  only in isolated cases.


SHORT-TERM DEVELOPMENT

Following a night of clear skies, it will be sunny on Saturday. The temperature will rise 
significantly, climbing to plus 4 degrees by midday at 2000 m. The wind is expected to shift to 
northwesterly and be blowing at strong velocity at high altitudes. Particularly in gullies and 
bowls, small but treacherous snowdrift accumulations are expected to form.


AVALANCHE DANGER FORECAST FOR SATURDAY

Northern flank of the Alps not including Prealps; Valais; central part of Main Alpine Ridge; 
Grisons not including lower Val Moesa and lower Val Calanca:
  Moderate avalanche danger (Level 2)
  
The avalanche prone locations are to be found primarily in gullies and bowls in all expositions. 
In the southern Valais not including Goms, the treacherous snowdrift accumulations are to be found 
above approximately 2800 m, in the remaining regions of this danger level they are found above 
approximately 2500 m. Both the freshly formed and older snowdrift accumulations are predominantly 
small sized but can be easily triggered. On backcountry skiing and freeriding tours, these places 
should be avoided whenever possible. Quite apart from the perils of being buried by snow, the 
hazards of being swept along and falling should be taken into consideration when evaluating routes.

Prealps; Ticino not including the regions along the Main Alpine Ridge; in addition, lower Val Moesa 
and lower Val Calanca:
  Low avalanche danger (Level 1)
  
Isolated avalanche prone locations are to be found particularly on wind protected slopes adjacent 
to ridge lines. The small sized snowdrift accumulations should be evaluated with great care. In high 
alpine regions, the avalanche danger level must be assessed somewhat more critically, due to the 
strong velocity winds.

On south facing slopes in all regions of the Swiss Alps, moist sluffs and generally small sized 
avalanches can be expected over the course of the day. This is particularly the case below about 
2500 m.


TREND FOR SUNDAY AND MONDAY

On both days, it is expected to be sunny in the mountains. It will remain mild. The winds will 
slacken off on Sunday. The danger of dry avalanches is expected to gradually subside. The danger 
of moist sluffs and avalanches will increase somewhat.