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      WSL Institute for Snow and Avalanche Research SLF
               NATIONAL AVALANCHE BULLETIN NO. 41
                for Sunday, 19 December 2010
             issue date 18.12.2010, 18:30 hours
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  WINDS AT STRONG TO STORM VELOCITY, FAR REACHING SNOW TRANSPORT
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CURRENT CONDITIONS

Between Thursday evening and Saturday morning on the northern flank of the Alps, in the Lower Valais 
and in Goms, there was 20 to 50 cm of snowfall. In the remaining Valais as well as north of the 
Anterior Rhine and in the Praettigau, there was 10 to 20 cm of fresh fallen snow; in the mountains 
of Ticino and in remaining Grisons, there was less than 10 cm. On Saturday in northern regions it 
was overcast in most areas, in the remaining regions predominantly sunny. The midday temperatures 
at 2000 m were approximately minus 15 degrees. The winds from a generally westerly direction were 
blowing at light to moderate velocity.

The snow depths correspond to long term averages. At 2000 m there is 50 to 100 cm of snow; in the 
eastern part of the northern flank of the Alps and in western Ticino as much as 130 cm. The snowdrift 
accumulations are lightly bonded and blanketed over with loosely packed snow.


SHORT-TERM DEVELOPMENT

On Sunday, weather conditions are expected to be variable. Towards evening in the southern regions, 
a few centimeters of snowfall is anticipated. The temperatures are expected to rise, particularly 
in northern regions. At 2000 m in northern regions it will be minus 6 degrees at midday, in southern 
regions minus 9 degrees. The wind will shift to southwesterly and be blowing at strong to storm 
velocity.

At the uppermost surface, the snowpack is lightly bonded in places. Through the strong to 
storm-strength winds and the corresponding snow transport which the winds will bring about, an 
escalation of avalanche danger is expected in all regions of the Swiss Alps over the course of the 
day. However, danger level 4, "high", will not be reached.


AVALANCHE DANGER FORECAST FOR SUNDAY

Jura; northern flank of the Alps; Valais:
  Considerable avalanche danger (Level 3)
The avalanche prone locations are to be found primarily on wind loaded slopes of western to 
northern to southern exposition. In the Jura, the danger zones occur above approximately 1400 m, 
amidst storm velocity winds. On the northern flank of the Alps, in the Lower Valais not including 
the Main Alpine Ridge, in northern Valais and in Goms, the hazardous zones occur above approximately 
1800 m. In the western part of the Main Alpine Ridge and in the remaining southern Upper Valais, 
they are to be found above approximately 2400 m. In isolated cases, naturally triggered avalanches 
are possible on wind loaded slopes. Even a single backcountry skier or freerider can trigger 
avalanches. In outlying terrain away from secured ski slopes, extensive experience in the evaluation 
of avalanche dangers is essential.

Ticino; Grisons:
  Moderate avalanche danger (Level 2)
The avalanche prone locations are to be found primarily on wind loaded slopes in all expositions. 
In northern Ticino and in northern and central Grisons, the wind loaded slopes above approximately 
1800 m, in central Ticino, Sotto Ceneri, the Engadine and the southern valleys of Grisons the wind 
loaded slopes above approximately 2000 m, are especially critical and must be evaluated with 
particular care. A prudent route selection and circumvention of freshly formed snowdrift 
accumulations are recommended.


TREND FOR MONDAY AND TUESDAY

On Monday, weather conditions in northern regions are expected to be variable, accompanied by a 
small amount of snowfall. In southern regions it will be sunny. On Tuesday in northern regions, it 
will be variably cloudy; whereas in southern regions some snowfall is anticipated. Temperatures are 
expected to rise. The avalanche danger will subside only incrementally.