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      WSL Institute for Snow and Avalanche Research SLF
               NATIONAL AVALANCHE BULLETIN NO. 38
                for Monday, 21 December 2009
             issue date 20.12.2009, 18:30 hours
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  VERY DELICATE AVALANCHE SITUATION, ACCOMPANIED BY STORMY WINDS
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CURRENT CONDITIONS

On Sunday it was predominantly sunny in the Swiss Alps. The 
midday temperatures at 2000 m were minus 15 degrees in eastern 
regions, minus 12 degrees in the remaining regions. By and large 
moderate strength westerly winds were blowing. 
Over the course of the last five days on the northern flank of 
the Alps, in the Lower Valais and in Goms, there has been 15 to 
30 cm of snowfall; in some areas in the northern regions of 
Central Switzerland there was more than 50 cm of very loosely 
packed snowfall, which fell predominantly from the fogbanks 
themselves. In the remaining regions, there was less snowfall, 
or none at all. Under the influence of very low temperatures, 
the snowpack has transformed intensely, and is now very loosely 
packed in general. In all regions, layers of snow near the 
surface are faceted due to the persistently low temperatures. 
Even firmly packed layers embedded in the snowpack have become 
loosened crystals. In areas with little snow, this is the case 
for the entire depth of the snowpack.


SHORT-TERM DEVELOPMENT

On Sunday evening, the wind velocity will increase noticeably 
from the west and a bit of snowfall is anticipated. Temperatures 
are expected to rise. On Monday the midday temperatures at 2000 
m in northern regions will be minus 7 degrees, on the southern 
flank of the Alps minus 10 degrees. On Sunday night, strong to 
stormy westerly winds are expected. The winds will subsequently 
shift to southwesterly to southerly and remain strong to stormy 
at high altitudes in western regions in particular. 
Large sized, brittle snowdrift accumulations are expected to 
form atop loosely packed intermediate layers. Influenced by the 
winds, the avalanche danger will rapidly escalate. Widespread 
naturally triggered avalanches are expected.


AVALANCHE DANGER FORECAST FOR MONDAY

Swiss Alps not including central Ticino and Sotto Ceneri:
  Considerable avalanche danger (Level 3)
On the northern flank of the Alps, in the Lower Valais, in 
the northern Valais and in Goms, the avalanche prone 
locations are to be found on steep slopes in all expositions 
above approximately 1600 m. Snowdrift accumulations in 
particular can be naturally released and/or easily triggered. 
In the central regions of the northern flank of the Alps, 
medium sized avalanches are expected. 
In the southern Upper Valais not including Goms, in Grisons 
and in northern Ticino, the avalanche prone locations are to 
be found primarily on wind loaded slopes in all expositions 
above approximately 2000 m. In these regions as well, 
avalanches can be easily triggered. The avalanche prone 
locations are widespread, but potential avalanches are 
expected to be predominantly  small in size. 
In all the indicated regions, extensive experience, as well 
as detailed knowledge of avalanche perils and assessment, are 
imperative in outlying terrain away from secured ski slopes.

Central Ticino and Sotto Ceneri:
  Moderate avalanche danger (Level 2)
The avalanche prone locations are to be found primarily in 
gullies and bowls in northern to southern to western 
expositions above approximately 2000 m. The snowdrift 
accumulations are brittle and can easily be triggered. The 
major hazard with small sized avalanches is that they cause 
backcountry ski tourers and freeriders to fall.


TREND FOR TUESDAY AND WEDNESDAY

On Monday night a small amount of snowfall is expected 
widespread. Whereas in southern regions it will continue to snow 
during the morning hours on Tuesday, the skies in northern 
regions are expected to clear up, accompanied by foehn winds. On 
Wednesday, the foehn winds will slacken off and a bit of 
additional snowfall is expected down to low lying areas. On the 
southern flank of the Alps, there may well be more than 30 cm of 
snowfall. The avalanche danger will gradually subside in 
northern regions. In southern regions it is expected to increase 
in some areas.