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      WSL Institute for Snow and Avalanche Research SLF
               NATIONAL AVALANCHE BULLETIN NO. 34
                for Thursday, 29 December 2011
             issue date 28.12.2011, 18:30 hours
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  CONSIDERABLE AVALANCHE DANGER FROM OBERALP PASS INTO ENGADINE
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CURRENT CONDITIONS

On Wednesday it was sunny. The midday temperature at 2000 m was a mild plus 4 degrees. The wind 
was blowing at light to moderate velocity, shifting from easterly to southwesterly. Particularly 
on the Main Alpine Ridge and in the Upper Engadine above approximately 2500 m, the most deeply 
embedded layers of the snowpack are faceted and inadequately consolidated in places. On north 
facing slopes there is surface hoar widespread.


SHORT-TERM DEVELOPMENT

On Thursday in western and southern regions it will be overcast, accompanied by bright intervals 
during the afternoon in particular. On the northern flank of the Alps, 5 to 10 cm of snowfall is 
anticipated down to low altitudes, elsewhere less snowfall is expected. The midday temperatures 
at 2000 m will drop noticeably: down to minus 8 degrees in northern regions and minus 6 degrees 
in southern regions. The wind will shift from westerly to northerly, be blowing at light to moderate 
velocity and will transport the new fallen snow.


AVALANCHE DANGER FORECAST FOR THURSDAY

Main Alpine Ridge from Oberalp Pass into Avers; region of Bivio; Upper Engadine and southern Lower 
Engadine:
  Considerable avalanche danger (Level 3)
  
The avalanche prone locations are to be found on steep slopes above approximately 2500 m in 
northwestern to northern to eastern aspects in particular. Avalanches can be triggered in the 
old snow, which is weak, even by one single backcountry skier or freerider and subsequently 
attain medium size. The avalanche prone locations are difficult to discern; whumpfing sounds 
are possible indicators. Caution is urged towards older snowdrift accumulations and in transition 
areas from shallow to deep snow. In outlying terrain away from secured ski runs, prudence and 
experience in evaluating avalanche dangers are imperative.

Northern flank of the Alps not including Prealps; Valais; northern Grisons; central Grisons not 
including the regions of the Main Alpine Ridge and not including the region of Bivio; in addition, 
northern Lower Engadine; Muenstertal; Puschlav; Bergell; lower Val Moesa; lower Val Calanca; 
central and northern Ticino not including Bleniotal:
  Moderate avalanche danger (Level 2)
  
The avalanche prone locations are to be found on steep slopes above approximately 2200 m in 
western to northern to eastern aspects in particular. Avalanches in the old snowpack are especially 
easily triggered where the snow is relatively shallow and in transition areas from shallow to 
deep snow. In southern Valais, in the indicated regions of the Main Alpine Ridge and in central 
Grisons, the avalanche prone locations occur more frequently above approximately 2500 m. The danger 
zones are difficult to recognize; whumpfing noises can indicate their presence. A cautious route 
selection is important.

Prealps; Sotto Ceneri:
  Low avalanche danger (Level 1)
  
Isolated avalanche prone locations are to be found primarily in shady spots with relatively shallow 
snow. Freshly formed snowdrift accumulations can be easily triggered, but are very small.

In all regions of the Swiss Alps, particularly on the northern flank of the Alps, in the Valais 
and in northern Grisons, full depth snowslides are possible below about 2400 m. They release in 
particular on steep grassy slopes and roadside embankments, especially in eastern to southern to 
western aspects.


TREND FOR FRIDAY AND NEW YEAR'S EVE

On both days it will be heavily overcast. Accompanied by storm strength northwesterly winds, 
snowfall down to low altitudes is anticipated, which will be prolonged in western and in northern 
regions. On New Year's Eve, the snowfall level will ascend up to intermediate altitudes. In the 
furthermost southern regions it will be partly sunny on both days. The avalanche danger is expected 
to escalate significantly, particularly in western and northern regions.