----------------------------------------------------------------
      WSL Institute for Snow and Avalanche Research SLF
               NATIONAL AVALANCHE BULLETIN NO. 28
                for Friday, 23 December 2011
             issue date 22.12.2011, 18:30 hours
----------------------------------------------------------------
  CRITICAL AVALANCHE SITUATION WIDESPREAD, ACCOMPANIED BY SNOWFALL AND WARMTH
----------------------------------------------------------------


CURRENT CONDITIONS

On Thursday there was snowfall; only in the furthermost southern regions were there bright 
intervals. Between Tuesday morning and Thursday evening on the northern flank of the Alps from 
the Bernese Oberland to Liechtenstein, in the Gotthard region and in northern Grisons there was 
60 to 100 cm of snowfall; in the Glarner Alps as much as 120 cm. Elsewhere there was 30 to 60 cm 
of new fallen snow widespread; in central Ticino and Sotto Ceneri, in the Engadine south of the 
Inn River and in the southern valleys of Grisons, markedly less fresh fallen snow. On Thursday 
the snowfall level ascended to approximately 1800 m in western regions, towards 1200 m in eastern 
regions. In the Alpine valleys of eastern regions it generally snowed down to the valley floor. 
The wind was blowing at moderate to strong velocity from the northwest.

Particularly in northern and eastern regions where snowfall was heaviest, the deep layers of new 
fallen snow and snowdrift are instable. From southern Upper Valais over northern Ticino into the 
Bernina region above approximately 2500 m, the lowermost layers of the snowpack are faceted and 
inadequately consolidated in places.


SHORT-TERM DEVELOPMENT

After midnight, the snowfall will slacken off in northern regions. By Friday morning on the 
northern flank of the Alps from the eastern Bernese Oberland into Liechtenstein and in the 
Praettigau, an additional 20 to 40 cm of snowfall is anticipated, from place to place even more. 
From the western Bernese Oberland over the Gotthard region to northern and central Grisons, 
10 to 20 cm of fresh fallen snow is expected, elsewhere less. The snowfall level will be at 1900 m 
in western regions, at 1000 to 1500 m in eastern regions. On Friday in northern regions it will 
turn increasingly sunny, in southern regions it will be predominantly so, and very mild. The midday 
temperatures at 2000 m will be plus 5 degrees in western regions, plus 1 degree in eastern regions. 
The northwesterly wind will be blowing at moderate velocity.


AVALANCHE DANGER FORECAST FOR FRIDAY

Northern Alpine Ridge from Wildstrubel to Liechtenstein; southern Obergoms; southern Urseren; 
northern Grisons: High avalanche danger (Level 4)

The avalanche prone locations are to be found on steep slopes of all expositions above 
approximately 2000 m. Increasingly frequent naturally triggered avalanches can be expected. 
These can even attain large size and place exposed transportation routes at risk. Due to the 
significant rise in temperatures and the solar radiation, naturally triggered avalanches can 
even be expected after the precipitation has come to an end. In outlying terrain away from 
secured ski runs, the greatest possible caution and restraint are called for.

Remaining northern flank of the Alps; remaining Valais; northern Ticino; central Grisons not 
including northern Tavetsch; Engadine: Considerable avalanche danger (Level 3)

The avalanche prone locations are to be found on steep slopes in all aspects above approximately 
1800 m. Avalanches can be triggered even by a single backcountry skier or freerider. Particularly 
in those regions bordering the regions with "high" avalanche danger, naturally triggered 
avalanches are possible. Due to the markedly higher temperatures and solar radiation, they can 
be triggered at any time throughout the day. In isolated cases the avalanches can attain large 
size and endanger exposed sectors of transportation routes. Extensive experience in evaluating 
avalanche dangers is imperative when skiing and freeriding in outlying terrain away from secured 
ski runs.

Central Ticino; Sotto Ceneri; southern valleys of Grisons: Moderate avalanche danger (Level 2)

The avalanche prone locations are to be found primarily on wind loaded slopes in all expositions 
above approximately 1800 m. Both freshly formed and older snowdrift accumulations should be assessed 
critically. A cautious route selection is important.

In all regions of the Swiss Alps, but particularly in the regions with heaviest snowfall along the 
northern flank of the Alps, in the Valais and in northern Grisons, full depth snowslides and full 
depth avalanches can be expected increasingly frequently below about 2400 m, due to the warmth and 
rainfall. They can be released with particular ease on steep, grassy slopes and roadside embankments. 
Due to the massive amounts of snow, they can attain dangerous size and endanger local transportation 
routes.


TREND FOR SATURDAY AND SUNDAY

On Friday night, temperatures will drop noticeably. In northern regions there will be snowfall down 
to the lowlands. In southern regions it will remain generally sunny. On Sunday, it will be 
predominantly sunny. The avalanche danger is expected to recede.