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      WSL Institute for Snow and Avalanche Research SLF
               NATIONAL AVALANCHE BULLETIN NO. 107
                for Sunday, 28 February 2010
             issue date 27.2.2010, 18:30 hours
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  AMIDST STORM-STRENGTH WINDS, ONGOINGLY CRITICAL AVALANCHE 
SITUATION
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CURRENT CONDITIONS

On Saturday it was sunny to begin with; increasingly during the 
afternoon it turned heavily overcast from the west. The midday 
temperatures at 2000 m were minus 3 degrees, in northwestern 
regions zero degrees. During the night a moderate to strong 
northwesterly wind prevailed in some regions, during the day 
today it shifted to southwesterly and blew at moderate strength. 
The layers of fresh fallen snow from this week are gradually 
settling and consolidating, particularly on south facing slopes. 
In all other expositions, the layers nearest the surface are 
still loosely packed, thus, they can be transported. Embedded in 
the old snowpack are loosely packed layers which are prone to 
triggering, especially prevalent below 2400 to 2700 m and in 
areas with shallow snowpack. At intermediate altitudes, the 
snowpack has become thoroughly wet, particularly so in western 
regions.


SHORT-TERM DEVELOPMENT

Tonight it will be heavily overcast and in southern regions 
precipitation is expected to set in. On Sunday in northern and 
eastern regions it will be variably cloudy, in western and 
southern regions heavily overcast. In southern regions the 
precipitation is expected to continue throughout the day. By 
Sunday afternoon in the western part of the Main Alpine Ridge 
and on the southern flank of the Alps, 10 to 20 cm of snowfall 
is expected. In the western and central parts of the northern 
flank of the Alps, in the remaining Valais, in the Gotthard 
region and in the Upper Engadine, 5 to 10 cm of snowfall is 
anticipated, in the areas bordering to the north only a few 
centimeters. During the night tonight, the freezing level in 
northern regions is expected to ascend towards 3000 m. During 
the day tomorrow, the temperature will drop: at 2000 m it is 
expected to be minus 1 degree at midday in northern regions, 
minus 4 degrees in southern regions. A strong to stormy 
southwesterly wind will be blowing. In the northernmost regions 
exposed to foehn influence, storm-strength foehn winds are 
expected to prevail. These will lead to compact, trigger 
sensitive snowdrift accumulations forming which in western 
regions in particular will be large sized.


AVALANCHE DANGER FORECAST FOR SUNDAY

Northern Alpine Ridge from Chablais to Wildstrubel; western 
Lower Valais:
  High avalanche danger (Level 4)
The avalanche prone locations are to be found on steep slopes 
in all expositions above approximately 2000 m. Naturally 
triggered, predominantly medium sized avalanches can be 
expected. The avalanche situation is very critical for 
backcountry skiers and freeriders in outlying terrain away 
from secured ski runs in particular. Exposed sections of 
transportation routes can be placed at risk.

Remaining regions of northern flank of the Alps and of Valais; 
Grisons; Ticino:
  Considerable avalanche danger (Level 3)
In the remaining western part of the northern flank of the 
Alps, the avalanche prone locations are to be found on steep 
slopes in all expositions above approximately 1600 m. In the 
remaining Valais, in Ticino and in Grisons, they are found on 
steep slopes in all expositions above approximately 1800 m. 
Avalanches which are capable of attaining medium size can be 
easily triggered even by a single backcountry skier or 
freerider. Naturally triggered avalanches are possible. 
Particularly in the inneralpine regions of the Valais and 
Grisons, avalanches can also sweep along deeply embedded 
layers of the old snowpack. In isolated cases, exposed parts 
of transportation routes can be endangered. 
In the central and eastern parts of the northern flank of the 
Alps, the avalanche prone locations are to be found primarily 
on southwest to north to southeast facing wind loaded slopes 
above approximately 1600 m. Freshly formed snowdrift 
accumulations can be released as avalanches even by one 
single backcountry skier or freerider. 
In outlying terrain away from secured ski runs, extensive 
experience in spotting and evaluating avalanche hazards, 
together with great restraint, are essential.

On the southern flank of the Alps below about 1500 m, in the 
remaining regions below about 2000 m, moist avalanches and full 
depth wet snowslides are possible. Such slides can attain medium 
size in some areas.


TREND FOR MONDAY AND TUESDAY

On Monday it will be variably cloudy in northern regions, in 
southern regions it will be sunny. On Tuesday, a bit of snowfall 
is expected. The avalanche danger will incrementally decrease.