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      WSL Institute for Snow and Avalanche Research SLF
               NATIONAL AVALANCHE BULLETIN NO. 105
                for Friday, 26 February 2010
             issue date 25.2.2010, 18:30 hours
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  AVALANCHE SITUATION IS CRITICAL IN SOME REGIONS, DUE TO 
SNOWFALL AND STORM
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CURRENT CONDITIONS

On Thursday in northern regions it was predominantly sunny to 
begin with, in southern regions it was partly so. In western 
regions it increasingly became heavily overcast, then during the 
afternoon above approximately 1500 m snowfall set in. The midday 
temperature at 2000 m was minus 1 degree. A moderate 
southwesterly wind was blowing, in western regions the wind was 
intermittently strong. The wind led to the forming of fresh 
snowdrift accumulations, particularly in western regions. 
Embedded inside the snowpack near the surface are hardened 
layers, often lying atop loosely packed layers of old snow. The 
bonding of these various layers to each other is poor in some 
regions. Aforesaid conditions are particularly pronounced below 
2400 to 2700 m as well as in areas with shallow snow. At 
intermediate altitudes, the snowpack has become thoroughly wet 
on south facing slopes in particular.


SHORT-TERM DEVELOPMENT

On Friday it will be heavily overcast. Between Thursday and 
Friday afternoon above approximately 2000 m, the following 
amounts of snowfall are expected: Vaud Alps, Fribourg Alps and 
Valais 20 to 30 cm; in the furthermost western regions as much 
as 40 cm; remaining western and central parts of the northern 
flank of the Alps as well as the southern flank of the Alps 10 
to 20 cm; eastern part of the northern flank of the Alps and 
remaining Grisons maximum 10 cm. The snowfall level will drop 
from about 1500 m down to low lying areas. The midday 
temperatures at 2000 m will be minus 6 degrees in western 
regions and minus 4 degrees in eastern and southern regions. A 
strong to storm-strength wind will be blowing; it will shift 
directions from southwesterly to westerly, then to 
northwesterly. The fresh fallen snow and loosely packed snow on 
the surface of the snowpack will be transported. At high 
altitudes in the major areas of precipitation, frequent and 
large sized snowdrift accumulations are expected to form. At 
intermediate altitudes, the snowpack will deteriorate, weakened 
by the rainfall.


AVALANCHE DANGER FORECAST FOR FRIDAY

Chablais; Trient; Champex; Great St. Bernard:
  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, generally medium sized avalanches can be expected. 
These can also sweep along the moist snow at intermediate 
altitudes. Exposed parts of transportation routes may be 
placed at risk.

Remaining regions of the northern flank of the Alps and of the 
Valais; northern Ticino; Grisons:
  Considerable avalanche danger (Level 3)
In the Vaud and Fribourg Alps, in the western Bernese 
Oberland, in the remaining Valais, in central Grisons, in the 
Upper Engadine, in southern Lower Engadine, in Bergell, in 
Puschlav and in Muenstertal, the avalanche prone locations 
are to be found on steep slopes in all expositions above 
approximately 2000 m. In the remaining regions of this danger 
level, the avalanche prone locations are found primarily on 
wind loaded slopes in western to northern to southern 
exposition above approximately 2000 m. 
In the western regions, naturally triggered avalanches are 
possible, which can attain medium size. The new fallen snow 
and the fresh snowdrift can be easily triggered by a single 
backcountry skier or freerider. Particularly in the 
inneralpine regions of the Valais and Grisons, avalanches can 
fracture in the weak old snowpack. Long and profound 
experience in spotting and evaluating avalanche hazards is 
imperative.

Central Ticino; Sotto Ceneri:
  Moderate avalanche danger (Level 2)
The avalanche prone locations are to be found on steep slopes 
in all expositions above approximately 1800 m. Through the 
course of the day, fresh snowdrift accumulations which are 
prone to triggering are expected to form. A cautious route 
selection is essential.

In all regions, below about 2000 m, particularly on very steep 
hillsides and grass covered slopes, wet snow avalanches and full 
depth snowslides are possible. They can, in some areas, attain 
medium size.


TREND FOR SATURDAY AND SUNDAY

On Saturday it will be quite sunny. On Sunday in western and 
southern regions, snowfall is anticipated. In northern regions 
there will be bright intervals accompanied by strong foehn 
winds. The avalanche danger will recede on Saturday in western 
regions in particular. It is not expected to change 
significantly on Sunday.