----------------------------------------------------------------
      WSL Institute for Snow and Avalanche Research SLF
               NATIONAL AVALANCHE BULLETIN NO. 104
                for Thursday, 25 February 2010
             issue date 24.2.2010, 18:30 hours
----------------------------------------------------------------
  CONSIDERABLE AVALANCHE DANGER IN SOME REGIONS
----------------------------------------------------------------


CURRENT CONDITIONS

On Wednesday in western and northern regions it was often 
heavily overcast, on the southern flank of the Alps and in 
Grisons predominantly sunny. The midday temperature at 2000 m 
was minus 1 degree. A light southwesterly wind was blowing 
during the day. 
Hardened layers of snow near the uppermost surface often lie on 
top of loosely packed layers of the old snowpack. The bonding of 
the layers to one another is poor in some places. These 
conditions are especially pronounced below 2400 to 2700 m and in 
places with shallow snow. Due to heavy wind influence throughout 
the winter, the snow layering is more favourable at higher 
altitudes. The situation is also more favourable in outlying 
terrain which has been regularly and heavily used. Through the 
mild temperatures, the snowpack at intermediate altitudes is 
more and more often becoming thoroughly wet, particularly on 
south facing slopes.


SHORT-TERM DEVELOPMENT

On Wednesday night in western regions, a few centimeters of 
snowfall is anticipated. In the morning hours of Thursday, it 
will be intermittently sunny. During the afternoon, 
precipitation will set in from the west. By Thursday evening in 
the furthermost western regions of the Lower Valais, as much as 
10 cm of snowfall is expected, elsewhere less. The midday 
temperature at 2000 m will be minus 1 degree. The southwesterly 
wind is expected to increase significantly in velocity over the 
course of the day.


AVALANCHE DANGER FORECAST FOR THURSDAY

Northern Alpine Ridge from Chablais to the Wildstrubel region; 
Champex; Main Alpine Ridge from Grand St. Bernard into 
Simplon region; southern Goms; northern and central Grisons 
not including the regions north of the Anterior Rhine; in 
addition, Engadine; Bergell, Puschlav; Muenstertal:
  Considerable avalanche danger (Level 3)
In central Grisons not including the northern Surselva, in 
addition in Upper Engadine, in Bergell, in Puschlav, in 
southern Lower Engadine 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 danger zones are to be 
found primarily on west to north to southeast facing wind 
loaded slopes above approximately 2000 m. Avalanches can be 
easily triggered by a single backcountry skier or freerider. 
Especially in the indicated regions of central Grisons and of 
the Engadine, avalanches fracture predominantly in the weak 
old snowpack. In isolated cases, avalanches which attain 
medium size are possible. Particularly in the indicated 
regions of the Valais, fresh, relatively small sized 
snowdrift accumulations are expected to form over the course 
of the day, which must be evaluated with great care. 
Extensive experience in spotting and assessing avalanche 
hazards is important.

Remaining regions of the northern flank of the Alps, of the 
Valais and of Grisons; Ticino:
  Moderate avalanche danger (Level 2)
The avalanche prone locations are to be found on steep slopes 
in all expositions above approximately 2000 m. Snowdrift 
accumulations which have been compressed and hardened by the 
wind can be triggered as avalanches primarily through large 
additional loading. In isolated cases avalanches can fracture 
down to deeply embedded layers of the old snowpack and 
thereby attain medium size. A cautious route selection is 
essential.

In all regions, below about 2000 m particularly on steep, grass 
covered slopes, moist full depth sluffs or snowslides can be 
unleashed which can attain medium size


TREND FOR FRIDAY AND SATURDAY

On Friday, snowfall is anticipated. Most of the fresh fallen 
snow, 30 to 40 cm, is expected in Lower Valais. A strong 
westerly wind will prevail. On Saturday it will be quite sunny. 
During the afternoon, clouds will move in. The avalanche danger 
will increase widespread on Friday. It is not expected to change 
significantly on Saturday.