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      WSL Institute for Snow and Avalanche Research SLF
               NATIONAL AVALANCHE BULLETIN NO. 99
                for Saturday, 20 February 2010
             issue date 19.2.2010, 18:30 hours
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  SNOW DRIFT ACCUMULATIONS MOSTLY COVERED BY FRESH SNOW
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CURRENT CONDITIONS

Friday was very cloudy in all regions of the Swiss Alps, and 
snow fell in the west, south and the Bernina region in 
particular. Since Thursday, approximately 5 cm of snow have 
fallen in the west, and 10 to 20 cm in the other forenamed 
regions. Larger quantities have fallen in some localities on the 
southern flank of the Alps and in the Bernina region. Further 
north and east, only a few centimetres of snow have fallen. The 
midday temperature at 2000 m was about minus 3 degrees. The 
southerly wind was occasionally strong at high altitude, but 
otherwise mostly light to moderate. During the afternoon its 
direction changed to northerly. Over three days and with some 
interruptions, 15 to 30 cm of snow have fallen in the extreme 
west of Lower Valais and on the main Alpine ridge and to the 
south, but less snow has fallen elsewhere. 
During Thursday night, the southerly wind will be strong at 
times and transport further snow, in particular above the tree 
line. Both the freshly transported snow and the snow drift 
accumulations that formed in midweek are prone to triggering. 
Underneath the snow drift accumulations, the snowpack is moist 
on south facing slopes up to intermediate altitudes. In other 
aspects, it is faceted and loosely bonded over a wide area down 
to low altitudes.


SHORT-TERM DEVELOPMENT

On Saturday, a further 5 cm to 15 cm of snow will fall above 
approximately 1000 m, in particular in the north and the 
mountains of Grisons. During the day the weather will brighten 
up temporarily from the west and south. The weather will become 
appreciably cooler. The midday temperature at 2000 m will be 
about minus 9 degrees. The northwesterly wind will be mostly 
moderate. 
The falling temperature will cause bonding of the slightly moist 
snowpack up to high altitudes, in particular on south facing 
slopes. Very little change will take place on north facing 
slopes.


AVALANCHE DANGER FORECAST FOR SATURDAY

Chablais; Vaud Alps; central part of the northern flank of the 
Alps excluding the prealps; western Lower Valais; main 
Alpine ridge; central Grisons; Upper Engadine; southern 
flank of the Alps:
  Considerable avalanche danger (level 3)
In Chablais, the Vaud Alps and the central part of the 
northern flank of the Alps excluding both the prealps and the 
Gotthard region, the avalanche prone locations are to be 
found, in particular, on southwest to north to southeast 
facing wind-loaded slopes above approximately 1800 m. In 
western Lower Valais, on the main Alpine ridge, in central 
Grisons and Upper Engadine and on the southern flank of the 
Alps, the avalanche prone locations are to be found on steep 
slopes of all aspects above approximately 2000 m. Visible and 
covered snow drift accumulations are particularly dangerous. 
The bonding of these accumulations with the old snowpack 
remains poor in places. They can be released easily. 
Avalanches can therefore release parts of the old snowpack as 
well. For those venturing off piste, a lot of experience in 
the assessment of avalanche danger is required.

Prealps; other regions on the northern flank of the Alps; Valais 
excluding both western Lower Valais and the regions on the 
main Alpine ridge; northern Grisons; northern Lower 
Engadine:
  Moderate avalanche danger (level 2)
In the prealps and the other regions on the northern flank of 
the Alps, the avalanche prone locations are to be found on 
steep slopes above approximately 1800 m. In the relevant 
regions of Valais and in northern Grisons and northern Lower 
Engadine, the avalanche prone locations are to be found on 
steep slopes above approximately 2000 m. Avalanches can be 
released in particular at transitions from a shallow to a 
deep snowpack. Fresh snow drift accumulations are also to be 
assessed especially critically. Careful route selection is 
still recommended.

The danger of full-depth avalanches, including slab avalanches, 
will decrease temporarily above approximately 1000 m.


TREND FOR SUNDAY AND MONDAY

Sunday will be generally sunny. Towards the evening, light 
snowfall may occur again in the west and south. On Monday, the 
weather will be changeable in the north and sunny in the south. 
The avalanche danger will decrease slowly.