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      WSL Institute for Snow and Avalanche Research SLF
               NATIONAL AVALANCHE BULLETIN NO. 141
                for Saturday, 3 April 2010
             issue date 2.4.2010, 18:30 hours
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  CAUTION IS URGED TOWARDS FRESH SNOWDRIFT
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CURRENT CONDITIONS

On Good Friday in the Swiss Alps, it was predominantly sunny. The midday temperature at 2000 m was minus 6 degrees. A generally light southwesterly wind was blowing, in western regions the wind was intermittently of moderate velocity.
On steep, sun bathed slopes, the snow has become sticky as far up as nearly 3000 m. On north facing slopes, loosely packed snow can be found far and wide. The layers of new fallen snow and snowdrift from Easter week have by now bonded with each other quite well in most regions. They have been deposited on top of a crust; beneath that crust, the snow is moist and soft below about 2200 m. The snow layering is fragile far and wide, except in the furthermost western regions and on the southern flank of the Alps; it is weakest of all in the inneralpine regions.


SHORT-TERM DEVELOPMENT

On Holy Saturday, a southerly foehn-dominated air current will reach the Alps. In the regions which are exposed to foehn wind influence, it will be partly sunny. Elsewhere it will be heavily overcast by and large and a few centimeters of snowfall are anticipated. The southwesterly wind will increase in strength and during the afternoon it will be blowing at strong velocity. The midday temperatures at 2000 m will be zero degrees in northern regions and minus 3 degrees in southern regions.
The loosely packed snow on north facing slopes will be transported and bonded. Snowdrift accumulations are expected to form which will be easily triggered.


AVALANCHE DANGER FORECAST FOR SATURDAY

Northern flank of the Alps not including Prealps; Valais; northern and central Ticino; Grisons:
  Considerable avalanche danger (Level 3)
The avalanche prone locations are to be found primarily on slopes of western to northern to southeastern exposition. In southern Valais from Val d'Herens into the Mattertal, in the inneralpine regions of Grisons, in the Lower Engadine and in the Muenstertal, the avalanche prone locations are to be found on steep slopes above approximately 2200 m for the most part; in the remaining regions of this danger level, predominantly on wind loaded slopes approximately 2000 m. Snowdrift accumulations are expected to expand in size and frequency during the day, so that the danger level "considerable" will be reached over the course of the day. Snowdrift accumulations should be avoided. Extensive experience in spotting and evaluating avalanche hazards is essential.
In addition to the above, in southern Valais from Val d'Herens into the Mattertal, in the inneralpine regions of Grisons, in the Lower Engadine and in the Muenstertal, avalanches of medium size can fracture in more deeply embedded layers of the old snowpack. It is also possible to trigger avalanches in terrain which has been heavily frequented. The situation is highly treacherous. Defensive conduct, such as maintaining distances between persons to relieve loads, and one-at-a-time descents, reduce the risks of groups being buried by an avalanche.

Prealps; Sotto Ceneri:
  Moderate avalanche danger (Level 2)
The avalanche prone locations are to be found primarily on wind loaded slopes in western to northern to southeastern exposition above approximately 2000 m. Freshly formed snowdrift accumulations can be easily triggered. They tend to remain relatively small sized, however.

In those areas exposed to foehn wind influence more than anywhere else, wet snow avalanches are possible below about 2200 m in isolated cases.


TREND FOR EASTER SUNDAY AND EASTER MONDAY

On Easter Sunday it will be heavily overcast in general and snowfall is anticipated widespread, most of which will fall between the Simplon region and western Ticino. On Easter Monday, conditions will become more pleasant from the west; on the southern flank of the Alps it will be predominantly sunny, accompanied by northerly foehn winds. On Easter Sunday in most regions of the Swiss Alps, the avalanche prone locations will become more widespread.