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      WSL Institute for Snow and Avalanche Research SLF
               NATIONAL AVALANCHE BULLETIN NO. 143
                for Monday, 5 April 2010
             issue date 4.4.2010, 18:30 hours
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  AVALANCHE DANGER IS CONSIDERABLE WIDESPREAD
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CURRENT CONDITIONS

On Easter Sunday it was heavily overcast and there was snowfall down to low altitudes from region to region. On the Main Alpine Ridge from the Gotthard region to the Bernina Pass and south thereof, there was 30 to 50 cm of snowfall; in the Chablais, the Vaud and Fribourg Alps, there was 10 to 30 cm, elsewhere 5 to 15 cm of snowfall. In the Lower Engadine it has thus far remained dry. The midday temperatures at 2000 m were minus 4 degrees in western regions and minus 2 degrees in eastern regions. A light to moderate southerly wind was blowing. On north facing slopes, the new fallen snow and snowdrift have been deposited on top of a snowpack surface which is often loosely packed; thus, they are consolidating poorly with the old snow. On south facing slopes, the snowpack surface has a crust up to approximately 3000 m. The snow below about 2200 m altitude is moist and soft in more deeply embedded layers. The snow layering is generally fragile, 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

By evening on Easter Monday in the eastern part of the northern flank of the Alps, in the Praettigau, the Silvretta and the Samnaun, 10 to 20 cm of snowfall is anticipated; in the remaining regions generally less than 10 cm of fresh fallen snow. As of about midday it will turn increasingly sunny in northern regions, as it already was previously in the Valais. On the southern flank of the Alps it will be predominantly sunny. The midday temperatures at 2000 m will be minus 6 degrees in northern regions, minus 3 degrees in southern regions. A moderate to strong northerly wind will be blowing, in Ticino the winds will blow down to low altitudes.
The loosely packed snow will be massively transported far and wide.


AVALANCHE DANGER FORECAST FOR EASTER MONDAY

Vaud and Fribourg Alps; western Bernese Oberland; northern Alpine Ridge; Valais; Ticino; Grisons:
  Considerable avalanche danger (Level 3)
On the Main Alpine Ridge from the Gotthard region to the Bernina Pass and south thereof, the avalanche prone locations are to be found on steep slopes in all expositions above approximately 1800 m. In outlying terrain away from secured ski runs, conditions are treacherous. Naturally triggered avalanches are possible, which in turn can attain medium size. A great deal of experience in spotting and evaluating avalanche hazards is essential.
In southern Valais from Val d'Herens into the Mattertal, in the inneralpine regions of northern and central Grisons, in the remaining Upper Engadine, in the Lower Engadine south of the Inn River and in the Muenstertal, the avalanche prone locations are to be found primarily on steep slopes in all expositions above approximately 2200 m. Freshly formed snowdrift accumulations should be assessed with great care and avoided. They can also be naturally triggered, more than anywhere else in the Engadine. Furthermore, avalanches of medium size can fracture in deeply embedded layers of the old snowpack. The situation is highly treacherous. Defensive conduct, such as maintaining distances between persons to relieve loads, and descending one at a time, reduce the risks of groups being buried by an avalanche.
In the Vaud and Fribourg Alps, in the western Bernese Oberland, on the northern Alpine Ridge as well as in the remaining regions of the Valais and Grisons, the avalanche prone locations are to be found primarily on wind loaded slopes in all expositions above approximately 2000 m. Freshly formed snowdrift accumulations can be easily triggered; they should be avoided.

Eastern Bernese Oberland not including the regions of the northern Alpine Ridge; in addition, central and eastern Prealps:
  Moderate avalanche danger (Level 2)
The avalanche prone locations are to be found primarily on wind loaded slopes in all expositions above approximately 1800 m. Freshly formed snowdrift accumulations can be easily triggered, although they are generally small sized. Through a cautious route selection, they should be avoided.

In southern regions more than anywhere else, wet snowslides are possible on very steep, grass covered, sun bathed slopes below about 2000 m.


TREND FOR TUESDAY AND WEDNESDAY

On Tuesday and Wednesday, quite sunny weather is anticipated. Following a clear and cold night on Monday, it will turn noticeably warmer in the course of the day on Tuesday. The danger of dry avalanches will diminish. The danger of wet snow avalanches will be subject to the daytime warming cycle.