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      WSL Institute for Snow and Avalanche Research SLF
               NATIONAL AVALANCHE BULLETIN NO. 144
                for Friday, 1 April 2011
             issue date 31.3.2011, 18:30 hours
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  DANGER OF WET AVALANCHES WILL RISE SWIFTLY TO LEVEL 3, "CONSIDERABLE"
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CURRENT CONDITIONS

On Thursday morning it was sunny. At midday, clouds moved in from the west and on the northern flank 
of the Alps above approximately 2000 m, there was a few centimeters of snowfall. The midday 
temperature at 2000 m was plus 3 degrees. The westerly wind was blowing at light to moderate 
strength.

On north facing slopes more than anywhere else, layers of bonded snow lie on top of a soft, faceted 
old snowpack. These bonded layers are prone to triggering from place to place, particularly in the 
southern sectors of the Valais and in the inneralpine regions of Grisons.


SHORT-TERM DEVELOPMENT

On Thursday night in eastern regions above 2000 m, a small amount of snowfall is anticipated. 
Thereafter, even there the skies will clear up from the southwest. On Friday it will be 
predominantly sunny. The temperature will increase; by midday it will be plus 6 degrees at 2000 m. 
The freezing level will ascend to 3000 m. During the night the northwesterly wind will be blowing 
at strong velocity in high alpine regions, elsewhere at light to moderate strength.
In northeastern regions, where the skies will only clear up in the latter part of the night, the 
snowpack surface will freeze but superficially. The snow surface will then rapidly soften up over 
the course of the day in all regions of the Swiss Alps, due to the warmth and the solar radiation.


AVALANCHE DANGER FORECAST FOR FRIDAY

Northern flank of the Alps not including Prealps, not including the Alpstein-Alvier region and not 
including Liechtenstein; in addition, Valais; northern Ticino; Grisons not including northern 
Praettigau:
  Moderate danger of dry avalanches (Level 2)
  
The avalanche prone locations are to be found primarily on steep slopes of northwestern to northern 
to eastern exposition above approximately 2200 m. In isolated cases, avalanches can be triggered in 
the old snowpack and attain dangerous size. In southern Valais, in the inneralpine regions of Grisons 
as well as in all seldom frequented backcountry skiing terrain in general, the avalanche prone 
locations occur more frequently. Whumpfing sounds or skiers breaking through the weakly consolidated 
old snowpack can be indicators of imminent danger. Through the daytime warming cycle, the likelihood 
of dry avalanches being triggered will escalate over the course of the day, even at high altitudes.
In high alpine regions in areas adjacent to ridge lines, there are freshly formed snowdrift 
accumulations. They should be avoided.

Prealps; Alpstein-Alvier region; Liechtenstein; northern Praettigau; central Ticino; Sotto Ceneri:
  Low danger of dry avalanches (Level 1)
  
In very isolated cases, avalanches can be triggered in the old snowpack. This applies particularly 
to seldom frequented, north facing slopes. Through the daytime warming cycle, the likelihood of dry 
avalanches being triggered is expected to escalate.

Danger of wet avalanches:
In all regions of the Swiss Alps, the danger of wet snow avalanches will swiftly increase to 
Level 3, "considerable", over the course of the day. Below about 2800 m on south facing slopes, 
and below about 2400 m on north facing slopes, wet avalanches can be naturally triggered. Yet 
moist or wet avalanches can also be triggered by skiers and freeriders, especially in regions 
with weak snow layering. This is particularly the case in southern Valais and in the inneralpine 
regions of Grisons. Skiing and freeriding tours in outlying terrain as well as ascents to huts 
should be launched very early in the morning and brought to an end early in the day.


TREND FOR SATURDAY AND SUNDAY

On both days, sunny weather conditions are expected. On Saturday, the freezing level will ascend 
to 3500 m. Above the treeline during clear nights, the snowpack will freeze and form a crust 
capable of bearing loads, bringing about favourable backcountry touring conditions widespread 
during the morning hours. Then, during the day, the danger of wet avalanches will escalate rapidly 
and significantly.