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      WSL Institute for Snow and Avalanche Research SLF
               NATIONAL AVALANCHE BULLETIN NO. 156
                for Sunday, 18 April 2010
             issue date 17.4.2010, 18:30 hours
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  A GENERALLY FAVOURABLE AVALANCHE SITUATION - CAUTION TOWARDS DAYTIME CYCLE
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CURRENT CONDITIONS

On Friday evening, local flurries above approximately 1800 m brought a few centimeters of new fallen snow widespread. Friday night then turned clear far and wide, although it was intermittently overcast in Grisons. During the day on Saturday there was general sunshine in the mountains. Towards evening, convective cloud built up and in Ticino showers set in. The midday temperature at 2000 m was plus 2 degrees. A light easterly wind was blowing.
The snow depths widespread are significantly below average for this juncture in the season, only on the southern flank of the Alps do they correspond to long- term mean depths. The snowline altitudes north of the Rhine-Rhone line on north facing slopes are at 1300 to 1500 m, south of the Rhine-Rhone line the snowlines are at 1500 to 1900 m. On south facing slopes the snowlines are approximately 200 m higher. On steep, north facing slopes above approximately 2500 m, there is still powder snow in some places. On steep, south facing slopes below about 3200 m, a melt freeze crust capable of bearing loads covers the surface. In the inneralpine regions of Grisons, to some extent in the Valais as well, the snow layering between 2200 and 3000 m is fragile.


SHORT-TERM DEVELOPMENT

Saturday night, clear skies  are expected widespread, although in Grisons and Ticino it will be intermittently overcast. On Sunday morning it will be predominantly sunny. During the afternoon, convective cloud will build up and showers can be expected. In northern regions above approximately 2200 m, and in Ticino above approximately 1800 m, a few centimeters of snowfall is anticipated. The midday temperatures at 2000 m will be plus 2 degrees in northern regions and zero degrees in southern regions. The winds will be light and shift to northwesterly.
On south facing slopes in particular, the snowpack is expected to become moist over the course of the day.


AVALANCHE DANGER FORECAST FOR SUNDAY

Inneralpine regions of northern and central Grisons:
  Moderate avalanche danger (Level 2)
The avalanche prone locations are to be found primarily on steep slopes in northwestern to northern to northeastern exposition above approximately 2400 m. For the most part, avalanches can be triggered through large additional loading, for example from a backcountry skier or freerider falling. The sliding snow masses can in isolated cases sweep along the old snowpack and subsequently attain medium size. Particularly on very steep, north facing slopes, maintaining distances between persons to relieve loads and one-at-a-time descents are recommended.

Remaining regions of the Swiss Alps:
  Low avalanche danger (Level 1)
Isolated avalanche prone locations are still to be found at higher altitudes primarily on extremely steep slopes in western to northern to eastern exposition. From region to region, the flurries of fresh fallen snow can amount to more than just a few centimeters. In such cases, avalanche hazards could well escalate to the danger level "moderate" locally. A cautious, on the spot assessment of the peril and adhering consequentially to the necessities are important, even when the overall avalanche danger level is low.

Because of the solar radiation, the danger of wet snowslides is expected to increase over the course of the day and reach the danger level "moderate". Such slides, and in isolated cases even wet snow avalanches, are expected particularly on steep, sun bathed slopes.


TREND FOR MONDAY AND TUESDAY

On both Monday and Tuesday it will be predominantly sunny during the morning hours. Over the course of the day, clouds will move in and on Tuesday, isolated snow flurries are possible. Caution is urged towards the daytime escalation of wet snow avalanche danger.