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      WSL Institute for Snow and Avalanche Research SLF
               NATIONAL AVALANCHE BULLETIN NO. 145
                for Wednesday, 7 April 2010
             issue date 6.4.2010, 18:30 hours
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  CONSIDERABLE AVALANCHE DANGER IN SOME REGIONS, INCREASE IN WET SNOW AVALANCHE DANGER DURING THE DAY
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CURRENT CONDITIONS

After a clear night, Tuesday was sunny. Temperatures rose significantly, reaching plus 5 degrees in the west and plus 3 degrees in the east at 2000 m at midday. The westerly wind was light to moderate.
The Easter weekend's fresh snow and snow drift accumulations are settling and bonding. At high altitudes, the fresh snow on north facing slopes is loosely bonded over a wide area. On south facing slopes the snowpack is moist up to approximately 3000 m in the west and up to approximately 2500 m in the other regions. In the deeper layers, the snow is moist and soft below approximately 2200 m. Except in the far west and on the southern flank of the Alps, the snowpack is generally weakly bonded. The bonding is weakest in the inneralpine regions.


SHORT-TERM DEVELOPMENT

On Tuesday night the skies will be clear. Wednesday will be sunny at first. In the afternoon, cloudbanks will build up from the southwest. Midday temperatures at 2000 m will be about plus 3 degrees in the north and 0 degrees in the south. The wind will be light to moderate from the southeast.
Warming during the day will reduce the strength of the near-surface layers of the snowpack.


AVALANCHE DANGER FORECAST FOR WEDNESDAY

Central part of the northern flank of the Alps excluding the Prealps; eastern part of the northern flank of the Alps from the Glarus Alps to the St. Gallen Oberland; Gotthard region excluding the Grimsel region; Grisons; northern and central Ticino:
  Considerable avalanche danger (level 3)
On the main Alpine ridge from the Gotthard to the Bernina region and to the south, excluding the Grimsel and Sotto Ceneri regions, the avalanche prone locations are to be found, in particular, on west to north to east facing steep slopes above approximately 2000 m. Avalanches can be triggered, mostly in near-surface layers of the snowpack, by a single person.
In the central part of the northern flank of the Alps excluding both the Prealps and Urseren, in the eastern part of the northern flank of the Alps as far as the St. Gallen Oberland, and in the other regions of Grisons, the avalanche prone locations are to be found, in particular, on west to north to east facing steep slopes above approximately 2400 m. Avalanches can be triggered by a single person. Especially in the inneralpine regions of Grisons and in southern Lower Engadine and Muenstertal, avalanches can even penetrate the weakly bonded old snow and reach medium size. Warming during the day will increase the likelihood of avalanche triggering in all regions. Isolated natural avalanches can also occur then.
In all the regions subject to this danger level, extensive experience in the assessment of avalanche danger is required.

Prealps; western part of the northern flank of the Alps; Alpstein region; Liechtenstein; Valais excluding Obergoms; Sotto Ceneri:
  Moderate avalanche danger (level 2)
In the Prealps, the western part of the northern flank of the Alps, and in the Alpstein region, Liechtenstein and Sotto Ceneri, the avalanche prone locations are to be found, in particular, on west to north to east facing steep slopes above approximately 1800 m. In Valais excluding Obergoms, they are to be found on steep slopes of all aspects above approximately 2000 m. Older snow drift accumulations in particular can be released easily in some places, but they are mostly small. They are to be avoided by way of careful route selection. In these regions as well, the likelihood of dry avalanche triggering will increase a little during the day. In Valais in particular, the avalanche prone locations are more widespread in the high Alpine regions.

In all regions, wet snow slides and avalanches are to be expected during the day, below approximately 3000 m in the west and 2600 m elsewhere. This applies in particular on south facing steep slopes. Backcountry touring and other off-piste activities are to be concluded early.


TREND FOR THURSDAY AND FRIDAY

Thursday will be changeable with light precipitation in the west. Friday will be sunny at times with high-altitude cloudbanks. The danger of dry avalanches will continue to decrease. The danger of wet snow avalanches will increase during each of the next two days.