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      WSL Institute for Snow and Avalanche Research SLF
               NATIONAL AVALANCHE BULLETIN NO. 83
                for Thursday, 4 February 2010
             issue date 3.2.2010, 18:30 hours
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  FOEHN WIND INFLUENCE, TEMPERATURES RISING MARKEDLY, CONSIDERABLE AVALANCHE DANGER WIDESPREAD
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CURRENT CONDITIONS

Following a night of stormy winds in northern regions, accompanied by a bit of snowfall on the northern flank of the Alps, the skies cleared up on Wednesday over the course of the day. By Wednesday midday on the northern flank of the Alps, there had been an additional 5 to 20 cm of snowfall, in the eastern part of the northern flank of the Alps as much as 30 cm of fresh fallen snow. Temperatures rose markedly. The midday temperature at 2000 m was minus 4 degrees. The westerly winds, which during the night had been strong to stormy, were at midday on Wednesday blowing predominantly at moderate velocity.
As a result of the westerly winds, the loosely packed snow of the last few days has been intensely transported, particularly in those regions north of the Rhine-Rhone line and in northern Grisons which had so much snowfall. Snowdrift accumulations have formed which are highly prone to triggering.


SHORT-TERM DEVELOPMENT

On Thursday it will be sunny. Over the course of the day, clouds will move in from the southwest. The temperature will continue to rise. The midday temperatures at 2000 m will be plus 2 degrees in northern regions, minus 3 degrees in southern regions. The wind will gradually shift from westerly to southerly and be strong in western regions, elsewhere it will blow at moderate velocity.
Through the strikingly higher temperatures and the influence of foehn winds, the snowpack will change swiftly up to intermediate altitudes. It is expected that at low and intermediate altitudes the snowpack will become thoroughly wet to an increasing degree on south facing slopes in particular. At altitudes above approximately 2000 m, the masses of fresh fallen snow from the last few days in particular will rapidly settle, especially on slopes with generally southern exposition. Particularly in northern regions, sliding snow masses can be expected on steep hillsides and grassy slopes at lower altitudes in some areas.


AVALANCHE DANGER FORECAST FOR THURSDAY

Regions north of the Rhine-Rhone line; Valais; Gotthard region; Praettigau:
  Considerable avalanche danger (Level 3)
The avalanche prone locations in the regions north of the Rhine-Rhone line as well as in Urseren and in Praettigau are to be found on steep slopes in all expositions above approximately 1800 m. In the southern Valais and in the remaining Gotthard region, the avalanche prone locations are to be found primarily on north to south to west facing wind loaded slopes above approximately 2200 m.
In all regions designated at the danger level "considerable", avalanches can be triggered even by a single backcountry skier or freerider. On south facing slopes in particular, naturally triggered avalanches can also be expected. In outlying terrain away from secured ski runs, extensive experience as well as very cautious assessment of avalanche hazards is imperative for backcountry skiers and freeriders.
Especially north of the Rhine-Rhone line and in particular in valleys exposed to foehn wind influence, naturally triggered wet snow avalanches and full depth snowslides can be expected below about 1800 m over the course of the day.

Ticino not including Gotthard region; remaining regions of Grisons:
  Moderate avalanche danger (Level 2)
The avalanche prone locations are to be found on steep slopes, particularly with northern to southern to western exposition above approximately 1800 m. The freshly formed and the somewhat older snowdrift accumulations can be easily triggered in some places, although the released snow masses are expected to remain small. Snowdrift accumulations should be avoided whenever possible. Naturally triggered, relatively small sized avalanches are possible on south facing slopes in particular.


TREND FOR FRIDAY AND SATURDAY

On Friday, weather conditions will be variable. Particularly in the Valais and in southern regions, snowfall is anticipated, amidst southwesterly winds. On Saturday, accompanied by northerly winds, it will be overcast in northern regions, sunny in southern regions. The avalanche danger will gradually recede in northern regions.