---------------------------------------------------------------- WSL Institute for Snow and Avalanche Research SLF NATIONAL AVALANCHE BULLETIN NO. 100 for Sunday, 21 February 2010 issue date 20.2.2010, 18:30 hours ---------------------------------------------------------------- TREACHEROUS AVALANCHE SITUATION WIDESPREAD, CONSIDERABLE AVALANCHE DANGER ---------------------------------------------------------------- CURRENT CONDITIONS On Saturday in most regions of the Swiss Alps there was 10 to 20 cm of snowfall. Somewhat less snow, 5 to 10 cm, fell in the inneralpine regions of the Valais and in the Grisons Oberland. The midday temperatures at 2000 m were minus 10 degrees in northern regions and minus 7 degrees in southern regions. In the central part of the Main Alpine Ridge and in Grisons, an often moderate to strong northwesterly wind was blowing, elsewhere the wind was light to moderate. On Thursday night, strong southerly winds led to snow being transported, primarily above the treeline, on north facing slopes in particular. On Saturday the northwesterly wind brought about small sized snowdrift accumulations, especially on south facing slopes. In general, the snowdrift accumulations are prone to triggering. Beneath the snowdrift masses, the snowpack on south facing slopes is moistened down to intermediate altitudes, in the remaining expositions it is faceted and loosely packed widespread down to low altitudes. SHORT-TERM DEVELOPMENT On Sunday it is expected to be predominantly sunny. Towards evening, clouds will move in from the west. The temperature will rise slightly. The midday temperature at 2000 m will be minus 6 degrees in western regions, minus 8 degrees in eastern and southern regions. The wind will shift to southwesterly and blow at moderate to strong velocity. The warmer temperatures and solar radiation will temporarily bring about a slightly deteriorating stability of the snowpack and a settling of the still loosely packed layer of fresh fallen snow on the surface. AVALANCHE DANGER FORECAST FOR SUNDAY Northern part of Alpine Ridge on the northern flank of the Alps from Chablais into the Wildstrubel region and from Haslital into the Pizol region; western Lower Valais; Main Alpine Ridge; Grisons; southern flank of the Alps not including Sotto Ceneri: Considerable avalanche danger (Level 3) The avalanche prone locations are to be found on steep slopes in all expositions above approximately 2000 m. Above all else, both the visible snowdrift and the snowdrift accumulations which are covered and concealed by new fallen snow are perilous. They are poorly bonded with the old snowpack in most regions. They can easily be triggered. Subsequently avalanches can sweep along even parts of the old snowpack. The most treacherous areas are to be found in central Grisons and on the southern flank of the Alps not including Sotto Ceneri. In all the indicated regions, conditions in outlying terrain away from secured ski runs are treacherous. Extensive experience in spotting and evaluating avalanche hazards is imperative in order to avoid triggering avalanches and being buried by them. Prealps; remaining regions of the northern flank of the Alps; Valais not including western Lower Valais and not including regions on the Main Alpine Ridge: Sotto Ceneri: Moderate avalanche danger (Level 2) The avalanche prone locations are to be found on steep slopes in all expositions. In the Prealps and in the remaining regions of the northern flank of the Alps as well as in Sotto Ceneri, they are found above approximately 1800 m; in the indicated regions of the Valais they are found above approximately 2000 m. Avalanches can be easily triggered, although avalanche prone locations are not widely spread. Freshly formed snowdrift accumulations must be assessed with particularly great care. A cautious route selection continues to be recommended. The danger of full depth sluffs and snowslides is again expected to escalate on south facing slopes in particular up to altitudes of approximately 2500 m. TREND FOR MONDAY AND TUESDAY On Monday it is expected to be partly sunny in northern regions, due to the influence of foehn winds. In southern regions it will be overcast and snowfall is anticipated. On Tuesday weather conditions will be variable. A bit of snowfall is expected. The avalanche danger will gradually subside.