Eastern Sierra - Tue Dec 29 2009
Eastern Sierra avalanche advisory
Posted December 29, 2009 by Sue BurakUpdated December 29, 2009 - 10:57 pm
Yesterday, small wind slabs formed above tree line and along exposed ridges in the Mammoth area. It is possible another 4-8 inches of snow will blow around today and create another round of wind drifted snow in the higher terrain. Extreme winds of up to 100 mph are expected today at ridge top level and slabs that form with high winds tend to be small and isolated from their neighbors. They form well beneath the tops of ridges sometimes up to a third of the way downslope. Some will be sensitive to a person’s weight on unsupported, mid-slope rollovers and convexities.
With severe winds, small slabs will form in places you might not expect slabs, particularly in openings in the trees on the way up to the Bardinis, Telebowl and the Mammoth Crest. These strong winds will also strip snow from the existing snowpack so expect to see sastrugi and wind rippled effects on exposed slopes after today.
Another avalanche concern is the general structure of the snowpack. The cold temperatures are turning the top few inches of the snow surface into sugary facets, also known as near surface faceted grains. Surface hoar is found around meadows and creeks, open areas in the forest and above tree line in sheltered areas on the lee side of small ridges on a slope. The wind will probably destroy it but remaining pockets could be a future problem.
The large change in snow temperature from the ground to the top of the snow is causing the snowpack to weaken. The rate and extent of this weakening is influenced by slope aspect, elevation and snow depth and whether you are in the Mammoth Lakes Basin, June Mountain area, Rock Creek or Bishop Creek. Even on a single slope, it is hard to predict where one can find low density depth hoar, ice lenses or high density Sierra depth hoar at the base of the pack.
The weather forecast calls for another high wind event for today with only a few inches of snow expected through the day. The Reno Office of the National Weather Service issued a wind advisory that will continue through this morning. Strong southwest winds will hammer the ridgetops with gusts over 100 mph. The wind will decrease later today as a high pressure ridge builds over the west coast for tonight and Thursday. By Friday, another storm passes north, bringing more wind and higher snow levels.
Snow is likely today with west winds 20 to 30 mph with gusts over 50 mph. Daytime highs could reach 30F at the 8,000 to 10,000 ft elevations. Higher elevation winds will be strong with average wind speeds 25 to 45 mph and gusts up to 100 mph. daytime highs will reach the mid teens to low 20‘s.
Please note that the avalanche danger rating in this advisory expires in 24 hours. This advisory is our best interpretation of snow pack conditions and NWS forecasts issued today. Backcountry travelers should be aware that elevation and geographic distinctions are approximate and that a transition zone exists between upper and lower elevations. Avalanches do not happen by accident and most human involvement is a matter of choice not chance. Most avalanche accidents are caused by slab avalanches that are triggered by the victim of member of the victim's party. Even small slides can be dangerous. Always practice route finding skills and carry avalanche rescue gear. Remember that avalanche danger ratings are only general guidelines. Distinctions between geographic areas, elevations, slope aspects and slope angles should be made.
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