Eastern Sierra - Sun Jan 24 2010
Eastern Sierra avalanche advisory
Posted January 24, 2010 by Sue Burak
For the Mammoth and June Mountain area, the avalanche danger rating is CONSIDERABLE in steep wind loaded terrain on northwest to east facing slopes. There is a MODERATE danger rating in non wind loaded areas. For the Rock Creek and Bishop Creek areas, the avalanche danger rating is CONSIDERABLE on all aspects above 7,000 ft. due to 4-10 inches of weak faceted snow at the bottom of the snowpack. Careful snowpack evaluation, cautious route finding and conservative decision making are required.
The mountains continue to ask for a fair amount of respect today. Many skiers and riders are enjoying the powder below the Sherwin Ridge.It's unfortunate that most people use tracks on the slope as the basis for their decision making process. While there are many tracks on a slope, don’t be fooled into thinking the slope is safe.
There is over five feet of new snow with about 10 feet of southwest winds that packed the snow into the firm slabs you are skiing powder on top of. Cross loading was likely responsible for some of the natural avalanche activity yesterday morning at the mid elevations on Mammoth. Geez, at least take the time to check the layers with a ski pole!
The snowpack in the mountains around Rock Creek and Bishop Creek is shallow compared to the Mammoth area. The general snow structure seems to be two distinct storm slabs resting on weak faceted crystals. Stability test results show columns collapsing on depth hoar while isolating the column. One observer reported a Rutschblock score of 2 on a southeasterly aspect at 9,600 ft.
This poor structure exists on all aspects and elevations. Obvious clues of instability like collapsing, cracking and whumpfing are widespread. A party remotely triggered a small slide on depth hoar at the 9,500 ft elevation in the Rock Creek area. A plow driver triggered a one foot crown on a steep road cut that partly buried his machine. This reports are all red flags telling us the snowpack is weak and susceptible to failing under a skier's weight.
For today, avoid steep, rocky terrain where the weak layer is more easily affected. Give yourself options for safety-choose terrain that gives you the room to ski or ride around terrain features where avalanches can be triggered. Watch for steep sparse trees, convex rolls and slopes that have terrain traps at the bottom.
A stream of subtropical moisture is seen on satellite imagery, http://www.met.sjsu.edu/weather/sat2/ir-10.jpg Expect light snowfall this afternoon and tonight. From Monday night through Tuesday evening, this storm could bring 6-10” of new snow to the higher elevations of the forecast area including the mountains in the southern Sierra. Cold temperatures will continue through most of the week. The towns of June Lake, Mammoth and Aspendell could reach 30-32F today. Daytime highs will be in the upper teens to low 20’s above 9,000 ft. Nights will be cold with lows in the single digits. Southwest winds will pick up by the afternoon, though not as strong as the winds that accompanied the storms of the past week. Gusts could reach 30-45 mph on the ridgetops.
Naturally occurring avalanches on Mammoth Mountain. A remotely skier triggered slide on depth hoar south of Mammoth. Steep road cut avalanched on a plow driver. At least two skier triggered avalanches on Table Mountain in the Bishop Creek drainage.
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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