Sierra Avalanche Center Advisory

Sierra Avalanche Center Advisory

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Sierra Avalanche Center Advisory

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Today’s Avalanche Advisory:

December 30th, 2010 at 07:01 AM

Near and above treeline, MODERATE avalanche danger exists on all aspects with pockets ofCONSIDERABLE danger on NW-N-NE-E-SE aspects on slopes steeper than 35 degrees. Below treeline, MODERATE danger exists on NW-N-NE-E-SE aspects with pockets of MODERATE danger on S-SW-W aspects on slopes steeper than 35 degrees.

This is the Sierra Avalanche Center’s Advisory for today, December 30th.  It’s a good idea to make checking their advisory a habit  before you head into the backcountry or other uncontrolled terrain.  Knowing the forecast and having the correct gear should be the minimum you take with you into the backcountry.
Forecast Disscussion:
Snow showers and winds continued through yesterday adding another 5-8 inches of snow to the mountains above 7000 ft. 20-27 inches of snow has fallen across the forecast area since Tuesday afternoon. As the low pressure moved out of the area last night, snow showers tapered off, the cold air settled in, and the winds decreased and shifted to the northwest and northeast. A high pressure ridge off the CA coast should allow this northerly flow to continue today. The winds should increase today then decrease again tonight and tomorrow as this ridge settles over the area. Temperatures should remain cold with daytime highs in the teens today and tomorrow and overnight lows in the single digits.

Observations:

On Andesite Ridge and along Donner Pass, natural avalanches released overnight on Tuesday and early Wednesday morning near the base of the storm snow on wind-loaded, N-NE-E facing slopes steeper than 37 degrees with 2 ft deep crowns (photos). Reports of other similar natural avalanches that also occurred in that time frame came in from along the Sierra Crest. No other natural avalanches that released during the day yesterday were reported. However, the wind slabs did remain sensitive to skier-triggering yesterday. Ski cuts and cornice drops on wind-loaded slopes caused similar sized avalanches on 38-40 degree, NE-E facing slopes near treeline on Andesite Ridge at noon (video). In the Mt Rosearea, a similar slide released due to a washing-machine-sized cornice dropped onto a wind-loaded, 38-40 degree, N facing slope at 12:15 (photos, video). Observers also reported shooting cracks and unstable test results in both these areas.

Primary Avalanche Concern: Wind Slabs

Even though natural activity due to the failure of the wind slabs formed during the storm has become less likely, large, deep, dangerous, human-triggered avalanches involving these wind slabs remain likely in wind-loaded areas today. The strong and consistent winds during the storm deposited these fragile wind slabs in unusual areas. Even traditionally protected areas like trees, farther down-slope than usual, and open areas below treeline could hold these fragile wind slabs. The most heavily wind-loaded N-NE-E and cross-loaded NW and SE aspects will hold the largest and most fragile wind slabs. As the north winds increase today, these slabs will become more stiff and stubborn. They could start to break above the person who triggers them or when the second or third person descends a wind-loaded slope.

The increased northerly winds will also transport snow onto the W-SW-S-SE aspects and form new, fragile wind slabs on these aspects. These wind slabs should remain smaller and limited to open slopes near ridgelines where the most wind transport occurs due to the northerly winds. Human-triggering of these new wind slabs will become possible today. Avalanches resulting from these slabs can push someone into an area that could have serious consequences like over a cliff, into a gully or creek, or into trees or other obstacles (terrain traps).

Secondary Avalanche Concern: Loose Snow

Human triggered loose snow sluffs will be possible on any steep slopes today. The colder NW-N-NE-E aspects will hold the best potential for larger loose snow sluffs. Even though these slides entrain less snow than slab avalanches, they can also push people into terrain traps.
The bottom line:

Near and above treeline, MODERATE avalanche danger exists on all aspects with pockets of CONSIDERABLE danger on NW-N-NE-E-SE aspects on slopes steeper than 35 degrees. Below treeline, MODERATE danger exists on NW-N-NE-E-SE aspects with pockets of MODERATE danger on S-SW-W aspects on slopes steeper than 35 degrees.
Andy Anderson – Avalanche Forecaster, Tahoe National Forest

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