This avalanche advisory is provided through a partnership between the Tahoe National Forest and the Sierra Avalanche Center. This advisory covers the Central Sierra Nevada Mountains between Yuba Pass on the north and Ebbetts Pass on the south. Click here for a map of the forecast area. This advisory applies only to backcountry areas outside established ski area boundaries. This advisory describes general avalanche conditions and local variations always occur. This advisory expires 24 hours after the posted time unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.


This Avalanche Advisory was published on November 22, 2010:


November 22, 2010 at 7:46 am

Near and above treeline, avalanche danger is MODERATE on NW-N-NE-E-SE aspects 35 degrees and steeper. For all other areas, avalanche danger is LOW with isolated pockets of MODERATE danger in open wind loaded areas on all aspects, 35 degrees and steeper. An increase in avalanche danger in expected to occur during the overnight hours into Tuesday.


Forecast Discussion:


A break in the weather will occur today as light snow shower activity holds on before the next major snowfall event impacts the forecast area tonight. An additional 6 to 9 inches of new snow has accumulated across the forecast area during the past 24 hours. This brings storm totals since Friday afternoon up to 21 to 48 inches, with the greatest snowfall amounts observed along the Sierra Crest. Ridgetop winds remain out of the southwest this morning and are moderate in speed. Moderate to strong southwest winds are expected for today. Air temperatures this morning at 8,500' are in the mid teens and are expected to warm only slightly towards the upper teens to low 20s during the day.

Observations:

Observations made yesterday on Silver Peak (Pole Creek area) noted shooting cracks up to 30 feet within the new snow in response to the weight of a skier on steep wind affected N aspect slopes below treeline (pit profile, more info). The existing slabs were so soft that traditional snowpit tests were unable to easily identify weak layers within the storm snow or isolate the slab. The old snow layers below the storm snow continue to show strength sufficient to handle the recent new snow load. Visual inspection of heavily wind loaded terrain near and above treeline revealed no signs of natural avalanche activity. A group of skiers touring at Alpine Meadows (a currently closed ski area with an uncontrolled, backcountry state snowpack) reported intentionally triggering three small avalanches (size D1) below treeline at 7,900' on N-NW aspect, 38 degree slopes. The largest avalanche was reported at 6 inches deep, 40 ft wide, 200 ft long. All of the avalanches involved very soft slabs.

Avalanche Concerns:

Recent observations have noted that the storm snow has bonded well to old snow surfaces below. Each of the past few days, short lived instabilities within the upper half of the storm snow layer have been observed. The lower portions of the storm snow are becoming increasingly more stable. Avalanche concerns for today continue to focus on new snow in wind loaded areas, mainly near and above treeline on NW-N-NE-E-SE aspects. Other more isolated areas of instability are expected in open wind loaded terrain at all other elevations on all aspects. Natural avalanches are unlikely today. Human triggered avalanches remain possible and should be anticipated by backcountry travelers venturing into wind loaded terrain where more cohesive slab formation has occurred.


The bottom line:

Near and above treeline, avalanche danger is MODERATE on NW-N-NE-E-SE aspects 35 degrees and steeper. For all other areas, avalanche danger is LOW with isolated pockets of MODERATE danger in open wind loaded areas on all aspects, 35 degrees and steeper. An increase in avalanche danger in expected to occur during the overnight hours into Tuesday.


Brandon Schwartz - Avalanche Forecaster, Tahoe National Forest


Weather Observations from along the Sierra Crest between 8200 ft and 8800 ft:

0600 temperature: 16 to 18 deg. F.
Max. temperature in the last 24 hours: 18 to 21 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 27 mph
Maximum wind gust in the last 24 hours: 46 mph
New snowfall in the last 24 hours: 6 to 9 inches
Total snow depth: 20 to 50 inches

Two-Day Mountain Weather Forecast - Produced in partnership with the Reno NWS

For 7000-8000 ft:

  Monday: Monday Night: Tuesday:
Weather: Mostly cloudy to cloudy skies with snow showers. Cloudy skies with high intensity snowfall after midnight. Cloudy skies with high intensity snowfall in the morning, snow showers in the afternoon.
Temperatures: 18 to 25 deg. F. 9 to 16 deg. F. 15 to 20 deg. F.
Wind direction: SW SW SW
Wind speed: 20 to 30 mph with gusts to 40 mph. 20 to 35 mph with gusts to 60 mph. 30 to 45 mph with gusts to 70 mph. Winds decreasing to 20 to 30 mph with gusts to 50 mph in the afternoon.
Expected snowfall: 1 to 4 in. 5 to 10 in. 10 to 16 in.

For 8000-9000 ft:

  Monday: Monday Night: Tuesday:
Weather: Mostly cloudy to cloudy skies with snow showers. Cloudy skies with high intensity snowfall after midnight. Cloudy skies with high intensity snowfall in the morning, snow showers in the afternoon.
Temperatures: 14 to 21 deg. F. 7 to 14 deg. F. 12 to 17 deg. F.
Wind direction: SW SW W
Wind speed: 30 to 45 mph with gusts to 70 mph. 40 to 60 mph with gusts to 100 mph. 55 to 75 mph with gusts to 110 mph. Winds decreasing to 35 to 50 mph with gusts to 80 mph in the afternoon.
Expected snowfall: 1 to 4 in. 6 to 12 in. 12 to 18 in.

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