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 December 2, 2012:


December 2, 2012 at 7:57 am

Both above and below treeline, avalanche danger is HIGH on all aspects on slopes 32 degrees and steeper due to rain on new snow up to 10,000'.Travel in or below avalanche terrain is not recommended.


Forecast Discussion:


A well advertised and very powerful storm system is impacting the forecast area today. In the last 24 hours, 1 to 2 inches of new snow accumulated above 8,000' followed by rising snow levels and 1 to 2 inches of rain with more on the way. Gale force (hurricane force) southwest winds will continue through today before decreasing significantly tonight into tomorrow. Ridgetop gusts well over 100 mph are expected today with gusts to 140 mph during the peak of the event this morning. Snow levels rose last night to around 9,500' and may climb to as high as 10,000' this morning. Snow level is forecast to fall during the late morning through evening hours, ultimately ending up around 6,000' at the end of the storm. An additional 1 to 3 inches of rainfall is expected in most areas today with up to an additional 6 inches along the Sierra Crest, especially in the northern portion of the forecast area. As snow level falls, new snow amounts of 12 to 24 inches are forecast today above 9,000'. Between 8,000' and 9,000' 6 to 12 inches of new snow are expected. Between 7,000' and 8,000', 3 to 6 inches of new snow are forecast.

Recent Observations:

Observations made yesterday in the Carson Pass, Donner Summit, and Mount Rose areas revealed that snowpack instability varied greatly around the forecast area. On Carson Pass, numerous avalanches that had occurred within the previous 12 to 24 hours were observed near treeline on N-NE-E aspects. These avalanche that were most likely triggered by cornice collapse, all failed within the storm snow. Snowpit data revealed dense wind slabs on top of significantly lower density storm snow. Snowpit test revealed continued instability on storm snow layers in areas with more wind exposure (videos, more info).

On Tamarack Peak (Mount Rose area) widespread dense wind slabs were observed in nearly all areas both above and below treeline. Ongoing wind transport was observed during the stronger wind gusts. These wind slabs were very difficult to trigger with either the weight of a skier or even with several hundred pound sections of cornice (pit profile, photo, more info). Nearby explosive control work produced numerous wind slab avalanches failing 1.5 to 3 feet deep.

On Andesite Peak (Donner Summit area) the snowpack below 7,500' was saturated with rain water. Above 7,500' snowpack depth increased and quickly became around 3 feet deep in the 7,500' to 8,000' range. Isolated wet snow roller ball activity was observed at 7,600' (photos, more info).

Today's Primary Avalanche Concern: Rain on new snow

Snow levels have risen above 9,000' in all areas and may go as high as 10,000' this morning. A widespread rain on snow avalanche cycle is expected for areas above 8,000'. This is where 3+ feet of new snow has fallen over the past 48 hours. Current heavy rainfall is adding significant weight and load to the snowpack. This is expected to cause snowpack failure to initiate on weak layers within the storm snow, with the possibility to entrain deeper snow or cause step down failure to layers of faceted snow that exist on northerly aspects above 9,000'. Naturally occurring slab avalanches are likely at this time. Upper elevation avalanches could be large and destructive.

Today's Secondary Avalanche Concern: Wind Slabs and Storm slabs

As snow level falls this afternoon, a new round of wind slab and storm slab development will occur. Wind loaded areas on NW-N-NE-E aspects will see the most rapid slab development. Due to gale force winds today, slab development is expected to extend far down slope below ridgelines. New snow amounts will be directly related to elevation. The greatest areas of instability for this avalanche character are expected to occur above 8,000'.


The bottom line:

Both above and below treeline, avalanche danger is HIGH on all aspects on slopes 32 degrees and steeper due to rain on new snow up to 10,000'.Travel in or below avalanche terrain is not recommended.


Brandon Schwartz - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: 34 to 37 deg. F.
Max. temperature in the last 24 hours: 34 to 37 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 74 mph
Maximum wind gust in the last 24 hours: 115 mph
New snowfall in the last 24 hours: Snow 1 to 2 inches, Rain 1 to 2 inches
Total snow depth: 36 to 54 inches

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

For 7000-8000 ft:

  Sunday: Sunday Night: Monday:
Weather: Cloudy skies with heavy rain this morning and early afternoon. Rain changing to snow between 7,500' and 8,000' after 1pm and down to 7,000' by 5pm. Mostly cloudy skies with scattered snow showers in the evening. Isolated snow showers after midnight. Mostly cloudy skies.
Temperatures: 37 to 41 deg. F., falling by mid afternoon to 31 to 36 deg. F. 27 to 32 deg. F. 39 to 44 deg. F.
Wind direction: SW SW SW
Wind speed: 45 to 60 mph with gusts 80 to 100 mph, decreasing to 25 to 40 mph with gusts 60 to 75 mph by mid afternoon. 15 to 30 mph with gusts to 45 mph, decreasing to 10 to 20 mph with gusts to 35 mph after midnight. 10 to 20 mph.
Expected snowfall: 3 to 6 in. 0 to trace in. O in.

For 8000-9000 ft:

  Sunday: Sunday Night: Monday:
Weather: Cloudy skies with rain and snow, changing to all snow by 1 pm. Mostly cloudy skies with scattered snow showers in the evening. Isolated snow showers after midnight. Mostly cloudy skies.
Temperatures: 33 to 37 deg. F., falling in the afternoon to around 30 deg. F. 22 to 28 deg. F. 35 to 40 deg. F.
Wind direction: SW SW SW
Wind speed: 80 to 95 mph with gusts to 140 mph, decreasing to 75 to 85 mph with gusts to 125 mph in the afternoon. 35 to 50 mph with gusts to 75 mph, decreasing to 25 to 35 mph with gusts to 50 mph after midnight. 15 to 25 mph with gusts to 40 mph.
Expected snowfall: Between 8,000' to 9,000' 6 to 12 in., above 9,000' 12 to 24 in. 0 to trace in. O in.

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