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 12, 2012:


December 12, 2012 at 8:00 am

Near and above treeline the avalanche danger will increase to MODERATE on NW-N-NE-E aspects steeper than 35 degrees due to new snow and wind. Pockets of MODERATE danger may form on those aspects in wind loaded below treeline terrain as well. Human triggered avalanches will be possible today. Use caution and evaluate snow covered slopes carefully when traveling in the backcountry.


Forecast Discussion:


The cold storm that reached the area last night has dropped 3-6 inches of cold light snow onto the forecast area. The forecast calls for another 2-4 inches above 7000 ft. and another 3-6 inches above 8000 ft. today. The winds shifted to the southwest yesterday and increased throughout the night posting average speeds in the 40-45 mph range and gusts as high as 90 mph. These winds should continue through today before decreasing significantly overnight as the storm departs. This cold storm also brought much colder temperatures to the area. Today's highs should only reach into the low 20's in the mountains with temperatures over the next 24 hours remaining in the teens and 20's.

Recent Observations:

Observations yesterday on Castle Peak (photos, snowpit, more info) continued to indicate a mostly frozen and stable snowpack. On shaded slopes and on all northerly aspects, the surface rain crust remained icy and supportable. As in other areas, a small layer of weak sugary snow (facets) exists between this surface crust and a much thicker, stronger frozen rain crust deeper in the snowpack. The facets between these two crusts could become a problem as the surface crust deteriorates over time or once a large load falls on top of the snowpack. The sun exposed slopes did soften some yesterday, but they quickly started to refreeze once the clouds moved in around 2-3 pm. Overall, this storm's new snow will accumulate on top of a variety of hard frozen crusts on all aspects.

Avalanche Problem #1: Wind Slabs

Wind slabs a few feet in depth could easily form today due to strong winds and new snow accumulation. Wind loading can multiply the depth of new snow by 2-10 times. These wind slabs will form on top of icy crusts and may not bond well to those crusts. Changes in wind speed or direction, snowfall rates, and other storm conditions could also create weaknesses within the storm snow. Human triggered avalanches involving these wind slabs will be possible today. The wind loaded NW-N-NE-E aspects in near and above treeline terrain will hold the largest and most fragile wind slabs. While the weight of these slabs may not add enough load for the facets below the old rain crust to break, avalanches involving these winds slabs could step down into those facets.

Avalanche Problem #2: Loose-Dry Avalanches

Cold dry snow resting on top of hard icy crusts provides a recipe for loose snow avalanches or sluffs. These sluffs could occur on any steep terrain with new snow on it. They could travel long distances downslope due to the efficient bed surface provided by the icy crusts. Most of these sluffs will remain shallow and should not entrain enough snow to bury a person; however, they could knock a person off balance or push a person off course into more hazardous terrain like cliffs, creeks, or other terrain traps. 


The bottom line:

Near and above treeline the avalanche danger will increase to MODERATE on NW-N-NE-E aspects steeper than 35 degrees due to new snow and wind. Pockets of MODERATE danger may form on those aspects in wind loaded below treeline terrain as well. Human triggered avalanches will be possible today. Use caution and evaluate snow covered slopes carefully when traveling in the backcountry.


Andy Anderson - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: 21-26 deg. F.
Max. temperature in the last 24 hours: 34-44 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 35-45 mph
Maximum wind gust in the last 24 hours: 90 mph
New snowfall in the last 24 hours: 3-6 inches
Total snow depth: 36-50 inches

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

For 7000-8000 ft:

  Wednesday: Wednesday Night: Thursday:
Weather: Snow Mostly cloudy with some snow showers in the evening. Snow tapering off after midnight. Mostly cloudy becoming partly cloudy in the afternoon with a slight chance of snow showers.
Temperatures: 22-29 deg. F. 12-19 deg. F. 21-28 deg. F.
Wind direction: Southwest Southwest Northeast
Wind speed: 20-30 mph with gusts to 50 mph 10-15 mph with gusts to 25 mph decreasing after 10 pm 10-15 mph
Expected snowfall: 2-4 in. up to 1 in. O in.

For 8000-9000 ft:

  Wednesday: Wednesday Night: Thursday:
Weather: Snow Mostly cloudy with some snow showers in the evening. Snow tapering off after midnight. Mostly cloudy becoming partly cloudy in the afternoon with a slight chance of snow showers.
Temperatures: 16-23 deg. F. 7-14 deg. F. 21-27 deg. F.
Wind direction: Southwest Southwest Northeast
Wind speed: 20-30 mph with gusts to 45 mph 15-20 mph with gusts to 30 mph decreasing after 10 pm 10-15 mph with gusts to 25 mph in the afternoon
Expected snowfall: 3-6 in. up to 1 in. O in.

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