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 23, 2010:


December 23, 2010 at 7:59 am

Near and above treeline, pockets of MODERATE avalanche danger linger on NW-N-NE-E aspects, 37 degrees and steeper. Below treeline, the avalanche danger is LOW in most areas.


Forecast Discussion:


Another 1-2 inches of snow fell across the forecast area during the day yesterday. By mid afternoon the showers started to taper off. A high pressure ridge should let warmer temperatures and drier weather to prevail over the forecast area through tomorrow. This ridge has started to slowly progress eastward allowing a low pressure in the Pacific to begin pushing some high clouds into the region today. The forecast calls for more widespread clouds by tomorrow. This low will also cause the southwest winds to increase during the next 24 hours.

Observations:

New, soft wind slabs did form on NW-N-NE aspects in the Castle Peak area (Donner Summit area) yesterday due to the increased winds and light snow. Most of these new wind slabs remained relatively small with the largest of them measuring about 1 ft in thickness. Some skier triggered cracks up to 3 ft long did occur on steep wind-loaded test slopes on Castle Peak and on Andesite Ridge (photo). Layer bonding tests and snowpit data indicated that the new wind slabs had not fully bonded to the snow below them (pit profile). Newly formed cornices remained tender during the day yesterday and broke easily in response to a person's weight. The oven-sized chunks of cornice did not trigger avalanches on the slopes below when they landed. Tests on the deeper layers in the snowpack in this area showed continued stabilization in the snowpack.

Primary avalanche concern: Wind Slabs

Pockets of instability due to the recently formed wind slabs will allow human triggered avalanches to remain possible today. These fragile wind slabs will most likely remain in near and above treeline areas on the most heavily wind-loaded slopes and in complex or extreme terrain (unsupported slopes, couloirs, cliffy areas, etc). As the winds increase again today, some additional wind-loading may occur adding a little more snow to these wind slabs. Avalanches due to failure of these wind slabs should remain within the top 12-18 inches of the snowpack; however, failures that step down into the older storm snow are not impossible especially in complex and extreme terrain. Cornices that exist above wind-loaded slopes will also remain sensitive to human triggering today.


The bottom line:

Near and above treeline, pockets of MODERATE avalanche danger linger on NW-N-NE-E aspects, 37 degrees and steeper. Below treeline, the avalanche danger is LOW in most areas.


Andy Anderson - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: 24 deg. F.
Max. temperature in the last 24 hours: 31 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 15-25 mph
Maximum wind gust in the last 24 hours: 44 mph
New snowfall in the last 24 hours: 1-2 inches
Total snow depth: 64-94 inches

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

For 7000-8000 ft:

  Thursday: Thursday Night: Friday:
Weather: High clouds thickening this morning and becoming cloudy. Mostly cloudy Considerable high clouds
Temperatures: 34-39 deg. F. 25-30 deg. F. 38-43 deg. F.
Wind direction: Southwest South South
Wind speed: 5-15 mph 10-20 mph 10-20 mph with gusts to 45 mph in the afternoon
Expected snowfall: O in. O in. O in.

For 8000-9000 ft:

  Thursday: Thursday Night: Friday:
Weather: High clouds thickening this morning and becoming cloudy. Mostly cloudy Considerable high clouds
Temperatures: 31-36 deg. F. 27-33 deg. F. 34-38 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: 15-25 mph with gusts to 40 mph 20-35 mph with gusts to 50 mph increasing to 60 mph after midnight 25-35 mph with gusts to 65 mph
Expected snowfall: O in. O in. O in.

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