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, 2011:


December 12, 2011 at 7:47 am

Avalanche danger is LOW for all elevations and aspects. Normal caution is advised.


Forecast Discussion:


A weather system will pass to the south of the forecast are today. Some very light snowfall may occur south of Lake Tahoe this afternoon. Mid to high level cloud cover will continue into tonight and maximum daytime air temperatures will once again be cooler than yesterday. Remote sensors around 8,500' are reporting air temperatures in the upper teens to low 20s this morning. Daytime highs in the 20s are expected today for areas above 7,000'. Ridgetop winds have remained light to moderate in speed over the past 24 hours, shifting from southwest to northeast yesterday evening. Continued moderate speed northeast to east winds are forecast for today. Decreasing cloud cover, decreasing winds, and slightly warmer air temperatures are expected for tomorrow.

Observations:

Recent observations from around the forecast area plus those made yesterday on Mt. Judah (Donner Summit area) all tell a similar story. The NW-N-NE-E aspects hold one of a few different snowpack structures. There is either bare ground from wind scouring by east winds, a very shallow snowpack with lots of anchoring, a slightly deeper snowpack that is highly faceted but lacks an overlying slab, or fairly stable hard slabs over facets and depth hoar. Occasionally, snowpit data has pointed towards isolated areas of ongoing instability due to the November 18 facet layer which sits near the bottom of the snowpack. This is not surprising as this is a persistent weak layer that will remain weak for some time to come (pit profiles, photos, videos, more info). Very little to no snow exists on SE-S-SW-W aspects.

Avalanche Concerns: Persistent slabs

Natural and human triggered avalanche activity is unlikely at this time. Unlikely is very different from impossible. While widespread stable snowpack conditions do exist, finding a very isolated pocket of instability where one could create a human triggered avalanche is not impossible. The snowpack contains a persistent weak layer so it does not matter that it has not snowed in weeks. The November 18 facet layer exists in most areas and certainly in those areas that hold a sufficiently deep snowpack for over snow recreation. Most of the remaining slabs that have not faceted away are hard slabs. Some of these hard slabs have sufficient anchoring, but isolated pockets that lack anchoring and have an underlying weak layer exist out there. They are generally few and far between and exist within surrounding areas of seemingly stable snow. Travel smart and exercise appropriate caution in the backcountry in order to minimize risk.


The bottom line:

Avalanche danger is LOW for all elevations and aspects. Normal caution is advised.


Brandon Schwartz - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: 18 to 21 deg. F.
Max. temperature in the last 24 hours: 29 to 34 deg. F.
Average wind direction during the last 24 hours: Southwest shifting to northeast.
Average wind speed during the last 24 hours: 12 mph
Maximum wind gust in the last 24 hours: 29 mph
New snowfall in the last 24 hours: O inches
Total snow depth: 6 to 20 inches

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

For 7000-8000 ft:

  Monday: Monday Night: Tuesday:
Weather: Mostly cloudy skies with a slight chance of snow in the afternoon south of Lake Tahoe. Mostly cloudy skies in the evening, becoming partly cloudy. Partly cloudy skies.
Temperatures: 23 to 30 deg. F. 13 to 19 deg. F. 25 to 33 deg. F.
Wind direction: NE shifting to E in the afternoon. E E shifting to NE
Wind speed: Around 10 mph with gusts to 20 mph. 10 to 15 mph with gusts to 25 mph. Around 10 mph.
Expected snowfall: 0 to trace in. O in. O in.

For 8000-9000 ft:

  Monday: Monday Night: Tuesday:
Weather: Mostly cloudy skies with a slight chance of snow in the afternoon south of Lake Tahoe. Mostly cloudy skies in the evening, becoming partly cloudy. Partly cloudy skies.
Temperatures: 19 to 26 deg. F. 13 to 19 deg. F. 25 to 31 deg. F.
Wind direction: NE shifting to E in the afternoon. E E
Wind speed: 10 to 15 mph with gusts to 30 mph, increasing to 15 to 25 mph with gusts to 40 mph in the afternoon. 15 to 20 mph with gusts to 40 mph. 10 to 15 mph. Gusts to 25 mph in the morning.
Expected snowfall: 0 to trace in. O in. O in.