THIS AVALANCHE ADVISORY EXPIRED ON December 27, 2013 @ 6:58 am
Avalanche Advisory published on December 26, 2013 @ 6:58 am
Issued by Andy Anderson - Tahoe National Forest

Near and below treeline NW-N-NE aspects still hold MODERATE avalanche danger on slopes steeper than 32 degrees because persistent slabs still remain in those areas.

Human triggered avalanches remain possible. Barely covered rocks, logs, stumps and other obstacles would increase the consequences of any avalanches that do occur.

1. Low

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Above Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

2. Moderate

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Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Persistent Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

The near and below treeline NW-N-NE aspects still hold persistent slabs. In some areas the myriad of anchors/obstacles poking through the snowpack have kept these slabs small. In some areas these slabs have become more difficult to trigger. However, in most places data and observations continue to indicate that the additional weight of a person on top of the slope can break the weak layer (Dec 7th facets) and that the resulting fracture can still travel along that weak layer. Human triggered persistent slabs avalanches remain possible. 

Any of these avalanches that do occur would drag a person through rocks, stumps, trees, and other obstacles because of the shallow snowpack and the fact that the near and below treeline terrain holds the most unstable snow as well as the greatest number of obstacles.

recent observations

We did not receive or make any observations yesterday. Data and observations from earlier this week show that the Dec. 7th facet layer still exists in near and below treeline terrain on the northerly aspects. The data also indicates that this layer remains weak. As persistent weak layers like this one gain strength, they strengthen sporadically with some areas showing no signs of instability while adjacent areas remain unstable and easy to trigger. This process leads to a great deal of variability on all scales and uncertainty about which slopes or even which parts of the same slope remain unstable. Recent observations seem to indicate some of this variability. Tests in some areas indicate that fractures will not travel along the persistent weak layer. In other areas the same tests indicate that fractures can still travel along the Dec. 7th facets, and in some places the additional weight of a person on the slope can still break this layer and trigger a collapse that propagates along this layer.

The weak and shallow snowpack will not support a person or snowmobile enough to prevent collisions with all of the barely covered rocks, stumps, logs, and other hard immobile objects. Collisions with these obstacles could easily break equipment and injure people.  

Weather and CURRENT CONDITIONS
weather summary

A strong and persistent high pressure ridge sitting over the forecast area will keep the weather clear, calm, and dry. This ridge will also keep temperatures in the mountains well above normal. Daytime highs above 7000 ft should continue to reach the mid to upper 40's today and tomorrow. The calm winds should also allow cooler air to remained trapped in the valleys and some of the valleys may remain colder than the mountains today. A front passing north of the region may push a few clouds into this area by tomorrow afternoon. The winds should also shift to the southwest and increase tomorrow afternoon. 

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 34 to 42 deg. F.
Max. temperature in the last 24 hours: 45 to 49 deg. F.
Average wind direction during the last 24 hours: East
Average wind speed during the last 24 hours: 5 to 15 mph
Maximum wind gust in the last 24 hours: 25 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 8 to 16 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Thursday Thursday Night Friday
Weather: Sunny Clear Sunny with some clouds developing in the afternoon
Temperatures: 42 to 49 deg. F. 23 to 33 deg. F. 42 to 49 deg. F.
Wind Direction: Variable Variable Variable
Wind Speed: Light Light Light
Expected snowfall: 0 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Thursday Thursday Night Friday
Weather: Sunny Clear Sunny with some clouds developing in the afternoon
Temperatures: 40 to 46 deg. F. 28 to 34 deg. F. 39 to 46 deg. F.
Wind Direction: Variable Variable Southwest
Wind Speed: Light Light 15-20 mph with gusts to 30 mph
Expected snowfall: 0 in. 0 in. 0 in.
Disclaimer

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.