Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Alpine
2 · Moderate
Treeline
2 · Moderate
Below
2 · Moderate
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Highlights

THIS ADVISORY WILL BE UPDATED ON JANUARY 2, 2017 at 7:00 am.

Human triggered avalanches will remain possible today and tomorrow on some slopes steeper than 32 degrees. MODERATE avalanche danger exists at all elevations. Evaluate the snowpack and terrain carefully; identify areas where instability may exist, and use this information to avoid those areas of concern. As more snow and wind imapct the region this week the avalanche danger will increase.

Avalanche problems

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1

    While triggering a persistent slab avalanche has become more difficult in some areas, data and observations indicate that human triggering of persistent slabs may remain possible on some slopes steeper than 32 degrees today and tomorrow. These kind of avalanches would fail at the base of the snow that fell last week on persistent weak layers near the 12/15 rain crust. In most areas near crust facets comprise this weak layer, but some areas may also have a buried surface hoar layer at this interface. Observations have shown the persistent weak layers to exist on W-NW-N-NE-E aspects at all elevations. Data also indicates that these persistent weak layers may remain weakest along the Sierra Crest in areas where a shallower snowpack exists or in areas where the snow above them has become more cohesive and slab-like. A great deal of variability exists concerning how much load these layers can support from slope to slope or even across the same slope. Some slopes may allow for humans to recreate on them without consequence and other slopes may break under the added weight of a person. As new snow provides additional load to the snowpack this week, persistent slab avalanches may become more widespread again. 

    Traditional obvious clues may not provide reliable data for persistent slabs. While digging into the snowpack and snowpit tests can reveal whether or not the persistent weak layers exist, an avalanche can sometimes be the first clue that a slope is unstable. Persistent slabs can break after several people have already used slope when one person finds the right trigger point and propagate farther than expected. They can be triggered remotely or low on a slope and away from the more common ridgetop wind slab start zones. In summary, persistent slab avalanches require different terrain management techniques. Use conservative decision making and travel techniques like traveling on low angle or well-anchored slopes or sticking to areas where the persistent weak layers do not exist.  

Avalanche Discussion

Yesterday observations from Lincoln Ridge (Yuba Pass), Castle Peak (Donner Summit), Silver Peak (North of Squaw), and Granite Chief (NW of Squaw) all found the 12/15 rain crust with facets near it below the snow that fell on 12/23-12/24. Tests targeting the facets in shallower snowpacks on Lincoln Ridge and Silver Peak below 8000 ft. indicated that it remains weak and that if it does break the resulting fracture could propagate along the facet layer. Some tests also indicated that the weight of a person may still be able to cause this layer to break in some places. On Granite Chief, observers saw a thicker layer of snow above the 12/15 facets and rain crust that has become more cohesive and slab-like over the last several days. Tests on low angle slopes in this area also indicated that the facets remained weak. On Castle Peak, tests on the 12/15 facets no longer showed signs of instability.  Overall observations over the last few days across the forecast area have shown a variety of results associated with the 12/15 persistent weak layer with some observations indicating that this weak layer remains weak and sensitive to human triggering and some observations indicating that it has started to adjust to the load above it. Most of the data indicates that it could still represent a problem when additional loading occurs. 

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