Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

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

Triggering an avalanche may be possible in areas where blowing snow piles up, especially on cold shady slopes where the snow was already weak. Unstable snow may lurk on a more sheltered slope if weak snow sits under stiffer snow. Watch out for places with signs of blowing snow, snow surface cracking, collapsing, or recent avalanches.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    Even though no new snow has fallen, new wind slabs could form and existing wind slabs could grow in size as the SW winds increase today. The winds should become strong enough that blowing snow will accumulate in areas near ridgelines on the NW-N-NE-E aspects. In some areas, it is possible that the added weight of a person could break the slabs of wind drifted snow and cause wind slab avalanches large enough to bury or injure a person. 

    Identify and carefully evaluate any slopes where signs of wind-loading exist. Common signs of wind-loading include blowing snow, cornices above a slope, drifted snow, or wind-created surface textures. In some locations especially above 8,500 ft. on northerly aspects, these wind slabs will exist on top of an already weak snowpack. If an avalanche occurs in such an area, it could behave more like the persistent slabs mentioned below breaking wider and traveling further than expected.

    (12/10/2021)

    Skier triggered wind slab on Tamarack Peak. 

    Photo: Blackbird Mountain Guides

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    Layers of weak sugary snow are buried in the snowpack above about 8500 ft on NW-N-NE aspects. Higher elevations and more shaded slopes hold weaker snow. In most places, the recent snow has not created enough of a slab to break these old weak layers. Still, there may be specific terrain features where a slab does exist above the weak layers like places where the wind slabs mentioned above exist or where the recent snow has grown more slab-like. In those areas, the weak snow (facets) may break under the additional load of a person. Significant uncertainty surrounds both the buried weak layers and whether or not a slab exists above them.  What is certain is that the consequences of an avalanche could be severe in this shallow early-season snowpack.   

    Due to the uncertainty and the potential consequences of getting caught in an avalanche, it is best to play it safe. Carefully evaluate any slope before you commit to it especially in areas where wind slabs may have formed on top of the buried weak layers. If any signs of instability like recent avalanches, whumphing, cracking, or collapsing exist, stick to lower-angle simpler terrain not connected to larger steeper terrain. Those lower angle slopes will offer more supportable snow conditions and will be easier to avoid buried obstacles on.  

    If a persistent slab avalanche does occur today,  it could break wider and travel farther than expected. Remote triggering from far away or from the bottom of the path could be possible as well. This persistent slab problem will become a more widespread and dangerous issue in the days to come as a major winter storm impacts the area. 

Avalanche Discussion

* On NW-N-NE aspects above 8000 - 8500 ft. 6 to 12 inches of light, cold snow exists above 1-3 ft of old snow comprised of crusts and weak layers of surgery snow (facets).  Before the recent snow, slopes below about 7,800'-8,000' were a mix of patchy snow cover and bare ground. Almost all SE-S-SW-W aspects were snow-free with continuous bare ground exposed before the storm.

* Most data and observations from around the region indicate that the recent snow was not quite enough to overload the buried weak layers. The exception to that may be on slopes where wind slabs accumulated on top of the buried weak layers.  

* Skiers triggered 2 wind slab avalanches yesterday on a NE aspect on Tamarack Peak. They reported crown heights of less than a foot but one of these avalanches still entrained enough snow to bury or injure a person. These avalanches left a firm icy bed surface behind so it is likely that the old faceted snow just above one of the rain crusts served as the weak layer.

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