Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Alpine
2 · Moderate
Treeline
3 · Considerable
Below
3 · Considerable
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Highlights

Pockets of CONSIDERABLE avalanche danger may still exist on slopes steeper than 30 degrees in near and below treeline terrain on NW-N-NE aspects above 9000 ft. due to recent snow slabs sitting on top of persistent weak layers. Human triggered avalanches could remain likely on these slopes today. MODERATE avalanche danger exists on similar slopes on those aspects below 9000 ft. due to buried wet persistent weak layers. MODERATE avalanche danger also exists and on the wind loaded NW-N-NE-E-SE aspects steeper than 35 degrees in near and above treeline terrain due to the presence of wind slabs.

The unusual conditions that exist in the mountains right now require extra caution and superb travel skills.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    Human triggered wet slab avalanches remain possible on near and below treeline NW-N-NE aspects due to the wet and weak facet layers buried under the new snow. During the storm rain soaked these persistent weak layers up to at least 8800 ft. These rain soaked layers will remain very weak until the water in them refreezes. Even though air temperatures have dropped below freezing, an insulating blanket of new snow now exists above these wet facet layers, so they may take a few days to refreeze. Once they do refreeze they will gain strength. Until that time wet persistent slabs will remain a problem. These avalanches could involve the entire snowpack. Fortunately as the wet layers refreeze not only will the snowpack stability improve, but the on the snow recreation quality will also improve.

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    In higher elevation areas, where rain did not penetrate to the ground and saturate the old persistent weak layers, human triggered avalanche activity may remain likely today. New snow slabs resting above old persistent weak layers that could barely support themselves prior to the storm create serious avalanche conditions. These kind of avalanches can be triggered from a distance or from "safe" slopes connected to steeper terrain. These avalanches could also involve the entire snowpack.

    Based on snow levels during the storm this avalanche problem should only exist on NW-N-NE aspects in near and below treeline terrain above 9000 ft. that had previous snow cover. Since these limitations greatly restrict the areas where this problem could exist, it remains an isolated and "pockety" problem. Digging snowpits to determine if the persistent weak layers exist below the surface and whether or not they have been soaked by rain is a must to determine where this problem could remain. We have very limited field data concerning locations where the persistent weak layers may remain dry; therefore, confidence is lower and uncertainty is higher concerning where and how widespread this problem may still be.

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    Some fragile wind slabs may still linger on the near and above treeline NW-N-NE-E-SE aspects. Human triggered avalanches involving these wind slabs remain possible. On the NW-N-NE aspects where these slabs exist above the old persistent weak layers, they will behave more like the persistent slabs and wet persistent slabs discussed above.

Avalanche Discussion

Yesterday observations from Silver Peak and Chickadee Ridge (Mt. Rose backcountry) showed a similar snowpack structure: 7 to 12 inches of new snow resting on top of wet facets. In both of these areas human-triggered collapsing and cracking occurred as the weight of a person on top of the snow caused the wet facets to fail. Snowpit tests also indicated instability and some tests showed that fractures can still travel through the wet facet layers. Observers on skis sank through the new snow into the rain soaked facets and would sink to the ground if they stepped off their skis. A large loose wet snow avalanche had naturally released during the early morning hours yesterday on Silver Peak, but we did not see any other avalanche activity nor did observers did not report any.

Regions covered