Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Highlights

CONSIDERABLE avalanche danger exists today both above treeline on NW-N-NE-E-SE aspects and near and below treeline on N-NE-E aspects on slopes 32 degrees and steeper due to a combination of wind slabs, storm slabs, and persistent slabs. For all other areas avalanche danger is MODERATE. Natural avalanche activity is possible during the morning hours. Human triggered avalanches will remain possible to likely all day. In some cases, avalanches could be large and destructive.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2

    New wind slabs began forming late yesterday afternoon and evening in near and above treeline areas. Gale force winds were reported as erratic in direction in some areas, with irregular wind loading patterns occurring. The vast majority of new wind slabs are expected to exist near and above treeline on NW-N-NE-E-SE aspects today with some more isolated wind slabs possible above treeline on S-SW-W aspects in areas where wind direction was erratic. Evidence of recently drifted snow in the form of cornice formations, wind pillows, and snow surface patterning will allow for identification of the areas where unstable wind slabs most likely exist.

  • Loose Dry

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    As new snow accumulates through the morning hours, new storm slabs may build in wind protected areas on all aspects near treeline and below treeline. New snow is falling on wet snow and weak rain crust below 8,000' and onto a drier old snow surface at the upper elevations. Hand pits will easily identify the strength of new snow bonding to the old snow surface as well as identify any easily shearing weak layers within the new snow.

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    3

    Persistent weak layers of reactive buried surface hoar exist 2.5 to 3.5 feet below the snow surface on some but not all near and below treeline slopes on N-NE-E aspects and potentially in isolated areas on NW and SE aspects. The number of slopes with unstable persistent slabs in diminishing as avalanche activity continues to occur and to some degree eliminate the hazard on specific slopes. In other areas the surface hoar layers have already collapsed or have been compressed and are now less reactive.

    In the areas where snowpit tests show lingering unstable buried surface hoar layers, recent rain and new snow has continued to load these weak layers. Avalanche activity failing on these buried surface hoar layers has the potential to be large and destructive with very wide propagation potential. Remote triggering remains possible. Snowpack collapse and signs of past avalanche avalanche activity in the form of crown lines and debris on nearby slopes are some of the obvious clues that may be present in areas of buried surface hoar layers. Otherwise multiple, highly targeted snowpits will be required to gain an insight on this avalanche problem.  

Avalanche Discussion

Signs of recent avalanche activity remain widespread in the backcountry. Evidence of a small natural avalanche believed to have been triggered by cornice collapse was observed yesterday near Carson Pass in the Frog Lake Cliffs area on a NE aspect (more info below). The last reported skier triggered avalanche activity occurred on Saturday on a NE aspect on Andesite Ridge in the Donner Summit area (more info below). Explosive control work performed yesterday on a backcountry snowpack on a cross loaded N aspect just above Donner Pass as part of an ongoing SAR mission produced small slab avalanche results on a slope with a start zone thought to be absent of buried surface hoar.

Observations made and received yesterday from the Donner Pass, Mount Rose Summit, Barker Pass, Luther Pass, and Carson Pass areas indicated that where buried surface hoar layers are found, snowpit tests produce either unstable results or mixed, less clear results. In areas where buried surface hoar layers are absent or have already been collapsed or compressed, little to no evidence of instability was observed in the snowpits.

In areas below about 8,000' where rain has fallen on the snowpack in recent days, the snow surface remained wet yesterday, with some new crust formation occurring as the day progressed.

Snowpit data from the Mount Rose Summit and Carson Pass areas yesterday also indicated some potentially problematic faceted snow layers 2 to 3 feet below the snow surface on NE aspects in the 8,700' to 9,200' elevation range.

Regions covered