Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Alpine
4 · High
Treeline
4 · High
Below
4 · High
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Highlights

HIGH avalanche danger exists at all elevations today. Natural and human triggered avalanches are likely in a variety of areas. Large destructive avalanches could occur. A complex mix of wind slab, storm slab, and deep slab avalanche problems exist. The majority of avalanche activity today is expected on slopes 35 degrees and steeper but could occur on slopes 30 degrees and steeper in isolated areas. A Backcountry Avalanche Warning is in effect until 7 am Thursday.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Very Likely
    Size
    2

    Naturally triggered wind slab avalanches occurred yesterday with indication of easy human triggering of wind slabs in steep wind loaded terrain. Continued new snow and wind will keep this avalanche problem ongoing today. This problem will be most prevalent on NW-N-NE-E-SE aspects in near treeline and above treeline areas, but could be found in more isolated areas on other aspects or below treeline.

    Numerous clues such as drifting snow, cornices, and wind pillows exist to identify areas of recent wind loading and subsequent wind slab formation. Identify and avoid areas where wind slabs are likely to exist. Maintin awareness to the proximity of runout zones for natural avalanches on slopes above.

  • Loose Dry

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Very Likely
    Size
    1

    Several feet of new snow has accumulated during this storm cycle with more on the way today. Snowpack failure associated with this problem is possible anywhere within the storm snow. With warming air temperatures and rising snow levels last night, the upper third of the storm snow is expected to have developed an upside down structure. This avalanche problem will be most prevalent in wind protected near treeline and below treeline areas on all aspects.

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    3

    Enough snow has been added to the snowpack to create the possibility of deep slab avalanches failing 3 to 7+ feet deep in the snowpack. Weak layers of concern are old near surface facets at the base of the storm snow and more well developed facets down at the Dec 15 rain crust. Any avalanches that either initially fail on or step down to these weak layers will be large and destructive and could run into mature forest. This deep slab avalanche problem is possible on W-NW-N-NE-E aspects above about 7,200', regardless of proximity to treeline. Deep slabs are not impossible in isolated areas on SE-S-SW aspects due to the depth of new snow accumulation.

Avalanche Discussion

Avalanche activity was reported yesterday from Andesite Peak (Donner Summit area) and from Rose Knob Peak (Mount Rose area). Wind slab and storm slab avalanches occurred with crown depths of 6 inches to 2 feet. Snowpit data continues to point to two areas of instability deeper in the snowpack, one at the base of the storm snow on the old/new interface and the other down on near crust facets around the Dec 15 rain crust. In many areas the old near surface facets at the old/new snow interface are 3+ feet down in the snowpack. Recent snowpit data has shown increasing variability on the degree of weakness in the Dec 15 facet layer from one location to another, but it is still a problem. Yesterday on Andesite Peak, steeping off of skis to dig a snowpit triggered a large collapse, likely on the Dec 15 facets about 4.5 feet down in the snowpack at that location.

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