Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

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

Ongoing dangerous avalanche conditions exist in many portions of the forecast area due to accumulating blowing snow near ridgelines and an existing weak snowpack with persistent instability, even in treed wind sheltered locations. Today you could trigger an avalanche from the bottom, side, or from other unusual portions of the slope. Increase margins for error by giving more room for avalanches to occur wider or travel further down slope than expected.

A period of HIGH avalanche danger is expected late tonight into tomorrow morning.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1.5–3

    Increasing SW winds today ahead of the next storm system are going to start blowing and drifting snow already on the ground from the last storm. This blowing snow is expected to accumulate into new wind slabs along and below ridgelines mainly above treeline and near treeline on NW-N-NE-E-SE aspects but could occur in other areas as well. If you see blowing snow, this avalanche problem exists nearby.

    Look for and avoid slopes where blowing snow is accumulating especially below cornice features or large wind drifts. Some avalanches in these areas could fail on deeply buried weak layers and behave like large persistent slab avalanches. Conservative terrain choices are highly recommended today.

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2–3

    Recent large avalanches and/or signs of ongoing persistent instability in the form of cracking, whumpfing, and snowpack collapse have been reported each of the last three days. Locations with known persistent instability include the Yuba Pass, Mount Rose, Luther Pass, and Carson Pass areas. This instability almost certainly exists in other locations as well, we just have yet to receive reports of persistent instability from other locations.

    There is weak faceted snow at and below the bottom of the recent storm snow, even in areas that had very little snow on the ground prior to this last big storm. The layer that has shown the most widespread instability is the Dec 11 layer of faceted snow that is found right at the bottom of the recent storm snow. It has been observed unstable above and below treeline on NW-N-NE-E-SE aspects as low as 6,800'. This is a much wider distribution than the Nov 9 facet layer that has more limited distribution on upper elevation NW-N-NE aspects.

    Getting back to the point, if you trigger an avalanche today on one of these deeply buried weak layers the avalanche will be big and potentially as wide as the terrain will allow, wrapping around numerous corners. You could also trigger this type of avalanche from the side, bottom, or other less common portions of the slope. Avoidance is the best tactic for dealing with this avalanche problem by sticking to sub 30 degree slope angle terrain. Choose slopes without connected steeper terrain where you could get hit by an avalanche from the side or from above.

    (12/14/2021)

    Persistent slab avalanches that most likely occurred during the early morning hours of Dec 14 on Relay Peak, NV in E to SE aspect terrain around 10,000'.

    Photo: Wendy Antibus

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