Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

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

Heavy snow and strong winds are loading onto an already weak snowpack creating very dangerous avalanche conditions.  Large and destructive avalanches are expected during this multi-day storm event.  Travel in, near, or below avalanche terrain is NOT recommended.

Avalanche problems

  • Wind Slabs

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

    Heavy snow with gale force SW winds have created wind slabs in near and above treeline areas. These wind slabs will continue to build today as snowfall rates increase and winds remain gale force.  Where wind slabs form on NW-N-NE aspects where an already weak snowpack structure exists, these wind slabs could break deeper into the snowpack and act like a persistent slab avalanche.  These avalanches could have wide propagation, be very large in size, and run down into below treeline terrain.  Wind slab avalanches are expected to occur throughout today as the storm continues.

  • Persistent Slabs

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

    A weak snowpack structure exists on NW-N-NE aspects above around 8500’.  Multiple layers of faceted snow are becoming buried deeper in the snowpack as storms continue to dump more snow.  Conditions have changed from last week.  The heavy load of the storm snow today could overload many of these weak layers and cause large and destructive avalanches that propagate wide and travel long distances.  Persistent slab avalanches can be remotely triggered, have wide propagation, and could wrap around terrain features.  This persistent slab problem exists in most areas that have enough snow coverage for recreating on.  Any avalanches associated with these weak layers could have severe consequences.  Travel in avalanche terrain is NOT recommended today.

  • Storm Slabs

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

    Multiple feet of snow, fluctuating snow levels, and high intensity snowfall will all make storm slab avalanches a problem for today.  Most storm slabs will form in areas with an existing snowpack, but as this large storm continues with multiple feet of snow-storm slabs may form on aspects with no previous snow coverage.  Storm slabs will exist in near and below treeline areas on all aspects.

Avalanche Discussion

* Early season snow from late October/early November became weak and faceted during our month long dry spell.  These weak layers are now buried down multiple feet below the snow surface in many areas.  These weak layers are especially concerning in high elevation areas of the Mt. Rose, Carson Pass, and Ebbetts Pass areas.  Near surface facets also exist on top of a firm rain crust at the base of the storm snow we received last week throughout the forecast region.  Any of these layers could potentially produce deeper, larger, and wider avalanches.

* 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.

* Wind slab avalanches have been active with human triggered avalanche activity reported every day since the Thursday storm.  Yesterday, wind slabs were touchy with large shooting cracks and wide propagation. 

Regions covered