Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

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

New storm snow today could begin to overload our already weak snowpack.  This is especially concerning in areas where blowing snow will add additional load onto shaded slopes with previous snow coverage.  Utilize lower angle terrain, with no steeper avalanche terrain above you, to avoid these avalanche problems if recreating today in the backcountry.

Avalanche problems

  • Wind Slabs

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

    New snow today with gale force SW winds will create wind slabs in near and above treeline areas.  These wind slabs will build throughout the day as more snow falls and winds continue.  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 be much larger and wider entraining more snow than the wind slabs that are forming today.  

    Signs that wind slabs are forming:  blowing snow, cornice formation on top of slopes, wind drifts or wind pillows, and snow surface cracking in wind loaded areas.  Avoid and move around areas below steep wind loaded slopes and slopes with cornices forming.  Early season conditions increase the consequences of even small avalanches.    

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    Weak faceted snow has formed in the mid to upper snowpack during our long dry period and has been getting buried deeper by recent storms.  This weak snow exists on NW-N-NE aspects above 8500' and has been most concerning in the Mt. Rose and Carson Pass areas.  The additional storm snow today could start to overload these faceted layers and cause failure.  Avalanches that fail on these weak layers could be much wider and larger than expected.

    Persistent slab avalanches can be remotely triggered, with wide propagation, and could wrap around terrain features.  This problem exists at all elevations, so being in the trees does not offer any protection.  It also exists in all of the same areas that currently have snow coverage for recreating.  Utilize lower angle terrain below 30 degrees with no steeper or connected terrain above you.  Collapsing, whumpfing sounds, shooting cracks, or recent avalanche activity would be warning signs that dangerous avalanche conditions exists.   

    (12/10/2021)

    Weak snow below the surface on Carson Pass

    Photo: Travis Feist

Avalanche Discussion

* On NW-N-NE aspects above 8000 - 8500 ft. 6 to 12 inches of light, cold snow exists above 1-3 ft of old snow comprised of crusts and weak layers of surgery snow (facets).  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.

* Multiple layers of weak faceted snow exist in the mid to upper snowpack, especially concerning in the Mt. Rose and Carson Pass areas.  Near surface facets also exist on top of a firm rain crust at the base of our recent storm snow throughout the forecast region.  Any of these layers could potentially produce deeper, larger, and wider avalanches.

Wind slab avalanches continue to be reported everyday since the Thursday storm.  Yesterday, a human triggered wind slab avalanche occurred near Round Top on Carson Pass.  On Friday, 2 skier triggered wind slab avalanches were reported on Tamarack Peak in the Mt. Rose area.  And on Thursday, natural wind slab avalanches were observed on Elephants Back in the Carson Pass area.  Some of these avalanches were large enough to injure or bury a backcountry user.

Regions covered