Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

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

The avalanche danger will rise quickly as a strong winter storm hits the region today. New slabs of wind-blown snow will form on exposed slopes near ridges and gullies, and storm snow layers could break loose on steep slopes in more sheltered areas. As conditions change, stepping back into gentler terrain away from steep slopes can help avoid today's avalanche problems.

 

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1.5–2.5

    New snow and SW winds will load snow onto leeward-facing slopes faster than it can bond to the snow below. These new wind slabs will start small but will quickly grow in size and extent as new snow accumulates. By this afternoon,  people would likely trigger wind slab avalanches on steep slopes where wind-blown snow piles up, and natural avalanches may be possible in areas with intense wind loading. The most fragile wind slabs will form on NW-N-NE-E-SE aspects near ridges, open wind-exposed slopes, and gullies but some may form in open areas below treeline due to the strength of the wind.

    Cracking in wind-blown snow, blowing snow, cornices above a slope, and recent avalanches indicate the presence of unstable wind slabs.  Blowing snow will also decrease visibility and make identifying avalanche terrain more difficult today. Traveling in terrain sheltered from the winds can help avoid the wind slabs, and should also provide better snow quality.

  • Storm Slabs

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

    As new snow intensifies today, layers of storm snow will accumulate. Some weaknesses will likely form within this storm snow due to changing conditions during the storm. During the periods of high-intensity snowfall this afternoon, avalanches of storm snow will become likely. Like the wind slabs mentioned above, these storm slabs should start out small and quickly increase in size and distribution as the storm intensifies. By this afternoon, unstable storm slabs could exist on any steep slope including below treeline slopes and other typically sheltered areas.

    Get out of existing tracks to look for cracking in the storm snow and feel for different layers of snow as you travel through the terrain.  If you see any signs of storm snow weakness or doubt a slope's stability, move to lower angle non-avalanche terrain. 

Avalanche Discussion

  • Check out our observation page for photos and details about the large avalanche cycle that occurred sometime on Tuesday or late Tuesday night. Many avalanches had crowns 4 to 8+ feet deep and wide propagation. A layer of weak faceted snow below a thin crust buried on Feb 21 provided the failure layer for many of these avalanches. At this point, strong consolidated snow has buried the Feb 21 weak layer that exists on NW-N-NE aspects more than 5 to 6 feet below the surface, making it unlikely for a person to trigger this layer. However, snowpack tests targeting this layer still indicate that fractures could travel along the layer if it does break. Getting force through the strong snow above it would take a very large trigger. Depending on the amount of snow accumulation, the next storm may add enough weight to the snowpack to possibly reactivate this layer in some places towards the storm's end. 
  • No avalanches have been reported or are thought to have occurred on the Feb 21 weak layer since the storm's end late Tuesday night.
  • Sun crusts have formed on E-SE-S-SW-W aspects at most all elevations. Thin sun crusts are also forming on lower-angle Northerly aspects. Tree drippings and bombs have refrozen around and beneath treed areas.

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