Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Tue
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
2 · Moderate
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Out of date — this forecast expired 3 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

People could still trigger avalanches 1 to 2 ft deep on slopes where weak snow lurks below the surface, or wind-blown snow piles up. In places where the wind-blown snow rests on the buried weak snow, avalanches could be several feet deep! Avalanches could come as a surprise in an area of seemingly stable snow. Carefully assess the snowpack before venturing into avalanche terrain.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    People remotely triggered multiple avalanches yesterday. In more sheltered areas, the avalanches measured 12 to 18 inches deep, and in wind-loaded areas, they got up to 3+ feet in depth. These likely failed on buried surface hoar or weak snow near crusts. Observers have reported these buried weak layers in most areas around the region. These weak layers persist longer than typical storm snow weaknesses, and triggering persistent slab avalanches remains possible today. Persistent slab avalanches could easily involve enough snow to bury or injure a person.  In some places near ridges, wind-loading could have piled even more snow on top of these weak layers, giving rise to larger avalanches. People can trigger persistent slab avalanches from above, below, or to the side of a slope, and they often break wider than expected.

    Clues like recent avalanches, snow surface cracking, snowpack collapse, or whumpfing sounds can indicate unstable, persistent slabs. Sometimes, these obvious signs of instability become less prominent as time passes, and triggering an unexpected, persistent slab avalanche represents the first and only clue that a slope remains unstable. They may also occur on slopes that already have tracks on them. If you see any signs of unstable snow or have any doubt about a slope, play it safe, assume it is unstable, and stick to low-angle, non-avalanche terrain not connected to steeper areas above or beside it.

    (1/7/2024)

    This 12-18 inch deep avalanche was remotely triggered from the tracks 20 feet to the side of the avalanche. Mt. Rose Wilderness.

  • Wind Slabs

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

    The NE winds continued moving snow through the night. These winds have scoured snow away from many of the slopes that got loaded by SW winds during the storm. The forecast calls for the winds to shift to the NW this afternoon, so some wind slabs may start to form again on slopes scoured by the NE winds later this afternoon. These shifting winds will leave a mix of scoured and wind-loaded areas in wind-exposed terrain today. Triggering a wind slab avalanche may remain possible in areas where wind-blown snow has piled up. Wind-blown snow on top of the persistent weak layers mentioned above will create larger persistent slabs.

    Blowing snow, cornices above a slope, and wind-created surface textures can help determine where wind slabs exist. Patterns of snow surface scouring can also help identify where wind-blown snow may accumulate. The shifting winds make locating wind slabs more challenging, so carefully evaluate any wind-exposed slopes, especially in areas that overlap with the persistent slab problem.

    (1/7/2024)

    Small reactive wind slab avalanche on a West aspect, 6 to 8'' deep.  Mt. Judah, Donner Summit area.

    Photo: SAC

Avalanche Discussion

More on our persistent slab avalanche problem:

The scope of the persistent slab problem has changed with weak layers higher up in the snowpack now the main concern and the problem is distributed across more aspects. The Jan 2 layers of crust/facet combo and/or buried surface hoar found 4 inches to 1.5 feet below the old snow surface are the layers to watch.

For much of the season, the persistent slab problem focused on faceted snow near the base of the snowpack. Substantial strength gains in the midpack and a deepening of the snowpack along the Sierra Crest have reduced concerns for deep weak layer failure until a major atmospheric river type storm with multiple inches of SWE returns to the forecast area. In the Carson Range along the eastern side of the forecast area where the snowpack has remained shallower and weaker, the situation is different and there remains a greater concern for deeper weak layer failure with incremental new snow loading over the next week or so.

Regions covered