Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Wed
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
2 · Moderate
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Highlights

People could still trigger large, dangerous avalanches today, especially in areas where weak snow lurks below the storm snow on steep slopes and where wind-blown snow has piled up near ridges, gullies, and other exposed terrain. Look for any potential signs of unstable snow like recent avalanches, cracking, collapsing, or even just feeling soft snow below more dense snow. Use travel plans that provide large safety buffers around any places where unstable snow might still exist. 

Avalanche problems

  • Persistent Slabs

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

    Old, weak snow buried under a slab of recent storm snow has resulted in a persistent slab avalanche problem on northerly aspects at all elevations. Many persistent slab avalanches have already occurred around the forecast area, including numerous natural releases and at least one human-triggered avalanche. As the snowpack slowly adjusts to its new load, obvious signs of instability like cracking, whumpfing, and collapsing may become less prevalent and avalanches may become less frequent. Unfortunately, triggering one of these persistent slab avalanches remains possible and could be the first and only clue that the snowpack remains unstable.

    Persistent slab avalanches can break much wider than expected, with fractures traveling around terrain features, resulting in large and very dangerous avalanches. People can also trigger these kinds of avalanches from nearby slopes or ridges or even from flatter terrain below the avalanche path. Due to uncertainty about where the weak layers are weakest and exactly what it would take to break them, "managing" persistent slab problems remains difficult at best. These avalanches often surprise snow professionals. The best way to deal with a persistent problem is to avoid terrain where the problem might exist by using low-angle slopes without exposure to steeper terrain.

    (12/15/2024)

    80’ wide 18” to 30” crown.  Snowmobile triggered avalanche on Sunday, Dec 15.  Relay Peak, Mt. Rose area.

  • Wind Slabs

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

    Yesterday's new snow and strong winds continued to pile slabs of wind-blown snow onto the leeward sides of ridges, gullies, and other wind-exposed terrain features. These wind slabs may not have fully bonded to the snow below them, and triggering a wind slab avalanche remains possible today. In places where the wind-drifted snow rests on old, weak snow from before the storm, larger avalanches could occur. The winds should shift direction today, but the forecast calls for them to remain lighter, leading to limited blowing snow. If the winds end up stronger than forecast, they might shift wind slabs into new locations.  The largest wind slabs should linger in near and above treeline terrain, but an isolated wind slab may also lurk in open areas below treeline. 

    Recent avalanche activity, cornices above a slope, piles of drifted snow, and wind features on the snow surface can help indicate where wind slabs have built up. Identify the wind-loaded areas and move around them carefully. Using sheltered, less steep terrain can help reduce exposure to avalanche problems and increase the potential to find softer, non-wind-affected snow. 

    (12/15/2024)

    An overview showing six different wind slab avalanches along the Castle-Basin Ridge, Donner Summit area.

    Photo: Sierra Avalanche Center

Avalanche Discussion

A persistent slab avalanche cycle started to occur at the end of the Thursday-Saturday storm that dropped 2 to 4 feet of snow.  Several reports have been received of large persistent slab avalanches with wide propagation that occurred naturally.  A report of a snowmobile-triggered avalanche in the Mt. Rose area is also suspected of having failed on the near-surface facet layer.  Many reports of collapsing and whumpfing sounds also came in, further pointing to this weak layer remaining active. The Dec 12 persistent weak layer is made up of near-surface facets but varies greatly in strength and location throughout the forecast region.  This weak layer will gain strength at different rates from area to area, and it will take time to adjust fully to the recent large storm load.  Where this layer is well developed, avalanche activity is expected to continue.

A large wind slab avalanche cycle also occurred at the tail end of the recent storm cycle.  Some wind slab avalanches were large and potentially involved the Dec 12 weak layer.

Regions covered