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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Out of date — this forecast expired 3 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

People could still trigger avalanches on slopes where weak snow lingers 1-2 ft below the surface or where strong winds create new wind-blown snow drifts. In places with piles of wind-blown snow AND buried weak snow, avalanches could be several feet deep! If signs of unstable snow, like recent avalanches or cracking exist, or if doubt about stability exists, using low-angle terrain not connected to steeper slopes can help reduce the chances of triggering an unexpected avalanche.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    A group of snowmobilers saw another persistent slab avalanche yesterday, adding to the persistent slabs reported since Sunday. People remotely triggered most, if not all, of these avalanches by recreating on less steep slopes nearby. A surface hoar layer buried ~1-2 ft deep represents the most likely failure layer, but observers have also reported weak snow near a crust at that depth. These weak layers persist longer than typical storm snow weaknesses, and triggering persistent slab avalanches that involve enough snow to bury or injure a person remains possible. More snow will exist above the weak layers in wind-loaded areas, allowing for 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.

    Recent avalanches, snow surface cracking, snowpack collapse, or whumpfing sounds indicate unstable, persistent slabs, but these obvious signs of instability can decrease as time passes. The weak layers remain, and hitting a trigger point lurking in an area of seeming stable snow could still release an unexpected avalanche. This scenario may even play out on a slope with tracks on it. If you see any signs of unstable snow or have any doubt about a slope, assume it is unstable. Using low-angle, non-avalanche terrain not connected to steeper areas above or beside it is a great way to enjoy the soft snow and reduce exposure to the persistent slab problem.

    (1/7/2024) Photo: Public

    Snowmobilers saw this persistent slab avalanche yesterday in the southern end of the forecast area near Highland Lake (Ebbetts Pass).

  • Wind Slabs

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

    Winds shifted back to the SW last night. They will transport the soft snow that remains near the ridges and use the few inches of new snow that fall today to form new wind slabs on the NW-N-NE-E-SE aspects. Most of these new wind slabs should remain small and not extend very far downslope due to the limited amount of snow available for transport. Still, a few wind slabs could grow large enough to cause problems for people in places with efficient wind loading. Wind-blown snow on top of the persistent weak layers mentioned above will create larger persistent slabs.

    Blowing snow, cornices above a slope, cracking in wind-blown snow, and wind-created surface textures can help determine where unstable wind slabs may exist. If you see signs of unstable wind slabs, bypass those areas, especially in places that might overlap with the persistent slab problem.

    (1/7/2024)

    This small, reactive wind slab avalanche formed on a West aspect in response to NE winds on Jan 7.  Today's SW winds will move snow onto NW-N-NE-E-SE aspects and could form wind slabs similar to this one on those new aspects. 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 surface hoar and weak snow near crusts buried on Jan 2 have become the most problematic layers. In sheltered areas, these are ~1-2 ft below the surface, but they could be deeper in places, especially where wind-loaded snow exists above them.

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, some concern remains that the incremental loading of several small storms this week could overload these layers since these areas hold a weaker and shallower snowpack.

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