Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

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

Human triggered avalanches remain possible today due to new wind slabs and old and new persistent slabs. Persistent slabs exist on open slopes in near and below treeline terrain on W-NW-N-NE-E aspects. Wind slabs will exist on wind loaded NW-N-NE-E-SE aspects in near and above treeline terrain. MODERATE avalanche danger exists in those areas on slopes 32 degrees and steeper. Remotely triggered avalanches and avalanches that propagate long distances are possible today especially in near and below treeline terrain.

Avalanche problems

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    Data indicates that the recent snow has consolidated and with that consolidation the lingering storm snow weaknesses have gained strength. However, the old surface hoar buried 20 to 40 cm in the snowpack on some slopes represents a persistent weak layer, and the more consolidated snow above now represents a persistent slab in areas where buried surface hoar exists. So far data has shown that this issue exists in some areas (like Andesite Ridge and possibly Red Lake Peak) but not in others. In addition to this lingering persistent slab issue, today's new snow will likely bury the widespread surface hoar that formed yesterday. This surface hoar will also serve as a persistent weak layer for another much more widespread crop of persistent slabs. These new persistent slabs will remain smaller and shallower since only 2 to 6 inches of new snow is forecasted for today, but human triggering of these of these new persistent slabs will be possible in any areas where new snow buried yesterday's surface hoar and may be likely in areas that receive the most new snow.

    Triggering an avalanche from a distance, triggering an avalanche from the middle of a slope, and avalanches that propagate farther than normal are all possible with either the old larger persistent slabs or the new smaller persistent slabs. These persistent slabs will exist on any open near and below treeline slope where the old buried surface hoar exists or where newly buried surface hoar exists. It does not take much of an opening for these slabs to exist in, and they may exist in areas typically thought of as safe.

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    New snow and wind transported old snow will combine to form new wind slabs on wind loaded near and above treeline slopes today. Human triggered wind slab avalanches will be possible. Even though only 2 to 6 inches of new snow may fall today, the leeward aspects still had soft snow on them yesterday afternoon that can also serve as material to build wind slabs. This combination of snow sources will mean that the wind slabs that form today may be larger than one would expect from only 2 to 6 inches of new snow. Avalanches resulting from the failure of these wind slabs may remain small in some areas but could involve enough snow to bury a person in other areas. In some near treeline areas wind slabs may form on top of yesterday's surface hoar and exhibit more persistent slab characteristics. The largest and most fragile wind slabs will exist on wind loaded N-NE-E aspects with smaller wind slabs on cross loaded NW and SE aspects. 

Avalanche Discussion

Widespread surface hoar existed across the forecast area yesterday. Observations from Jakes Peak, Incline Lake Peak, Andesite Ridge, Negro Canyon, and Donner Summit all found large feathery surface hoar up to 1.5 cm in size on the snow surface. This surface hoar persisted into late yesterday afternoon on NW-N-NE-E aspects and on the upper elevation SE aspects. Below the surface snowpit tests showed another buried surface hoar layer that remains fragile on Andesite Ridge. On Jakes Peak snowpit tests did not reveal any signs of a buried weak layer. Ski cuts and snowpit tests on wind loaded test slopes on Jakes Peak and Incline Lake Peak also did not reveal any signs of instability associated with the old wind slabs. 

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