Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

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

Human triggered avalanches are likely today on NW-N-NE-E aspects and some W and SE aspects at all elevations on slopes steeper than 32 degrees due to newly formed wind slabs and the possibility of deep persistent slabs. The avalanche danger is CONSIDERABLE today. Today's avalanches could be large and have severe consequences and may occur in areas traditionally considered safe especially in the case of the deep persistent slabs. Careful snowpack evaluation, cautious route-finding and conservative decision-making are essential.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    3

    Strong south and southwest winds and 6 to 13 inches of dense new snow have formed new wind slabs on wind loaded NW-N-NE-E aspects and on some cross loaded W and SE aspects. Most of these new wind slabs will exist in exposed near and above treeline terrain, but some isolated open areas in below treeline terrain may also hold smaller wind slabs due to the strength of the winds. Human triggered wind slab avalanches are likely today on wind loaded slopes steeper than 34 degrees. These new wind slabs will consist of heavy dense snow and could result in destructive avalanches that will easily involve enough snow to bury or injure people. 

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    3

    Buried surface hoar still exists on open slopes on some near and below treeline N-NE-E aspects. This layer could still exist on some isolated NW and SE aspects as well. This persistent weak layer was still reactive before this storm, and now the storm has added another 6 to 13 inches of heavy wet snow on top of the 2-3 ft. slab that already existed on top of the surface hoar layer. Triggering an avalanche on these buried surface hoar layers will be possible on slopes steeper than 32 degrees where the surface hoar exists. These deep persistent slab avalanches would be large and have severe consequences. Avalanches that fail on buried surface hoar can propagate long distances, can occur in places traditionally considered safe, and can be triggered from lower angle terrain. 

    Due to the consequences of these types of avalanches, avoiding steep slopes where reactive surface hoar still exists represents the best way to manage this avalanche problem. Unfortunately, determining where the buried surface hoar still exists and where it is still reactive is difficult. Recent avalanche activity, shooting cracks, collapsing, whumpfing, and snowpit tests can provide some clues. However, a persistent slab triggered by a party on the slope sometimes represents the first and only clue to avalanche instability. 

Avalanche Discussion

Yesterday observations from Red Lake Peak on Carson Pass and the Fireplug in the Mt. Rose backcountry found strong winds and blowing snow already starting to occur as of midday. In each of these areas old avalanches existed with recent snow and wind transported snow obscuring the details of these older avalanches. Snowpit data and general observations on the N-NE-E aspects in the Fireplug area did not find any lingering signs of instability or persistent weak layers in the snowpack. On Red Lake Peak some splitboarder triggered cracking did occur. In both areas southerly facing slopes had breakable and supportable crusts on the surface, and northerly aspects above 7500 ft. had a heavy layer of soft snow sitting on top of a lighter layer of recent snow. Below 7500 ft. rain crusts existed on all aspects on Red Lake Peak. 

Regions covered