Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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Pericolo valanghe

Sun
Alta quota
4 · Forte
Limite del bosco
4 · Forte
Sotto il limite
4 · Forte
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In sintesi

A powerful storm is bringing high intensity snowfall to the forecast area along with gale force winds. Widespread areas of unstable snow and natural avalanche activity are expected today. Travel in, near, or below avalanche terrain is not recommended.

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Very Likely
    Dimensione
    2–3

    As ridgetop winds and blowing snow got started yesterday morning ahead of the current storm, a skier was partially buried on Stevens Peak (Carson Pass area) in the first reported wind slab avalanche of this storm cycle. Ongoing gale force SW winds today combined with the addition of high intensity snowfall will build larger slabs of wind drifted snow and further increase the hazard. Wind slab avalanches are expected today mainly near treeline and above treeline on NW-N-NE-E-SE aspects. Wind slabs may also form to a lesser extent in below treeline areas due to the strength of the winds.  Wind slab avalanches could be very large in size today, traveling down into below treeline terrain and areas of mature timber.

    Travel in, near, or below avalanche terrain is not recommended at this time.

    (1/13/2023)

    Wind slab avalanche that resulted in the partial burial of a skier. Stevens Peak, Carson Pass, CA 

    Photo: Public

  • Storm Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–2

    Periods of high intensity snowfall today and tonight are expected to deposit new snow faster than it can bond to itself and to the old snow surface. Slabs of unstable storm snow are expected to form today in wind protected areas on all aspects. Crust, graupel, and/or rime on the old snow surface may enhance storm slab instability today. Snow surface cracking created while moving through untracked areas is a sign that snow in the area is unstable.

    If attempting to navigate in low angle, non-avalanche terrain in wind protected areas below treeline today, take great care not to wander beneath areas of steeper avalanche terrain above. Expect poor visibility and difficult seeing the surrounding terrain. Natural avalanches from above could easily travel down into lower angle terrain today and into treed areas. 

Avalanche Discussion

There are numerous rain crusts buried in the snowpack that could have small amounts of faceting around them due to melt layer recrystallization. Some of these crusts are now buried 7 to 15 feet deep in the snowpack making them very difficult to assess with snowpit tests. The vast majority of avalanches during and immediately after the current storm cycle are expected to occur near or above the old/new snow interface. An isolated avalanche event up to D4 in size and failing on a crust deep in the snowpack would not be a major surprise during this storm cycle. It would be impressive, but not a representation of the vast majority of expected avalanche activity.

Recent Observations

  • A size D2 wind slab avalanche caught and partially buried a skier yesterday on a NE aspect in above treeline terrain near 8,800' on Stevens Peak (Carson Pass area).
  • Prior to the current snowfall, rain crust was widespread on the old snow surface across all aspects below 8,200' to 8,600'.
  • At higher elevations above the areas of rain crust, sun crust was observed on the old snow surface yesterday across SE-S-SW aspects up to at least 9,700'.
  • Some areas of rime and graupel were observed yesterday on the old snow surface.

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