Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Alpine
4 · High
Treeline
4 · High
Below
4 · High
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Highlights

Dangerous avalanche conditions are expected in near treeline and below treeline areas today caused by rapid new snow loading of a problematic weak layer in the upper snowpack.  New snow and wind will also form unstable slabs of wind drifted snow in near treeline and above treeline areas. A rapid increase to HIGH avalanche danger is forecast for all elevations today with travel in avalanche terrain not recommended.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Very Likely
    Size
    2

    Gale force winds combined with high intensity snowfall today will create unstable slabs of wind drifted snow. These unstable wind slabs will mainly exist in near treeline and above treeline areas on NW-N-NE-E-SE aspects. Unstable wind slabs may form in isolated areas on other aspects or below treeline due to gale force winds. Avalanche size is anticipated at D2 with up to D3 possible if failing on the Jan 4 crust/facet sugary snow weak layer.

    Travel in or below avalanche terrain is not recommended today. If traveling non-avalanche terrain in the backcountry today, make a detailed plan for how to navigate in limited visibility conditions in order to successfully avoid exposure to avalanche terrain when you cannot see it above or around you. Natural avalanches are expected today.

  • Loose Dry

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Very Likely
    Size
    2

    In my mind, this is the scarier of the two avalanche problems today. High intensity snowfall will cause rapid loading and stressing of the weak sugary snow around Jan 4 crust/facet layer today. Avalanches that fail on this weak layer could be naturally triggered, human triggered from afar (remotely triggered), are expected to have wide propagation, have the ability to wrap around corners, and fracture above and behind a person who is a direct trigger on the slope.

    The first known avalanche on this weak layer occurred yesterday, well before today's rapid loading event has even begun.  This Jan 4 crust/facet weak layer exists near treeline and below treeline on NW-N-NE-E aspects. Avalanche size is expected at D2. High intensity snowfall is expected to spread storm slab instability to other aspects as well simply due to weakness within the new snow itself. 

    "Staying in the trees" will not avoid this avalanche problem today. This can be a way to avoid a wind slab problem, but not a way to avoid this storm slab problem. Travel in or below avalanche terrain is not recommended today. If traveling non-avalanche terrain today, avoid areas connected to avalanche terrain above or to the side. Anticipate poor visibility limiting your ability to see the terrain.

      Photo: First reported avalanche on Jan 4 weak layer.

Avalanche Discussion

* The first report of an avalanche failing on the Jan 4 crust/facet layer was reported to have occurred yesterday on Donner Ridge (Donner Summit area) in wind loaded near treeline terrain on a N to NE aspect around 7,500'. Crown depth was reported at approximately 2 feet. Trigger unknown.

* SAC has been tracking the weak sugary snow around the Jan 4 rain crust for a while now. Snowpit tests using an artificial new snow load have indicated that snowpack failure and propagation along this layer is likely under new snow loading conditions.

* In most locations the Jan 4 crust/facet layer is 4 to 6 inches below the old snow surface, but buried deeper under wind drifted snow in isolated areas. It exists in near treeline and below treeline terrain on NW-N-NE-E aspects. It is most prevalent in the 7,000' to 8,500' elevation range but has been observed with concerning weakness in more isolated distribution at higher elevations.

Regions covered