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

There is a high degree of uncertainty today in regards to snowpack instability near and below treeline. A significant loading event onto a weak layer in the upper snowpack has occurred near treeline and below treeline. Significant drifting of new snow has occurred near treeline and above treeline. CONSIDERABLE avalanche danger is forecast for all elevations.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2

    This is the simpler of today's two avalanche problems to figure out. The combination of wind and new snow during the last 24 hours has created some slabs of wind drifted snow in lee areas. These wind slabs exist mainly near treeline and above treeline on NW-N-NE-E-SE aspects. Avalanche size is expected mostly at D2 with up to D3 possible in the most heavily wind drifted areas.

    The wind direction has not changed so use the clues of cornice formations, wind pillows, snow surface cracking, and any signs of recent avalanches to determine where unstable wind slabs are most likely to be encountered. Travel around these specific areas with caution, anticipating instability.

  • Loose Dry

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2

    This is the more complicated of the two avalanche problems today and there is great uncertainty here. A rapid new snow loading event occurred on all aspects in wind protected areas near treeline and below treeline. At a basic level, weakness within the new snow may allow for continued instability. More complex is how the sugary weak snow of the buried Jan 4 crust/facet layer in adjusting to the new snow load. Information at this time is very limited and we simply do not know if this will stabilize like a typical storm slab or if there will be persistent instability. For now, anticipate instability and account for uncertainty by increasing margins of safety until the situation becomes more clear. Avalanche size is expected at D2.

    The Jan 4 crust/facet weak layer exists near treeline and below treeline on NW-N-NE-E aspects. In most below treeline locations, it is found 4 to 6 inches below the old/new snow interface. In some near treeline locations, it is found deeper under older wind drifted snow.

    "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. If you encounter signs of recent avalanches near to below treeline, evidence of snow surface cracking, snowpack collapse (possible whumpf sound), or unstable snowpit test results, avoid travel on or below steep slopes in the area.

Avalanche Discussion

* With the delayed start to yesterday's storm, evidence of instability observed during the midday hours was limited.

* Snowpit tests from the Donner Summit, Ward Canyon, Brockway Summit, and Luther Pass areas produced a mix of results. Snowpit test results showing failure around the Jan 4 crust/facet weak layer and indicating likely propagation were reported late day from Ward Canyon. Tests from other areas and those performed closer to midday provided limited to minimal indications of instability.

* With poor visibility and the peak in snowpack instability having occurred around or after sunset yesterday, it is unknown how much avalanche activity has occurred.

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