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

Spatial variability for the persistent slab avalanche problem is increasing. This makes it more difficult to observe evidence of instability prior to triggering a persistent slab avalanche. Triggering a large avalanche remains possible, especially near treeline and below treeline. Additionally, small unstable slabs of wind drifted snow (wind slabs) exist above treeline and near treeline. MODERATE avalanche danger continues at all elevations.

Avalanche problems

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    This avalanche problem is found near treeline and below treeline and in isolated areas above treeline on W-NW-N-NE-E aspects. A slab of around 1 foot+ of recent new snow sits on top of a weak layer of sugary, faceted snow. This sugary snow is the Dec 11 persistent weak layer (PWL). Given the current setup, any persistent slab avalanches will likely be large.

    Spatial variability is increasing for this avalanche problem. The challenge this presents is finding evidence of instability for the slope(s) of interest prior to triggering an avalanche. While collectively snowpack data from around the forecast area suggests that instability associated with this problem is decreasing on a regional scale, data comes in daily from specific locations where instability is still very much present. Please continue to exercise caution. Cracking, whumpfing (audible snowpack collapse), or snowpit tests provide clues to the presence of instability. Since spatial variability is increasing, do not underestimate the significance of any signs of instability, even if they seem isolated. Limiting exposure to avalanche terrain by using only sub 30 degree angle slopes without steeper connected slopes above or to the side is the easy way to manage this avalanche problem.

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1

    Recent and ongoing SW to W ridgetop wind has drifted small amounts of snow above treeline and near treeline. Small unstable wind slabs exist in these areas on NW-N-NE-E-SE aspects. Slopes below or adjacent to cornice features and/or pillows of wind drifted snow are the most suspect. Move around these areas with caution.

Avalanche Discussion

* The faceted Dec 11 persistent weak layer (PWL) continues to produce snowpack collapse with audible whumpfing and increasingly variable unstable vs. non-propagating snowpit test results. Spatial variability is increasing for this PWL.

* Small reactive wind slabs were reported yesterday from a variety of near treeline and above treeline locations.

* Surface hoar exists in many areas. Any surface hoar that survives warm air temperatures today and strong winds this afternoon/evening could be buried under light snowfall tonight.

* Snow surface melt is occurring on southerly aspects. Shaded northerly aspects remain unaffected by melt.

 Reported Saturday from south of Carson Pass, exact occurrence day/time unknown. Small terrain somewhere between The Nipple and Blue Lakes Rd on an N to NE aspect at around 8,200'. Unverified weak layer but this has the signature of a persistent slab avalanche. Shallow wind slab crown on looker's right half likely stepped down to deeper faceted snow on the left side. Long fractures in the snow on the far right side likely go down to faceted snow (Dec 11 PWL).

Regions covered