Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Alpine
2 · Moderate
Treeline
2 · Moderate
Below
1 · Low
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Highlights

The new snow has made the rocks and other obstacles harder to see and easier to hit. This storm's new snow and wind have also added potential avalanche concerns. Unstable slabs of wind drifted snow (wind slabs) may exist on open slopes near ridgelines on almost any aspect today. Human triggered avalanches will be possible. MODERATE avalanche danger exists in near and above treeline terrain. 

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1

    On its own, the 2 to 4 inches of new snow that fell last night may not seem like much, but the gale-force winds can transport that snow and form slabs of drifted snow deep enough to cause problems. With the winds forecasted to shift to the NE and E today, these wind slabs may move during the day. These newly formed wind slabs may not bond well to the existing snow surfaces. In some places, the possibility exists that the additional weight of a person on them could cause the slabs to break away from the slope resulting in an avalanche. Most of these would remain relatively small, but any size avalanche could have severe severe consequences in the current shallow snowpack. There are still numerous obstacles close to the snow surface and a collision with them would hurt.  

    Clues like blowing snow, cornices above a slope, snowdrifts, and wind created surface textures can all help identify where wind slabs may exist. Use this information to adjust travel plans to minimize exposure to potential wind slabs. 

Avalanche Discussion

* Observations collected across the forecast area have shown a weak and shallow snowpack in many places. Of particular concern are the open northerly aspects in near and below treeline terrain. Weak sugary snow (facets) have formed on these slopes during the long dry spell since our last storm on Nov. 18. As new snow accumulates on top of this potential weak layer, it could overload the layer, and avalanche activity may result.

* Prior to this snowfall, a somewhat supportable snowpack 1-2 ft. deep exists on northerly aspects above 8600 ft. in the Mt. Rose area, above 7000 ft. along the northern Sierra Crest, and above 8000 ft. along the southern Sierra Crest. Coverage and snow surfaces varied widely. Many southerly aspects had melted back to bare ground. The near and below treeline northerly aspects hold the most snow. More sheltered areas hold a mix of firm melt-freeze snow, breakable crusts, and some areas of soft surface snow (near-surface facets) on sheltered northerly aspects. Wind-scoured and sculpted surfaces existed in exposed near and above treeline terrain.

Last night's new snow accumulated above old weak snow on many open slopes sheltered from the winds in near and below treeline terrain. While this old snow is weak, an additional 2 to maybe 4 inches of new snow on top of it is unlikely to overload it. This new snow could represent the beginning of a slab layer on top of that old weak faceted snow. As more storms impact the area in the coming days enough snow could accumulate on top of the old weak sugary snow (facets) to create an avalanche problem in near and below treeline terrain. 

Regions covered