Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
View on map

Avalanche danger

Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
Open full map
Out of date — this forecast expired 13 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

CONSIDERABLE avalanche danger exists at all elevations on NW-N-NE aspects 30 degrees and steeper due to a combination of lingering wind slabs and widespread persistent slabs. Human triggered avalanches remain likely today especially in near and below treeline areas where the persistent slabs exist and in above treeline areas where wind slabs sit on persistent weak layers. Remotely triggered avalanches can occur under these conditions. In addition to the avalanche danger a myriad of season-ending obstacles still lurks just below the surface. Don't let "powder-fever" cloud your judgement. Conservative terrain selection, careful snowpack evaluation, and conservative decision-making are essential.

Avalanche problems

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2

    Any areas that had snow cover prior to this storm remain suspect. The weak layers below the new snow still exist and will not go away anytime soon. As the new snow starts to consolidate, it will become more cohesive and slab-like. More of a slab on top of those buried Dec. 7th facets means triggering avalanches remains likely. In fact it may be easier to trigger these kind of avalanches today. Since fractures can travel so easily through the persistant weak layer, people can trigger persistent slabs remotely while standing in low angle terrain or on ridgelines or anywhere that the snowpack under thier feet connects to steeper and/or deeper terrain. Wide propagation and avalanche activity on lower angle slopes to around 30 degrees remains likely today below treeline.

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2

    The wind slabs that formed during the storm still remain on the near and above treeline NW-N-NE-E-SE aspects. In many areas these winds slabs formed on top of wind scoured slopes that held a mix of rocks, dirt, and a few inches of old snow. However, some of these wind slabs could sit on top of the persistent weak layers mentioned above on the NW-N-NE aspects that did not get as scoured. Human triggered avalanche activity resulting from the failure of these wind slabs remains likely on slopes where the persistent weak layers exist and possible on slopes that just hold wind slabs.

Avalanche Discussion

Yesterday widespread cracking, whumphing, and collapsing occurred on N-NE aspects in the Mt. Rose area (photo, more info) and on Silver Peak (photos, snowpit, more info). These cracks, whumphs, and other failures occurred as the layer of weak sugary snow grains (facets formed before the storm - now the "Dec. 7th facets") collapsed under the combined weight of the new snow and a person. In some areas enough anchors (stumps, bushes, logs, trees, rocks, etc.) existed to prevent the collapses from traveling too far. Of course these same barely covered anchors also prevented people from traveling easily in those areas. In places with more snow cover these collapses propagated along the Dec. 7th facets for much longer distances, and in several cases shooting cracks shattered entire low angle slopes (photos). These failures could initiate in areas where less snow existed or in low angle terrain and travel into deeper and steeper terrain. On Silver Peak one of these collapses occurred as a skier weighted a shallow area of snow on a ridgeline that the E winds had partially scoured before the storm. This collapse traveled down slope and fractured an area below the ridge where more snow existed.

Regions covered