Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
View on map

Avalanche danger

Alpine
2 · Moderate
Treeline
2 · Moderate
Below
2 · Moderate
Open full map
Out of date — this forecast expired 6 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Within the snowpack, lurks old weak sugary snow that can barely support the soft slabs of recent snow. People recreating on steep slopes where this weak snow exists could break this layer possibly triggering large avalanches. This problem is widespread near and below treeline and isolated above treeline. MODERATE avalanche danger exists at all elevations. Traveling on slopes less steep than 30 degrees that are not connected to steeper slopes can provide fun snow and better safety margins.

Avalanche problems

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    Triggering persistent slab avalanches remains possible today. The layer of weak sugary snow that was buried on December 11th continues to fail as people travel over the snow. Despite no known avalanches on this layer, collapsing, whumpfing, and unstable test results have been widespread near and below treeline on W-NW-N-NE-E aspects. Observers have also found this layer below wind drifted snow in a few isolated areas above treeline. All the right ingredients still exist for persistent slab avalanches. The foot + of recent snow represents the slab. The Dec 11 facets represent the weak layer. A person recreating on the snowpack could provide the trigger. Failures in the facet layer could travel across terrain allowing remote triggering from below, above, to the side, or from any connected terrain. These avalanches could fracture wider than expected and start from unexpected spots on a slope (like midslope instead of the top of the slope). These avalanches would also have serious consequences as many obstacles still exist in the early season snowpack.

    Persistent slabs represent one of the most challenging avalanche problems to manage. Sometimes cracking, whumpfing, or snowpit tests can provide clues to instability. Sometimes triggering a large avalanche is the first and only clue to instability you see. Avoiding travel in avalanche terrain where persistent slabs might lurk represents a good strategy for dealing with persistent slabs. The best travel advice is: stick to slopes less steep than 30 degrees that are not connected to steeper slopes for fun snow and better safety margins.

Avalanche Discussion

Observations across the forecast area show that the Dec 11 persistent weak layer remains widespread in near and below treeline terrain on W-NW-N-NE-E aspects. This layer also has been reported in isolated areas above treeline. These facets formed during the extended dry period from Nov 18 through Dec 11. Now recent snowfall has buried them more than a foot below the snow surface in many areas.

Since they were buried on Dec. 11 observers have reported whumpfs and unstable snowpit test failing on this layer every single day in multiple locations. Yesterday was no exception with widespread whumpfs and unstable test results in various areas between Bear Valley and Donner Summit.  

Regions covered