Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Alpine
1 · Low
Treeline
2 · Moderate
Below
2 · Moderate
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Out of date — this forecast expired 13 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Avalanche danger remains MODERATE near treeline and below treeline on NW-N-NE aspects on slopes steeper than 32 degrees due to lingering persistent slab instability.

Ongoing signs of snowpack instability have been observed on a regular basis. Human triggered avalanches remain possible. 

Avalanche problems

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    Persistent slab instability remains ongoing near treeline and below treeline on NW-N-NE aspects. The snowpack structure and the faceted Dec 7 persistent weak layer are slowly changing in these areas, but not enough to make much difference in terms of reducing overall avalanche danger.

    Dealing with the spacial variability that now exists around the forecast area presents a challenge for anyone desiring to travel on NW-N-NE aspects that are steeper than 32 degrees. When dealing with a persistent weak layer, slope instability is assumed. Significant amounts of snowpack study and data collection are necessary to determine why a particular slope that meets the aspect and slope angle criteria for suspected instability is showing an absence of the weak layer and/or overlying slab.

Avalanche Discussion

Observations made yesterday on Tamarack Peak (Mount Rose backcountry) revealed continued faceting at all heights within the snowpack on northerly aspects. As a result, the slab of once cohesive snow that sat on top of the faceted Dec 7 persistent weak layer has lost strength. Without a cohesive slab on top of the weak layer, propagation no longer occurred upon weak layer collapse. In essence, the snowpack in this area has turned into one faceted weak layer without an overlying slab. This differed significantly from observations made Friday on Silver Peak (Pole Creek area) where a reactive slab remained in place on top of the Dec 7 weak layer. This allowed for skier triggered cracking and weak layer collapse propagating out to 60 feet away from the trigger point.

This further illustrates that spacial variability is increasing around the forecast area in regards to the current persistent slab avalanche problem. In some areas the Dec 7 facet layer has gained strength, in other areas the overlying slab has eroded away, and in some areas the right combination of slab and weak layer exist to keep human triggered avalanches possible.

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