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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Out of date — this forecast expired 9 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Avalanche danger is MODERATE for all elevations. Human triggered avalanches have become possible in isolated areas. Curious as to how the avalanche danger went from LOW to MODERATE without a storm? Read up on the terms faceted snow and persistent slab.

Avalanche problems

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1

    Faceting, faceting, and more faceting has weakened the existing snowpack during this current dry spell. This has created isolated areas of persistent slab instability that are reactive to the weight of a person on the snowpack. It doesn't matter that it hasn't snowed since Dec 20th, metamorphic change of snow crystals on the ground is making a loose sugary weak layer get weaker by the day. NW-N-NE aspects near and above treeline hold isolated hard slabs over this weak layer. NW-N-NE aspects below treeline hold isolated soft slabs over this weak layer.

    Faceted snow is widespread on NW-N-NE aspects around the forecast area, but the reactive combination of slab over faceted weak layer is isolated. This distribution makes human triggered avalanches possible in isolated areas while avalanches remain unlikely in the majority of areas on a regional scale.

    Isolated persistent slabs are challenging to manage from a travel standpoint as signs of instability to aid in decision making are not readily available. Previous tracks are not an indication of stability, especially when the weak layer is becoming progressively weaker. Use best practice travel techniques for minimizing exposure near and in avalanche terrain. Keep in mind that persistent slabs are often triggered much lower in the start zone than wind slabs.

Avalanche Discussion

* Signs of persistent slab instability on northerly aspects including snow surface cracking, whumpfing, and/or unstable snowpit test results have occurred during the past several days in the Carson Pass, West Shore Tahoe,  Deep Creek, and Pole Creek areas. In all cases failure occurred on faceted snow beneath either a hard or soft slab.

* Mt. Rose Ski Patrol reported unstable snowpit test results on a similar slab over facet weak layer in closed, uncontrolled terrain (natural state snowpack that is analogous to a backcountry snowpack).

* Near surface facets without an overlying slab are fairly widespread on northerly aspects in wind protected areas below treeline.

* Snow surface conditions range from wind scoured snow and firm ice near and above treeline to lingering areas of soft unconsolidated snow on sheltered northerly aspects below treeline.

* Northerly aspects above 8000 ft in the Mt. Rose area and along the Sierra Crest north of Emerald Bay hold the best snow coverage at 2 to 3+ feet. Overall less snow cover exists south of Emerald Bay, but decent coverage exists above 8,500 ft. on northerly aspects. Coverage becomes patchy on other aspects and large areas of bare ground exist on many southerly aspects at all elevations.

Regions covered