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Out of date — this forecast expired 6 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

In most areas, triggering an avalanche is becoming more difficult. Some exceptions include areas with a shallow snowpack where old weak snow remains closer to the surface or places where new snow and wind may form small slabs of drifted snow today especially in the northern part of the forecast area. In these places, human-triggered avalanches may remain possible. The avalanche danger remains MODERATE at all elevations. 

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1

    Strong SW winds last night and today may form some small slabs of drifted snow on leeward aspects (NW-N-NE-E-SE) near ridgelines. In many areas, not much snow exists for transport so these wind slabs should remain small. In the areas north of I80, more widespread wind slabs may form if new snow accumulates today. Even in these areas, the wind slabs should remain relatively small since only up to 2 inches of new snow is forecast. 

    Look for blowing snow, new cornice development, and wind pillows as clues to where wind slabs may exist. Even though they may remain small, wind slab avalanches could still have consequences, especially in complex or extreme terrain. 

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    Old weak snow still remains buried in the snowpack. Fortunately, data targeting the persistent weak layer (Dec 11 facets) indicates a strengthening trend, and triggering a persistent slab is growing more difficult in most places. Unfortunately, persistent weak layers are notoriously variable and do not gain strength uniformly. Some areas where the snowpack remains shallow still exhibit signs of instability like whumpfs, shooting cracks, and unstable snowpit tests. Specific areas with reports of a shallow unstable snowpack include the Carson Pass and Ebbetts Pass areas as well as lower elevation areas in other parts of the forecast region. Triggering a persistent slab avalanche may remain possible on a steep slope where the weak layer sits closer to the surface. A person hitting just the right spot (trigger point) on a slope or larger triggers like multiple people on a slope may still be problematic.  

    Even though the likelihood of triggering one of these avalanches has decreased, the consequences of being caught in one have not. This uncertain, high-consequence, low-probability problem still warrants caution. Terrain less steep than 30 degrees not connected to steeper slopes or terrain without the weak layer can provide fun recreation opportunities with significantly higher safety margins. The snowpack conditions are improving, and bigger, steeper terrain will still be here after the persistent slab problem goes away. 

Avalanche Discussion

* Observers reported more unstable test results on the Dec 11 facet layer in areas where the snowpack remains shallow. Unstable test results came in from Elephant's Hump (Carson Pass) and on Signal Peak (lower elevation peak west of Donner Summit). 

* On Silver Peak and on Castle Peak in areas with a deeper snowpack few signs of instability were reported. Snowpit data from Silver Peak indicated that the Dec 11 facets continue to slowly gain strength.  

* Observers reported variable conditions yesterday with some soft surface snow on sheltered upper elevation slopes around Elephant's Hump, Castle Peak, and Silver Peak. In more exposed areas wind-scoured, wind-packed, and rain crusted surfaces existed.  At lower elevations, a sometimes breakable sometimes supportable rain crust existed on most slopes.

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