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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Highlights

MODERATE avalanche danger exists on slopes steeper than 32 degrees on NW-N-NE-E-SE aspects in near and above treeline terrain and on NW-N-NE aspects in below treeline terrain. Wind slabs will linger on wind loaded slopes, and the potential for persistent slabs still remains on some sheltered northerly aspects today. Evaluate snow and terrain carefully, and identify features of concern before committing to a slope. While avalanches will be more difficult to trigger today, human triggered avalanches will remain possible.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    The wind slabs that formed during the storm still remain on wind loaded N-NE-E aspects and cross loaded NW and SE aspects. Data and observations have indicated that these wind slabs have formed good bonds within themselves and with the old snow surfaces in many places, but not in all places. Triggering an avalanche involving one of these wind slabs has become more difficult, but human triggered wind slab avalanches remain possible today. Open, exposed, near and above treeline terrain will represent the most common locations for these wind slabs. The most fragile wind slabs will exist in steep, wind-loaded couliors, on unsupported slopes above rocks or cliffs, in very heavily wind loaded areas, or in other complex or extreme terrain. Avalanches involving these wind slabs could still involve enough snow to bury a person. If these wind slabs release on slopes where the persistent weak layers mentioned below exist, they could overload that deeper layer and involve more snow. Today's shifting winds may also deposit some very small and isolated wind slabs on other aspects (like the S-SW-W).

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    Observations continue to show a persistent weak layer lurking near the base of the snowpack on NW-N-NE aspects above 8000 ft. Data still indicates that if this layer does break the resulting fracture can travel through the persistent weak layer and that a person in the right spot on the right slope could break that layer. In many areas these persistent slabs remain small and isolated since anchors poking through the snowpack interrupt the weak layer. However, these persistent slabs could fracture across wider areas and involve more snow on slopes with smooth ground cover and less anchors. Cracking, collapsing, whumphing, and recent avalanche activity can all provide clues as to where these persistent slabs may be an issue, but persistent slabs often release on slopes and days with none of these obvious clues. Digging into the snowpack to find this persistent weak layer and avoiding slopes where it exists, traveling on lower angle terrain, or traveling on well anchored terrain represent the best ways to avoid this avalanche problem.

Avalanche Discussion

Yesterday observations on NW-N-NE aspects above 8000 ft. from Indian Valley (between Carson Pass and Bear Valley) and Castle Peak (Donner Summit area) showed new snow resting above an old snowpack that consisted of crusts surrounding loose weak snow (facets). In both areas tests targeting the persistent weak layers failed and indicated that if the weak layer breaks the resulting fracture can travel through the persistent weak layer. In the Indian Valley area, one low angle slope near a creek collapsed twice: first under the weight of snowmobiler when he stepped off his sled and second when another snowmobiler approached the slope on foot. These whumps, snowpit tests, and the upside down structure of the snowpack represented the only signs of instability concerning the persistent weak layer on Castle Peak and in the Indian Valley area. On Silver Peak (Pole Creek area) in terrain below 8000 ft. the persistent weak layer had become very moist and would easily clump together. Observations near ridglines on Silver Peak and Castle Peak showed some wind slabs that had bonded well to themselves and the old snow surfaces. Some minor cracking occurred in both places on wind loaded test slopes and one small 4 to 8 inch wind slab did release on one wind loaded test slope on Silver Peak. Overall signs of instability remained limited and hard to find yesterday. 

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