Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Highlights

Dangerous avalanche conditions continue in the backcountry at all elevations with wind slab, storm slab, and deep slab avalanche problems. Terrain and route selections should be made deliberately and conservatively with the specific intention to manage today's avalanche problems. Avalanche danger is CONSIDERABLE at all elevations.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2

    As of this writing, ridgetop wind speeds remain strong out of the S to SW and have yet to decrease as forecast. A shift to SE and a decease to moderate speed is expected today. This should decrease drifting and new wind slab formation as compared to the past 36 hours. Continued instability of wind slabs formed yesterday and last night remains the major concern above treeline and near treeline on NW-N-NE-E-SE-S aspects. Avalanche size on these aspects is expected mostly at D2 with up to D3 possible today. Once the forecast wind shift occurs, smaller wind slabs may form near treeline and above treeline on SW-W aspects that held previous snow cover.

    Identify cornices and pillows of wind drifted snow. Wind slabs are on the slopes below these features. Route find around these areas and do not linger below them. Avoidance is a better tactic today than trying to determine if wind slabs are stable or unstable.

  • Loose Dry

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1

    Ongoing snowfall is continuing to introduce new weak layers within the storm snow. Warming air temperatures today may create an upside-down storm slab layer in the upper snowpack. This will keep a storm slab avalanche problem ongoing today in wind protected areas near treeline and below treeline on all aspects. With 2 to 5 feet of new snow in most areas, anchoring does not exit for upper snowpack storm slabs in the areas that had little to no previous snow cover.

    Use snow surface cracking triggered while breaking trail as a clue to the presence of this avalanche problem. Specifically choose terrain and routes that give you options for avoiding avalanche terrain if storm slab instability is present.

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    In many areas, feet of new snow has been loaded on top of weak faceted (sugary) snow a few inches below the old/new snow interface near treeline and below treeline on NW-N-NE aspects. This has created a deep slab avalanche problem with an expected avalanche size of D2 to D3. Any deep slab avalanches will involve all of the accumulated new snow, plus the additional depth of wind drifted snow if occurring near treeline. Very large avalanches may occur in below treeline areas on slopes with a history very infrequent avalanches.

    Becoming caught in a deep slab avalanche near or below treeline may not be survivable. If you hear a whumph or experience snowpack collapse, unstable snow associated with this problem is present in the immediate area and an avalanche would have just occurred if the slope were steeper. Practical ways to avoid this avalanche problem are aspect avoidance or conservative terrain selection.

Avalanche Discussion

* Due to the high danger level yesterday and numerous road closures, observations were collected in very conservative terrain. On Donner Ridge (Donner Summit area) fragile cornices and wind slab avalanches up to 10 inches deep were intentionally skier triggered on test slopes along the ridgetop. Multiple layers of wind slab were noted on these test slopes. Digging a snowpit in wind protected terrain below treeline on a N aspect at 7,720' we unintentionally collapsed the facet layer just below the old/new snow interface with deep foot penetration. An audible whumph and a visible drop of the snow surface with this collapse gave evidence of the suspected deep slab avalanche problem.

* Other observations submitted from around the forecast area, also from conservative terrain gave indications of weakness within the new snow and possible storm slabs.

Regions covered