Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Alpine
4 · High
Treeline
4 · High
Below
3 · Considerable
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Highlights

CONSIDERABLE avalanche danger exists at all elevations today due to likely wind slab and storm slab avalanches and possible deep slab avalanches. Another round of intense snowfall and wind impacting the region this afternoon and tonight should cause the avalanche danger to rise to HIGH in near and above treeline terrain. Most avalanche accidents happen at CONSIDERABLE or MODERATE, avoiding avalanche terrain and travelling on slopes less than 30 degrees without any exposure to steeper terrain above is recommended today.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Very Likely
    Size
    2

    During this morning's lull in the storm, human-triggered wind slab avalanches will remain likely and natural wind slab avalanches will still be possible. As more snow and wind impact the forecast area this afternoon and tonight, the wind slabs will become deeper, more widespread, and easier to trigger with natural avalanche becoming likely again and human-triggered avalanches becoming very likely. Wind slabs could exist at any elevation today due to the strength of the winds so far during this storm. Wind slab avalanches that do occur could be large and destructive and could run into more sheltered areas like dense stands of trees. 

    Use clues like recent avalanche activity, blowing snow, drifted snow, cornices above slopes, and shooting cracks to identify where wind slabs may exist and avoid wind loaded slopes. Wind slabs could exist in unexpected places today. 

  • Loose Dry

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2

    This storm's heavier snow rests on top of older lighter snow creating an upside-down snowpack. Changing conditions during this storm also have created weaknesses within the storm snow. As a result, human-triggered storm slab avalanches will remain likely today on steep sheltered slopes. These avalanches could involve all of the new snow and could occur in areas traditionally thought of as safe like tree-covered slopes.  As another round of intense snowfall impacts the region this afternoon and tonight, storm slab avalanches should become more widespread and easier to trigger with natural storm slab avalanche again becoming likely. 

    Recent avalanche activity, human-triggered cracking, and probing into the snow to feel for softer layers below more dense layers can help identify areas where storm slabs may exist. 

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    3

    A loose weak layer of snow exists near a crust on NW-N-NE aspects across the forecast area. This layer is now buried several feet below the surface. The loading provided by this storm may be enough to cause this layer to fail making deep slab avalanches a possibility today. These deep slab avalanches would be large and destructive, could propagate across large distances, could release below traditional starting zones, and are difficult to predict.

    Snowpit data and probing can help identify where the persistent weak layer may exist. Due to the uncertainty and dire consequences associated with these deep slab avalanches avoiding slopes where they may exist represents the best course of action.  

    Due to the amount of new snow that has fallen and the additional snow expected today wind slabs and possibly storm slabs could also exceed 3 feet in slab thickness allow these avalanche problems to behave more like deep slabs (failing deeper, wider, and more destructive than typically anticipated) and not necessarily triggered by the first person on the slope.

Avalanche Discussion

* Widespread wind slab avalanches occurred on Andesite Peak yesterday at all elevations. Some released naturally, some released as a person stepped on low angle terrain above them, and some released as small cornice pieces were dropped onto the wind-loaded slopes. They propagated long distances and crowns measured up to 2 ft. deep by noon yesterday.

* Tender cornices that failed easily in response to ski kicks and more stubborn wind slabs existed on Tamarack Peak. Due to poor visibility, observers could not tell whether or not the cornice collapses triggered wind slab avalanches. Wind loading and wind drifts 3 to 6 ft deep were reported on Relay Peak.

* Widespread storm slab avalanches also occurred on sheltered near and below treeline terrain on Andesite Peak yesterday. Several natural storm slab avalanches had released and triggers ranging from ski cuts on test slopes, to lightly stepping onto test slopes, to touching steep test slopes with a ski pole caused storm slab failures on any steep slopes. Lower angle slopes also cracked and shattered.

* Skier triggered cracking and unstable snowpit test results occurred in below treeline terrain on Andesite Peak and on Tamarack Peak with the interface between the older lighter snow and the new heavier snow acting as the failure layer. 

* Snowpit tests on Relay Peak targeting the now deeply buried persistent weak layer of loose weak snow near an old crust (near crust facets) yielded unstable results. 

While days like this account for the majority of avalanche accidents, the conditions don't mean that you cannot get out into the mountains. These kinds of conditions require experience and proper planning to manage terrain to avoid the hazards. The lull in the storm may have allowed the avalanche danger to decrease this morning, but today's avalanche problems are real and complicated. The mix of large deep wind slabs, widespread storm slabs and possible deep slabs will make finding safe terrain challenging. All of these avalanche problems could have serious consequences, could fail unexpectedly, could be remotely triggered, and could run into unexpected terrain. For example wind slabs may linger in below treeline areas, storm slabs may exist on tree covered slopes traditionally thought of as safe, and deep slabs could propagate around corners or break below traditional starting zones. Carefully identifying and then avoiding steep areas where instability may exist and lower angle more sheltered areas where you may be exposed to avalanche runout zones should be part of today's travel plan.  As more snow and wind impact the area the avalanche danger will increase again and travel will become even more challenging.  

Regions covered