Lawinenlagebericht

Central Sierra Nevada

Sierra Avalanche Center
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Lawinengefahr

Hochgebirge
3 · Erheblich
Waldgrenze
3 · Erheblich
Unterhalb
3 · Erheblich
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Veraltet — dieser Lagebericht ist 8 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

CONSIDERABLE danger exists at all elevations due to likely wind and storm slabs and possible deep slabs. Most avalanche accidents happen at CONSIDERABLE or MODERATE danger. Don't let "powder fever" corrupt good judgment. Make a plan before going into the backcountry that focuses on thorough and careful terrain and snowpack evaluation. Avoiding avalanche terrain and traveling on slopes less than 30 degrees without any exposure to steeper terrain above is recommended today. Avalanches could be large and destructive.

Lawinenprobleme

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Likely
    Größe
    2

    Wind slabs that have formed during this storm have grown large and are widespread. Human-triggered wind slab avalanches will remain likely today and natural wind slab avalanches will still be possible. The most sensitive wind slabs should exist on wind-loaded N-NE-E aspects and cross-loaded NW and SE aspects in near and above treeline terrain, but some wind slabs could also linger in below treeline terrain due to the strength of the winds early in this storm. Wind slab avalanches could be larger and more destructive than expected and could run into more sheltered areas like dense stands of trees. 

    Recent avalanche activity, blowing snow, drifted snow, cornices above slopes, and shooting cracks can help identify where wind slabs may exist. Use this information to avoid wind-loaded slopes. Wind slabs may exist in unexpected places today. 

  • Loose Dry

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Likely
    Größe
    2

    Changing conditions during this storm have created several potential weaknesses within the storm snow including graupel layers and denser layers of snow on top of softer layers of snow (upside-down layering). These storm snow weaknesses mean that human-triggered storm slab avalanches will remain likely today on steep sheltered slopes. These avalanches could involve all or part of the new snow and could occur in areas traditionally thought of as safe like tree-covered slopes.  While most of these should break in the upper layers of the storm snow, some of them could still be large and destructive due to the amount of new storm snow that exists.  

    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

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    3

    Human-triggered deep slab avalanches will be possible today. If they do occur they would likely fail on a loose weak layer of snow buried several feet below the surface near a crust. Observations have found this layer on NW-N-NE aspects across the forecast area. These deep slabs are most likely to react to larger triggers like several people on a slope, large cornice failures, or other avalanches or the right trigger in the right spot on a slope (a trigger point). 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 accumulated during the storm, deep wind slabs and possibly deep storm slabs could behave more like a deep slab avalanche problem (failing deeper, wider, and more destructive than typically anticipated) and not necessarily triggered by the first person on the slope.

Avalanche Discussion

* Yesterday on Mt. Judah (Donner Summit area), wind-loading had reloaded slopes that had already avalanched and ski kicks on lower angle sections of the reloaded wind slabs up to 30 ft away triggered wind slab avalanches and cornice collapses on the steeper sections of the slopes. Widespread evidence of previous natural avalanches existed. 

*  Observations from the Blue Lakes area (near Carson Pass) found evidence of weaknesses within the storm snow layers including graupel and density changes. 

* The persistent weak layer of loose weak snow near a crust (near crust facets) is now buried several feet below the surface, but observations on Mt. Judah and in the Blue Lakes area indicated that snowmobiles and skiers could still affect that layer. Snowpit tests targetting the persistent weak layer on Mt. Judah did not yield unstable results.  

Most avalanche fatalities happen on days like today. Human-triggered avalanches remain likely and dangerous avalanche conditions exist in the backcountry, but the danger has become more subtle. This doesn't mean that you cannot get out into the mountains. Safe backcountry travel requires training, experience, and proper planning to manage terrain to avoid the hazards. 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, could fail after other tracks already exist on a slope, 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 or fail after multiple people have used the slope. 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. 

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