Lawinenlagebericht

Central Sierra Nevada

Sierra Avalanche Center
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Lawinengefahr

Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
2 · Mässig
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Kernpunkte

MODERATE avalanche danger exists at all elevations due to newly formed wind slabs and unlikely but not impossible persistent slab avalanche problems lingering in isolated areas. In the unlikely event that an isolated human-triggered persistent slab avalanche does occur, it could be large with significant consequences.

Lawinenprobleme

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1

    Small wind slabs sensitive to human-triggering formed near ridgelines on wind-loaded slopes by yesterday afternoon. After a night of continued wind-loading, wind slabs may now exist on wind-loaded N-NE-E aspects and some cross-loaded NW and SE aspects in near and above treeline terrain. Most of these wind slabs should remain small, not involve enough snow to bury a person, and not extend very far from ridgelines. In the most wind-loaded areas or in areas where terrain traps could magnify the consequences of any size avalanche, some of them could have grown large enough to bury a person.

  • Deep Persistent Slabs

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

    While triggering a persistent slab avalanche may be unlikely, snowpit tests continue to yield unstable results indicating that persistent slab avalanches are not impossible for just the right trigger in just the right spot on some isolated near and below treeline NW-N-NE aspects sheltered from previous NE winds. If a person does trigger one of these isolated, difficult to trigger (stubborn) persistent slabs, the resulting avalanche could be large (up to D2 or D3) and destructive. Which isolated slopes may still hold a persistent slab avalanche problem is subject to significant uncertainty and variability with no clear explanation for the variations in stability across similar terrain.

    Due to the uncertainty, variability, and potential high consequences of this difficult to trigger (low probability) avalanche problem, avoiding terrain where persistent slabs may exist represents a prudent choice. Collapsing and whumpfing, unstable snowpit test results, and handpits/probing into the snowpack to feel for a loose layer of weak snow under firm slab layers can help identify where persistent slab problems may still exist. Keep in mind that snowpits dug high on the slope may miss evidence of this problem since it is more likely in the mid to lower portions of the slope.

Avalanche Discussion

* Yesterday, a snowpit test on a N aspect on Tamarack Peak (Mt. Rose backcountry) and several tests on N-NE aspects on Red Lake Peak (Carson Pass area) indicated that while a persistent slab may be difficult to trigger if the weak layer does break the fracture could still travel through it. Other tests from Elephant's Hump (Carson Pass), Upper Blue Lake and Twin Lake areas (South of Carson Pass), and Cold Stream Drainage (Donner Summit area) in the last few days agree with the tests from yesterday. This evidence of stubborn instability exists in near and below treeline terrain on some NW-N-NE aspects sheltered from NE winds. 

* Other tests and observations from additional similar locations on Tamarack Peak (Mt. Rose backcountry) and Castle Peak (Donner Summit) yesterday, as well as observations and data from Andesite Peak (Donner Summit) and National Geographic Bowl (Granite Chief Wilderness) on Saturday, did not reveal any signs of instability associated with the persistent weak layer and snowpit tests indicated a well-consolidated snowpack.

* Ski cuts yesterday afternoon on a small NE facing wind-loaded test slope on Tamarack Peak triggered a small wind slab failure. Winds started to transport snow in that area in the late morning.

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