Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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Pericolo valanghe

Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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In sintesi

On slopes steeper than 32 degrees, MODERATE avalanche danger exists on NW-N-NE-E-SE aspects at all elevations due to lingering persistent slabs and wind slabs sitting on persistent weak layers. Large human triggered avalanches remain possible today. Unusual avalanche conditions exist. Avalanches on steeper slopes could be remotely triggered by a person standing in lower angle terrain or may be triggered in places traditionally considered safe. Evaluate snow and terrain carefully; identify features of concern.

Problemi valanghivi

  • Deep Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    3

    1 to 3 layers of buried surface hoar still lurk within the snowpack. Several avalanches have occurred on these buried surface hoar layers with 2 more possibly occurring yesterday. People should not be surprised if they trigger persistent slab avalanches that fail due to one of these surface hoar layers collapsing today. Human triggered persistent slab avalanches large enough to bury people are still possible on any slope steep enough to slide at all elevations where buried surface hoar exists. Avalanches that fail on buried surface hoar can propagate long distances connecting trees or rocks, they can wrap around corners, and they can occur in places traditionally considered safe. People can also trigger failures in the surface hoar layer on lower angle terrain and that failure can propagate into steeper avalanche terrain and cause avalanches. In near and above treeline terrain wind slabs that have formed on top of buried surface hoar layers will behave like large persistent slabs and may look like wind slabs on the surface. In near and below treeline terrain persistent slabs will also exist, but they will be harder to identify. 

    Buried surface hoar does not exist on every slope so this avalanche problem will not exist on all slopes. Unfortunately, determining which slopes the buried surface hoar does exist on is difficult. Recent avalanche activity, shooting cracks, collapsing, whumpfing, and snowpit tests can provide some clues, but sometimes the first clue is an avalanche triggered by the party on the slope. So far observations have shown buried surface hoar layers on open N-NE-E aspects and a few NW and SE aspects.

    On some slopes where rain falls on the snowpack, the rain on snow could trigger some instabilities like loose wet avalanches or even the persistent slabs. Since the snow level should stay below 7500 to 7000 ft and not much rain is expected, this issue should not become widespread. If more rain falls than forecasted or snow levels climb higher than forecasted, rain triggered persistent slabs could be more of an issue. 

     

Avalanche Discussion

Yesterday observations, snowpit data, and test slopes from Hidden Peak and Crystal Bay found some storm snow weaknesses in the recent snow but did not reveal buried surface hoar layers. Some minor cracking and small wind loaded test slope failures occurred as a result of this weakness on Hidden Peak. Evidence suggested that the recent snow was gaining strength on Hidden Peak. On Luther Pass a low elevation snowpit found one of the surface hoar layers but snowpit tests did not show as much lingering weakness associated with it in that spot. On the other hand, we did receive two vague reports of avalanche activity. One was reported as a remotely triggered avalanche on a NE aspect somewhere on Donner Summit which sounds similar to other avalanches that have resulted from failure of the buried surface hoar. The other was reported on a E-SE aspect of Ralston Peak. We don't have any other details about those two.

If you would like to report observations to the avalanche center to help us create better forecasts and help get more information out to more people, please use our submit observation forms:

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