Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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Pericolo valanghe

Sat
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
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Non aggiornato — questo bollettino è scaduto 4 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Even though chances for natural avalanches may have lessened, dangerous and unusual avalanche conditions still exist. Triggering avalanches would be easy on steep slopes in exposed and sheltered areas where new snow rests on top of weak snow or where wind-drifted snow exists near ridges. Avalanches may be triggered from places below or near steep slopes. Using flatter terrain away from steep slopes can help avoid today's problems. 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    2–3

    Yesterday's storm slab problem has become today's persistent slab problem. This problem will linger and create dangerous and unusual avalanche conditions for our area.

    Sunny skies, fresh powder, and early-season stoke will lure people into the mountains and could impair our judgment. The decrease in natural avalanche activity today will help to mask the danger. Weak, faceted snow buried under new snow is just waiting for someone to break it. Large human-triggered avalanches are LIKELY today. Weak snow exists at all elevations, with the most widespread and weakest snow on NW-N-NE-E aspects. Some weak snow is also buried on other aspects. Once a fracture starts in the facet layer, it can propagate long distances like a falling row of dominos. Avalanches can break wider than expected, wrap around terrain features, and travel farther downhill than usual. People can also trigger these avalanches remotely from nearby slopes or flat areas at the bottom of a hill. 

    It would be easy to get caught in an avalanche today. Shooting cracks, whumpfing, and collapsing represent clear signs of instability. Digging into the snow and probing can help identify the weak layer under the new snow. Increase safety margins by steering clear of any terrain steeper than 30° or terrain connected to/below steep terrain if you see signs of instability or feel weak snow. If there is any uncertainty about whether or not the weak layer exists, play it safe and assume the snowpack is unstable. 

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    2–3

    The storm also piled deep slabs of wind-drifted snow onto leeward-facing slopes. In some areas, these wind slabs rest on top of the weak layer mentioned above and would behave like deeper versions of the persistent slab problem. In other areas, wind slabs landed on previously wind-scoured snow surfaces or bare ground. Large human-triggered wind slab avalanches will be likely today. They could run longer than usual or trigger additional avalanches as they move downhill. The largest and most unstable wind slabs will exist on NW-N-NE-E-SE aspects. 

    Use clues like cornices above a slope, drifted snow, or other wind-created surface textures to help evaluate which slopes hold wind slabs. Choose travel routes that avoid these wind-loaded areas. Cautious route-finding focusing on lower-angle terrain not connected to steeper terrain can help avoid both avalanche problems today. 

    (12/1/2022)

    Small wind slab failure on a wind-loaded test slope in exposed upper elevation terrain on Andesite Peak. This failed on a weakness within the storm snow.

    Photo: Andy Anderson

Avalanche Discussion

  • Yesterday, observers and forecasters triggered several long shooting cracks in low-angle terrain that failed on the near-surface facet layer below the new snow. The near-surface facets are widespread across the forecast area and remain weak. 
  • Intense wind loading was reported from exposed ridgelines, and some wind slab failures occurred on small test slopes. 
  • Before this storm, the snowpack was weak and shallow. Numerous rocks, trees, stumps, and other obstacles existed on the surface or just below the surface. These obstacles remain close enough to the surface to hit and would increase the consequence of getting caught in an avalanche.

Regioni coperte