Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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Pericolo valanghe

Alta quota
1 · Debole
Limite del bosco
1 · Debole
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 13 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

The avalanche danger remains LOW at all elevations and on all aspects. Even though avalanches have become unlikely on a regional scale, some unstable snow could still lurk on isolated terrain features or in extreme terrain.

Problemi valanghivi

  • Cornices

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    1

    Across the forecast area a layer of soft, weak snow sits on top of a strong refrozen snowpack in most places. Since no slabs exists on top of the soft, weak snow this weakness does not currently pose a problem, and avalanches remain unlikely in most places today. However, two potential problems could arise concerning this layer of soft snow once more snow accumulates above it:

    1. A buried surface hoar layer exists in the upper few inches of the snowpack in some areas. Surface hoar often forms during the calm, clear, cold nights here in Tahoe, but it rarely gets buried. Last Sunday's light snowfall buried a surface hoar layer under 2-4 inches of light, soft snow in some areas.
    2. The snow above the rain crust has started to weaken and facet and take on the characteristics of a persistent weak layer.

    For now keeping track of where these layers exist and how they are changing will help determine which slopes may pose future problems. While traveling in the backcountry, stop and dig into the snow from time to time. Sift soft snow above the rain crust through your hands. If it feels sugary and you can't make a snowball out of it, it is probably facets. If you see any triangular shaped, striated snow grains in the top few inches of the snowpack, a shallowly buried surface hoar layer exists in that area.

  • Deep Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    1

    In areas above 9,500 to 9,800 ft, rain did not percolate all the way through the old facetted snow layers on near treeline and below treeline NW-N-NE aspects. In these upper elevation areas, human triggered persistent slab avalanches are not impossible. Weak layers of well developed facets and depth hoar exist below the overlying slab of recent snow in these areas. Since there are very few areas in the forecast area with terrain above this elevation where these conditions exist, this problem remains isolated in distribution.

Avalanche Discussion

Yesterday on Tamarack Peak, 8 to 12 inches of soft, cold, unconsolidated snow existed above a 4 to 6 inch refrozen rain crust on the N-NE aspects below 9500 ft. On other aspects much less of the soft snow remained. Ski cuts on test slopes, snowpit data, and other observations did not reveal signs of instability in the upper soft snow layers in most places. Data did indicate that this soft snow layer has begun to facet and weaken. One wind loaded NE facing test slope at 9200 ft did release a 3 to 4 in deep wind slab in response to a ski cut (photos). This small slab was about 15 ft wide and ran downhill for about 20 to 30 ft.  Below the rain crust an unconsolidated layer of large snow grains (what used to be the Dec. 7th facets) still remained. Tests and observations indicated that it is difficult to get force through the thick rain crust.

Above 9500 ft. the rain crust thinned along with the thickness of the old snow layers below it. On many of the near and above treeline slopes in this area the recent soft snow sat on top of the ground with no old snow below it (see photo from the top of the Hourglass).

Unlike Silver Peak and Rubicon Peak earlier this week, no signs of buried surface hoar existed in this area.

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