Bulletin d’avalanche

Central Sierra Nevada

Sierra Avalanche Center
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Danger d’avalanche

Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 11 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Areas of MODERATE avalanche danger remain near and above treeline on NW-N-NE-E-SE aspects on slopes 35 degrees and steeper due to recently formed wind slabs. Below treeline, there is LOW avalanche danger with pockets of MODERATE avalanche danger in open areas below treeline and near treeline on N-NE-E aspects on slopes 32 degrees and steeper due to deep persistent slabs.

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    2

    Lingering wind slabs that formed from the most recent storm remain in near and above treeline areas on NW-N-NE-E-SE aspects. Human triggered avalanches 12 to 18 inches deep occurred yesterday in some wind loaded areas (more info below). Wind slabs will be more difficult to trigger today than yesterday, but could still bury or injure a person. Use clues such as recent avalanche activity, cornice formations, wind pillows, and snow surface texture to identify suspect slopes.

  • Storm Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    3

    Areas where deep persistent slabs remain a problem are becoming more isolated. Buried surface hoar layers are now 2.5 to 4.5 feet deep in the snowpack and remain unstable in some areas. A widespread persistent slab avalanche cycle occurred Jan 5 through 19 as these weak layers collapsed. Distribution of these weak layers are changing. In some areas, avalanche activity has cleaned them out, in some areas they have already collapsed or been compressed and are no longer reactive, in other areas the buried surface hoar crystals are rounding and gaining strength. In some areas snowpit data indicates that instability is ongoing. Open areas near treeline and below treeline on N-NE-E aspects is where this avalanche problem is found, not up near the ridgetops in the traditional avalanche start zones.

    Even though avalanche occurrence has become unlikely, consequences remain high. Due to the depth of the weak layer and density of the snow above, any deep persistent slab avalanches that do occur could be large, destructive, and difficult to survive. As with all weak layers, slab avalanches are most likely to be triggered in areas where the weak layer is closer to the snow surface. This often occurs near isolated rocks and trees. This is certainly the type of avalanche problem where a slope could avalanche after it has already been traveled by numerous individuals who left tracks without finding the shallow spot trigger point. Perform your own evaluation, keeping in mind that previous tracks are not a sign of intelligent life. For more info on managing deep persistent slabs, see the Avalanche Problems Toolbox.

Avalanche Discussion

Human triggered wind slab avalanches 12 to 18 inches deep occurred yesterday on Andesite Ridge (Donner Summit area) in above treeline areas on NE aspects. A small natural avalanche was observed in a creek bed in terrain south of Carson Pass between Blue Lakes and Indian Valley (more info on these avalanches in the specific obs below).

Encountered snowpack stability varied widely yesterday in the backcountry with some parties seeing little to no signs of instability and others seeing avalanches. Significant amounts of blowing snow and wind transport were observed across the forecast area. Most evidence of instability that was observed occurred within the top 2 feet of the snowpack. In isolated areas, snowpit data continues to point to ongoing deep instability down 2.5 to 4.5 feet where the Jan 5, Jan 9, and Jan 13 buried surface hoar layers linger. Numerous rain crusts now exist within the top 3 feet of the snowpack, especially below 8,000'. Problematic near crust faceting has yet to be reported, but these crusts warrant monitoring for potential weak layer development.

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