Bulletin d’avalanche

Central Sierra Nevada

Sierra Avalanche Center
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Danger d’avalanche

Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 12 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

CONSIDERABLE avalanche danger exists on near and above treeline NW-N-NE-E-SE aspects on slopes 35 degrees and steeper. Human triggered avalanches are likely in these areas. For all other areas avalanche danger is MODERATE on slopes 35 degrees and steeper. Wind slabs, storm slabs, and persistent slabs are all possible today.

Problèmes avalancheux

  • Wind Slabs

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

    Human triggered wind slab avalanches are likely today. The best window for natural wind slab avalanches occurred last night, but some natural wind slab avalanches may remain possible in heavily wind loaded areas today. During the last 24 hours, new snow and southwest winds created wind slabs on the wind-loaded aspects. In exposed near and above treeline terrain, fragile wind slabs 2 to 4 ft in depth could exist on the wind loaded N-NE-E aspects and cross loaded NW and SE aspects. Wind slab avalanches could step down to the older weak layers mentioned below in some cases.

  • Loose Dry

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

    Changing conditions during the storm allowed weaknesses to form in the storm snow. As a result of these storm snow weaknesses, human-triggerable storm slabs could exist on any slope with new snow that is not wind affected. These storm slabs should remain less than 2 ft in depth, but if they release they could add enough load to snowpack to cause a failure in the loose weak snow layer under the old crust at the base of the recent snow.

  • Deep Persistent Slabs

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

    The loose weak layer below the old crusts on the NW-N-NE aspects now has 2-3 ft of snow on top of it in most places. This additional load could bring it close to its breaking point, and human triggered persistent slab avalanches may be possible today. Other larger triggers like the wind slabs or storm slabs mentioned above, cornice failures, or multiple people on a slope are more likely to cause a failure in this layer. This weak layer has been observed in many locations around the forecast area and is most well developed in near treeline and below treeline areas above 8,000' on NW-N-NE aspects. Any avalanches that fail on this weak layer will be large, involving all of the snow accumulated since March 25th. These kind of avalanches can fracture across longer distances and in areas that do not typically slide.

Avalanche Discussion

Yesterday, observations on northerly aspects on Becker Ridge and in Candy Land (Bear Valley backcountry) showed a thin crust on the snow surface prior to the onset of last night's storm. Below this surface crust, 8-15 inches of recent storm snow existed on top of an older and thicker crust. A layer of loose weak snow (near crust facets) exists just below this older crust in some areas. In the Candy Land area, this weak layer resided on northerly aspects between 7500 and 8500 ft, and tests on this weak layer indicated that if it breaks the resulting crack can propagate along the layer. On Becker Ridge this weak layer was only well developed above 8200 ft, and tests did not reveal any signs of instability associated with this layer. Other observations around the forecast area since 3/27 have shown similar results and variability concerning this weak layer. Data seems to indicate that it is weakest and most well developed on northerly aspects above 8000 ft.

Régions couvertes