Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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Pericolo valanghe

Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
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In sintesi

Widespread areas of CONSIDERABLE avalanche danger exist near and above treeline on NW-N-NE-E-SE aspects 30 degrees and steeper due to newly formed wind slabs and below treeline on NW-N-NE aspects 30 degrees and steeper due to persistent slabs. Natural avalanches are possible, human triggered avalanches are likely today. Significant avalanche danger exists below treeline. --- Everyone is chomping at the bit to get out there today. Only a small percentage have a handle on the weakness of the existing snowpack and the reality of avalanches today. Conservative terrain selection and effective group communication are imperative.

Problemi valanghivi

  • Deep Persistent Slabs

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

    The areas that held the greatest amount of snow cover prior to last night's snowfall also hold the greatest hazard today. The near treeline and below treeline areas have many weak layers in the form of basal facets, near crust facets, near surface facets, and surface hoar. New snow loading on top of these persistent weak layers will allow for the possibility of natural and human triggered avalanches today. Wide propagation and avalanche activity on lower angle slopes to around 30 degrees are likely today below treeline. Anyone who is thinking that they will avoid avalanches today simply by staying in the trees is greatly mistaken. 

  • Wind Slabs

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

    Near and above treeline, wind loading on NW-N-NE-E-SE aspects is expected to have created fresh wind slabs. Due to previous wind scouring, these new slabs are forming on top of just a few inches of old snow in most areas. Avalanche activity that occurs in these areas is most likely to occur within the storm snow. In isolated areas below cliff bands on NW-N-NE aspects, more old snow exists above treeline with the potential for more active weak layers within the older snowpack.

Avalanche Discussion

The snowpack so far this season has been built a few inches at at time, not feet at a time in typical Sierra fashion. As a result the existing snowpack is weak with many weak layers. It is more like a snowpack that is commonly experienced in the Rockies, Tetons, or Wasatch than in the Sierra Nevada.

Observations collected from around the forecast area over the past few weeks have shown that the existing snowpack is weak and in poor condition to handle new snow loading. Near treeline and below treeline areas that are above 7,500' on NW-N-NE aspects held 1 to 2 feet of weak faceted (sugary) snow, eroding crusts, and surface hoar prior to the current snowfall. The vast majority of above treeline areas were significantly scoured by ENE winds Nov 21-23, removing most of the previous snow cover.

Observations made yesterday below Grouse Rocks (Ward Canyon area) revealed multiple weak layers within the snowpack on northerly aspects. Surface hoar was prevalent in open areas below treeline. Near crust facets on top of the upper most crust in the snowpack produced propagating failures in snowpit tests. For the most part, a significant slab was absent from above the most problematic weak layers yesterday (video, photo, pit profile, more info). Now that significant amounts of new snow have been added overnight, the slab is now there and additional load has been placed on these weak layers.

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