Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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Pericolo valanghe

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

In sintesi

Avalanche activity is unlikely today. Travel with a mindset and actions that will minimize the consequences of triggering an avalanche unexpectedly. Avalanche danger will increase tonight and tomorrow as new snowfall and wind impact the forecast area.

Problemi valanghivi

  • Persistent Slabs

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

    Upper elevation NW-N-NE aspect terrain above about 8500 ft holds a layer of weak faceted snow near the ground that is buried underneath 1 to 3 feet of snow.  This weak layer appears to have become dormant, waiting patiently for a new snow load to reactivate instability. The amount of new snow load that accumulates by Saturday afternoon could be enough to increase the likelihood of triggering a persistent slab avalanche.

    For today, signs of persistent slab instability will be hard to come by without digging into the snowpack and testing the layers (PST is likely the best test until it snows). Several options exist for minimizing exposure to this avalanche problem which could come in handy for this weekend. You can choose to travel on specific aspects or below specific elevations where this persistent slab avalanche problem does not exist. You can also choose the lower angle, non-avalanche terrain without steeper connected terrain above or to the side in areas where this problem exists.

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    1–2

    Gale force SW winds on Dec 27 drifted and deposited snow into cohesive wind slabs up to 2 feet thick. There is a slight chance one of these wind slabs could still be triggered today, potentially in a shaded spot where a wind slab formed on top of softer, near surface facet snow crystals. Shaded areas directly below cornice features are the most suspect for today. Move around these areas with caution.

    In the wake of the Dec 27 wind event, very little snow remains available for drifting today as winds increase ahead of the incoming storm. It is unlikely that new wind slabs will form before the onset of snowfall tonight. Once new snowfall begins tonight, a new round of wind slab formation will occur with the likelihood of triggering an avalanche increasing tonight and tomorrow.

    (12/28/2023)

    A snowboarder triggered wind slab avalanche from Dec 27 on an ENE aspect of Peak 9,750 above Crater Lake, Carson Pass area, CA. Start zone in the shade, indicating the potential for near surface facets on the bed surface.

    Photo: SAC

Avalanche Discussion

Extended periods of clear weather and a shallow snowpack have left various weak layers on the cold, northerly aspects that do not see much sun this time of year. Facets exist near the bottom of the snowpack, on the surface, or near/between crusts. Recent warm weather has allowed the weak, faceted (sugary) snow to start clumping together in some areas. In other places, the weak layers remain dry and loose. In general, the snowpack gets weaker as elevation increases and as you move east of the Sierra Crest. The resulting persistent slab problem may linger for some time and could reactivate with future storms.

The snowpack is usable, mainly on NW-N-NE-E aspects above 7,500 to 8,000 ft. Melt since the last storm has caused the lower limit of the usable snowpack to creep upwards in elevation. Expect some rocks to hit and open creeks to deal with.

Regioni coperte