Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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Pericolo valanghe

Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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Non aggiornato — questo bollettino è scaduto 11 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

MODERATE avalanche danger exists on slopes 32 degrees and steeper above treeline on all aspects, near treeline on NW-N-NE aspects, and below treeline NW-N-NE aspects. For other areas LOW avalanche danger exists. Today's avalanche problems are a combination of persistent slabs and wind slabs. Natural avalanches are unlikely today. Human triggered avalanches are possible and to a significant degree expected today near treeline and below treeline on NW-N-NE aspects where areas of buried surface hoar have not yet collapsed.

Problemi valanghivi

  • Deep Persistent Slabs

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

    Persistent slab avalanche problems remain an issue today following yesterday's rapid new snow loading and subsequent avalanche activity. Areas of buried surface hoar linger near treeline and below treeline on NW-N-NE aspects, most likely above 7,000' to 7,500'. This is an uncommon weak layer for this region and it certainly does not abide by any mythical "24 hour rule". The potential for human triggered avalanches involving all of the new storm snow with the potential for remote triggering and wide propagation, wrapping around corners, etc requires a more conservative approach to both uphill and downhill travel. Question traditional route placement and give large steep open areas below treeline a wide birth until the presence or absence of the weak layer is verified. Ongoing easy shears in hand pits at the old/new snow interface are a potential indication of buried surface hoar that remains intact as a weak layer. Surface hoar crystals may be visible on the lower side of the snowpack failure interface.

    Developing near crust facets around the Dec 10 rain crust also pose a potential persistent slab avalanche problem. These facets have been noted as most well developed in the Carson Pass and Ebbetts Pass/Bear Valley areas on NW-N-NE aspects above about 8,000'. In other areas this weak layer has not been seen.

  • Wind Slabs

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

    Last night's wind shift from SW to NE will redistribute snow in above treeline areas. New wind slabs have formed on SE-S-SW-W-NW aspects. Wind scouring will occur on N-NE-E aspects, but pockets of wind slab will remain on N-NE-E aspects in complex terrain. Look for areas of blowing snow, recent cornice formation, and wind pillows below ridgelines. These are all indications of potential wind slabs on the slopes below.  

Avalanche Discussion

Observations made yesterday in the Deep Creek area revealed natural and skier triggered avalanche activity occurring on buried surface hoar in open areas near treeline and below treeline on N-NE aspects. In any areas where buried surface hoar had not yet collapsed, it was readily identified at the failure interface in hand pits and was reactive in snowpit tests. On Becker Ridge (Echo Summit area) better stability was observed with an absence of buried surface hoar. Very difficult to trigger but still potentially problematic rounded and moist facet remnants at the base of the snowpack were noted to produce sudden collapse failure characteristics in snowpit tests on northerly aspects in this area. On Castle Peak (Donner Summit area) a party reported sensitive wind slabs near treeline on N aspects with unstable snowpit tests leading to test slope failure.

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