Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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Pericolo valanghe

Sun
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
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

Ridgetop winds are expected to drift and deposit new snow into cohesive slabs along ridgelines. Identify areas of concern below cornice formations and wind drifts. Move around these wind loaded slopes with caution. The areas of greatest avalanche hazard today are expected to focus along the Sierra Crest.

Problemi valanghivi

  • Wind Slabs

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

    New snow falling this morning combined with winds over the ridgetops out of the SSW is drifting and depositing snow onto the downwind side of ridgetops. New wind slabs are expected to form this morning where drifting snow is deposited. This is expected mainly along the Sierra Crest where new snowfall amounts will be highest.

    Look for areas where wind drifting is actively occurring, where cornices or wind drifts have formed, and for any areas of snow surface cracking. Use these clues to identify areas of potentially unstable wind slab. Move around these areas with caution and consider opting for wind protected terrain.

    (12/28/2023)

    An avalanche that occurred on Dec 27, likely naturally triggered during an intense period of wind loading. This may have been an instance of deeper persistent weak layer failure in a wind loaded area or just a natural wind slab avalanche. The Sisters, Carson Pass area, CA

    Photo: Public

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2–2.5

    Upper elevation NW-N-NE aspect terrain above about 8500 ft holds layers of weak faceted snow near the ground that is buried 1 to 3 feet below the old snow surface.  This weak layer appeared to have become dormant leading up to the current storm. In locations where wind drifted snow is depositing on top of a weak snowpack (NW-N-NE aspect terrain above about 8500 ft) this deeper weak layer could reactivate. Any persistent slab avalanches that occur will have a high density slab and expose rocks and logs at the bed surface both of which will increase the likelihood of trauma.

    Any signs of snowpack collapse or whumpfing sounds are indications that this avalanche problem is reactive. Be wary of wind loaded areas across NW-N-NE aspects near and above 8,500'. Several options exist for minimizing exposure to this avalanche problem. In wind protected locations that receive just the forecasted up to 4 inches of new snow, it is expected that this weak layer will remain dormant with triggering unlikely. If in doubt, choose aspects or elevations where this persistent slab avalanche problem does not exist. If encountering signs of instability or just want the safest option, choose the lower angle, non-avalanche terrain without steeper connected terrain above or to the side.

Avalanche Discussion

Other weak layers exist in addition to the faceted snow at the bottom of the snowpack that is outlined in the persistent slab problem. Weak faceted snow has been observed at the snow surface and just below thin surface crusts that formed across many aspects over the past week. These weaker snow crystal types that are now buried at the old/new snow interface could increase the chances of triggering a wind slab avalanche or allow wind slab reactivity to linger longer than usual.

The snowpack is usable, mainly on NW-N-NE-E aspects above 7,500 to 8,000 ft. Melt over the past week 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