Bulletin d’avalanche

Central Sierra Nevada

Sierra Avalanche Center
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Danger d’avalanche

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

Points saillants

A rapid increase in avalanche danger is expected this afternoon during a period of high intensity snowfall and gale force winds. The chances of triggering a large avalanche in a variety of areas will increase as the day progresses. A few hours of HIGH avalanche danger may occur starting just before sunset and lasting into the evening hours.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    2–2.5

    New persistent weak layers in the form of a facet/crust combo and/or surface hoar were buried on Jan 2 across a wide range of aspects and an avalanche cycle occurred on these layers on Jan 3. These weak layers are generally 4 to 18 inches below the old snow surface and have been observed in numerous locations around the forecast area including Carson Pass, West Shore Tahoe to Donner Summit, and the Mount Rose zone. They likely exist in other areas as well. As new snow loads these weak layers, another avalanche cycle is expected to occur starting around midday today and last through tomorrow. It might continue longer given the specific type of weak layers we are dealing with.

    New snow will load and stress these weak layers today. Signs that these weak layers have hit critical loading will include snowpack collapse, whumpfing sounds, snow surface cracking, and avalanche activity.  These avalanches could be triggered from above, below, or from off to the side of a slope. This narrows the margin for error, making terrain selection and perfect route finding more critical than is often the case. If in doubt, consider avoiding avalanche terrain for the time being. Low density new snow will make for great riding/skiing conditions on lower angle terrain.

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–2

    Ridgetop winds have shifted around to the SW and are increasing in speed this morning. New snowfall combined with strong to gale force winds today will drift and deposit snow into cohesive slabs in leeward (downwind) areas. The vast majority of new wind slabs are expected to form today in above treeline and near treeline areas, but may also form to a lesser extent in open areas below treeline due to the strength of the wind.

    Look for areas where blowing snow is depositing and for any areas of snow surface cracking.  Clues such as cornice areas, wind drifts, and loaded vs scoured snow surface conditions can help identify the areas of greatest concern. Exercise increased caution today around wind loaded areas, especially if there are signs that any of the above mentioned Jan 2 persistent weak layers are present in the area. 

    (1/4/2024)

    Wind slab avalanche, likely naturally triggered on Jan 3. This is classic terrain for a wind slab avalanche to occur. NE aspect, Peak 8,506, Upper Pole Creek, CA

    Photo: SAC

Avalanche Discussion

More on our persistent slab avalanche problem:

The scope of the persistent slab problem has changed with weak layers higher up in the snowpack now the main concern and the problem is distributed across more aspects. The Jan 2 layers of crust/facet combo and/or buried surface hoar found 4 inches to 1.5 feet below the old snow surface are the layers to watch.

For much of the season, the persistent slab problem focused on faceted snow near the base of the snowpack. Substantial strength gains in the midpack and a deepening of the snowpack along the Sierra Crest have reduced concerns for deep weak layer failure until a major atmospheric river type storm with multiple inches of SWE returns to the forecast area. In the Carson Range along the eastern side of the forecast area where the snowpack has remained shallower and weaker, the situation is different and there remains a greater concern for deeper weak layer failure with incremental new snow loading over the next week or so.

Régions couvertes