Bulletin d’avalanche

Central Sierra Nevada

Sierra Avalanche Center
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Danger d’avalanche

Fri
Alpin
4 · Fort
Limite forestière
4 · Fort
Sous la limite
4 · Fort
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Points saillants

Storm slabs will form today in both wind exposed and wind protected areas as another round of heavy snow and strong winds impact our forecast region.  Uncertainty continues to exist with ongoing reactivity of buried weak layers under the large storm snow load.  Potential continues to exist for large to very large avalanches occurring in the backcountry.  Travel in, near, or below avalanche terrain is not recommended today.  

Problèmes avalancheux

  • Storm Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Very Likely
    Taille
    1.5–3

    Another round of snow and strong winds will occur today, continuing to load our existing snowpack.  Large to very large avalanches remain a very real concern with ongoing uncertainty with how weak layers will respond to additional storm snow load.  Travel in, near, or below avalanche terrain is not recommended.

    Storm snow instabilities could take the form of slabs or sluffs today in many locations.  Slabs of wind drifted snow will continue to form in wind exposed areas below ridges, in gully features, and on open slopes as wind speeds remain strong to gale force through tonight.  Weaknesses within the storm snow have been observed with multiple graupel layers forming yesterday.  Colder temperatures could potentially slow settlement rates and stabilization of recent storm snow.  

    The Feb. 10 facet layer exists on NW-N-NE aspects in wind protected terrain above around 7500'.  The avalanche accident that occurred on Tuesday is suspected to have involved this weak layer.  Whumping and unstable snowpack test results were reported yesterday indicating that this weak layer could still be reactive.  See observer video here:  https://photos.app.goo.gl/nimKWgtn4onF4pe88  Avalanches involving this weak layer would be large to very large in size, could break wider than expected and run long distances, and could be triggered mid-slope or below.     

Avalanche Discussion

The avalanche that occurred on Tuesday February 17 at the Frog Lake/Castle Peak area is a major blow to our community and the human toll is immeasurable.  Our condolences go out to all that have been affected by this tragedy.

Weak sugary snow (near-surface facets) formed on many NW-N-NE aspects during the recent, extended dry spell in January. This weak layer was buried by new snow starting on Feb 10, and is now under 3 to 7+ feet of recent storm snow. Most areas have a thin rain crust along the top of this facet layer up to 8,600 to 8,800 ft.  This weak layer still represents a concern with uncertainty surrounding the ease of triggering large avalanches.  Weak layers, similar to the Feb 10 facet layer, have contributed to large avalanches and avalanche fatalities within our forecast area in the past.

Lower elevation areas and sun-exposed slopes experienced significant snowmelt in late January and early February, exposing areas of dirt, rock, and downed trees.  These hazards and obstacles now lie hidden, covered by low density new snow. This creates a hazard situation similar to early season coverage in some areas.  Additionally, deep open holes, open creeks, and ponded water are widespread in low-lying areas, especially below 8,000'. 

Régions couvertes