Bulletin d’avalanche

Eastside Region

Eastern Sierra Avalanche Center
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Danger d’avalanche

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

Points saillants

Very dangerous and unpredictable avalanche conditions exist as a result of heavy snow and wind loading combined with a buried persistent weak layer. It will be difficult to maintain adequate safety margins today. Travel in avalanche terrain is not recommended.

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Very Likely
    Taille
    2–3.5

    Over the past twenty-four hours, strong southwest winds have been loading leeward terrain features at all elevations. Wind-transported snow can accumulate and overload the snowpack much more quickly than storm snow alone. Large and very large natural avalanches are expected to release today in leeward avalanche paths that sit just below ridgelines. It's important to avoid the runouts of large avalanche paths today. With good coverage at lower elevations, very large natural avalanches could run down to valley floors and into low-angle below treeline terrain. An additional consideration is that with high rates of wind loading, avalanche paths that have already avalanched during this cycle may refill and release again.

    (2/26/2023)

    Large natural wind slab avalanche. Released on the morning of 2/26 on a NE aspect at 8700'

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    2–3.5

    From late January through late February, the Eastern Sierra saw mostly clear weather and very little snowfall. During this dry spell a persistent weak layer of faceted snow developed in the upper 40cm of the snowpack and has been buried by this week's storm. Up to 60" of snow combined with wind loading from southeast and southwest winds have pushed persistent weak layers to the breaking point. Wind loading today in terrain where the persistent weak layer is buried means the likelihood of persistent slab avalanches is increasing. Persistent slab avalanches can be remotely triggered from adjacent slopes and flat terrain below a steep slope. If triggered, persistent slabs can propagate across and around terrain features that would otherwise confine new snow instabilities. The best management strategy is to put a wide margin between yourself and the terrain where the problem may exist.

    ASr, D3, NW, 9000' at the entrance to Rock Creek. Remote-triggered avalanche from 2/26/23. No injuries and no involvements.

  • Storm Slabs

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

    Storm instabilities will grow in size and become more sensitive as the day progresses. Avoid being on or under terrain over 30° today and be cognizant of your exposure to terrain features that increase the consequences of avalanches. This includes road cuts, gully features cliff bands, and mid-slope depressions. Be wary of overhead hazards today and avoid being in or underneath avalanche terrain.

    (2/25/2023)

    Natural avalanches on the slopes above Highway 395 in the Lee Vining area. These seem to have slid in the morning on 2/25/23.

Avalanche Discussion

Yesterday's observations highlighted two patterns in recent avalanche activity:

Many large natural wind slab avalanches were reported occurring from the early morning through late afternoon. These were triggered by rapid and efficient wind loading from southwest winds despite the break in snowfall.  Ridgeline avalanche paths ran into flat terrain below treeline including on highways. The large natural avalanche observed on Panorama Dome is a good example.

A very large (D3) persistent slab avalanche was remote-triggered from safe terrain in the Rock Creek area mid-day yesterday (2/26). The avalanche  NW aspect at 9000' that had been previously loaded by south-east winds. There were no involvements or injuries.

Evidence of multiple natural, large to very large (D2.5), persistent slabs was observed in Convict Creek.  These avalanches were located on slopes between 9,000' and 10,000' on N through NE, mid-slope 35 to 40-degree start zones. These avalanches ran up to 1000' (full path), entraining loose snow in the tracks.

Snow events like this one, especially when paired with an active persistent weak layer, are notorious for producing avalanches that surprise even professionals. Usual "safe routes" should be carefully re-evaluated for exposure to overhead hazard. Some avalanche paths may exceed max run-outs over the next two days.

 

 

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