Bulletin d’avalanche

Central Sierra Nevada

Sierra Avalanche Center
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Danger d’avalanche

Thu
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

Backcountry travelers venturing into avalanche terrain today should expect to trigger avalanches. Yesterday's storm added a substantial new load onto existing weak layers in the snowpack in addition to creating the usual instability associated with wind drifted snow and rapidly accumulating new snow. Highly conservative terrain selection is encouraged for today.

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    2–3

    Observations made yesterday morning revealed unstable wind slabs that were rapidly increasing in size. SW winds strong enough to drift snow into cohesive wind slabs remain ongoing. The vast majority of wind slabs exist in above treeline and near treeline terrain across NW-N-NE-E-SE aspects. Wind slabs exist to a lesser degree in below treeline areas due to the strength of yesterday's winds. Any wind loaded areas where the slab fails deeper in the snowpack as a persistent slab will allow for more destructive avalanches with wider propagation to occur.

    Identify and consider avoiding slopes and gullies where wind drifted snow has accumulated, especially below or adjacent to cornice features and wind pillows. Clues such as recent avalanches and snow surface cracking in wind exposed areas indicate the presence of unstable wind slabs.

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    2–3

    Any persistent slab avalanches that are triggered today are expected to fail 2 to 4 + feet deep and have wide propagation. Clear signs of weak layers of graupel persisted right up to yesterday's snowfall on NW-N-NE aspects both above and below treeline. A substantial new snow load was rapidly added to the upper snowpack over the past 24 hours. Don't expect that this avalanche problem went away.

    Consider avoiding avalanche terrain on NW-N-NE aspects today, especially along the Sierra Crest where most of the persistent instability has occurred. Deep snowpack failure and wide propagation often lead to deep burials and/or multiple people caught in one avalanche. Don't mess around with this one today. Avoidance is the only reliable tactic. Traveling persistent slab avalanche terrain today without incident is just getting lucky, not a repeatable tactic for long term success.

    (3/27/2023)

    Unstable snowpit test results on a graupel layer 2 feet below the snow surface and adjacent to an avalanche that occurred on March 24. The Nipple, South of Carson Pass, CA

    Photo: SAC

  • Storm Slabs

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

    Small storm slab avalanches were reported yesterday afternoon failing at the old/new snow interface in wind protected areas below treeline. Relatively cold air temperatures last night and this morning are expected to slow the stabilization of this avalanche problem.

    Recent avalanches and snow surface cracking in wind protected areas near treeline and below treeline are signs of unstable storm slabs in the area. Stick to lower angle, non-avalanche terrain without connected steeper terrain above when signs of unstable snow are present.

Avalanche Discussion

  • Skiers on Echo Peak yesterday afternoon reported triggering storm slabs anytime they ventured into short pitches of steeper terrain. One skier was briefly caught and carried without burial or injury.
  • Unstable wind slabs were observed around cornice areas near treeline yesterday morning in the Yuba Pass area. At the time these wind slabs were rapidly growing in size.
  • Signs of ongoing persistent slab instability in the form of snow surface cracking, unstable snowpit test results, and skier triggered avalanches with persistent slab characteristics were reported on Monday. Snowpack failures were observed 6 inches to 2 feet below the old snow surface. They are now buried deeper.

Régions couvertes