Bollettino valanghe

Eastside Region

Eastern Sierra Avalanche Center
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Pericolo valanghe

Sun
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
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In sintesi

The avalanche danger will rise as dense blowing snow loads buried persistent weak layers. You can trigger dangerous avalanches from mid-slope, or areas you may think of as “safe.” Unstable slabs of wind-drifted snow will also grow in size throughout the day. Step back from steep terrain and the slopes below as new snow accumulates.

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1.5–3

    New loading will likely make weak layers of sugary snow more sensitive. These layers are responsible for shooting cracks, collapsing, and recent avalanches at all elevations. Warmer temperatures and wind have stiffened the overlying slab. You may not get signs of instability until you’ve wandered onto the wrong slope. Persistent slab avalanches can be triggered from low-angle terrain below or adjacent to avalanche starting zones. Tracks on a slope are not evidence that it’s safe. Once triggered, these avalanches often break in unexpected ways and could involve the full depth of the snowpack. Choose travel routes with lower slope angles and without exposure to overhead hazards.

    (1/12/2024)

    A large human-triggered persistent slab avalanche occurred on 1/12/24. This avalanche resulted in a full burial. Following a fast and efficient rescue, the party was able to self-rescue. Thankfully, this incident only resulted in minor injuries.

    Photo: Steve Mace

  • Wind Slabs

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

    Strong winds at all elevations will redistribute dense new snow onto leeward slopes throughout the day. You’ll likely find dense slabs getting larger and more sensitive as new snow falls. The first places to develop fresh drifts will be the leeward sides of ridgelines and the sidewalls of cross-loaded gullies. As winds increase and new snow falls, you may find drifts further down the slope than you’d expect, and downwind of tight trees. Watch for blowing snow and growing cornices. Shooting cracks and the sound of the snowpack collapsing under your weight are warning signs that wind slabs are becoming unstable in your area. Keep in mind that even a relatively small wind slab avalanche breaking in the upper snowpack may be enough to step down to persistent weak layers, creating a larger avalanche.

    (1/5/2024)

    Small wind slab avalanche failed mid-storm. Northeast aspect, 10,000 ft. Sherwin Ridge

    Photo: Clancy Nelson

Avalanche Discussion

The approaching storm will affect the East Side later today. Snowfall and winds will peak overnight. The most snow should fall near the Sierra Crest and in the northern reaches of the forecast area. Places like Tioga Pass could see more than a foot of new snow by tomorrow morning.

Yesterday, a rider was buried by a large avalanche that broke widely across a steep, rocky slope below San Joaquin Ridge. Fortunately, their partner performed a quick rescue and there were only minor injuries. Another party looks to have triggered a small avalanche below treeline while riding past on a snowmobile. Observers experienced collapses and shooting cracks near and below treeline on multiple aspects. Over the last 72 hours, slope-scale collapses, natural, explosive, and skier-triggered avalanches have run at all elevations near Mammoth. Weak old snow hidden below the surface is the main cause of all this instability.

New loading will add stress to the fragile snowpack structure. A layer of sugary facets is buried around an old crust 1 to 2 feet below the surface. A fragile layer of large, loose, depth hoar exists near the ground on shady slopes. Both of these layers have been actively failing at the slope scale. Warm temperatures yesterday stiffened the overlying slab. Because the weak snow is hidden below the surface the hazard can be hard to track. The consequences are potentially deadly.

Shifting winds over the past week have alternately stripped and hardened slopes above treeline. Slopes near treeline have the most continuous coverage and may have the most consistent distribution of avalanche problems.

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