Avalanche forecast

Eastside Region

Eastern Sierra Avalanche Center
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Avalanche danger

Thu
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Highlights

Rising avalanche danger combined with damaging wind, blowing snow, and variable conditions will complicate backcountry travel today. Expand safety margins to account for unpredictable avalanches breaking at the base of the snowpack.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    West winds will steadily increase today and reach speeds up to 100 mph overnight tonight. Expect wind slab development at all elevations today. Wind slabs are typically found on the sheltered side of terrain features that block or re-direct winds. Be suspect of steep terrain features that encourage drifting, such as the leeward sides of ridgelines, alpine couloirs, gully features, and cross-loaded depressions. Due to the extreme wind speeds unstable drifts may be found lower on slopes than usual. Shooting cracks and the sound of the snowpack collapsing under your weight are the best indications that wind slabs are 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 near the ground, creating a larger avalanche involving the full depth of the snowpack.

     

    (1/9/2024)

    Gale force winds moving snow in the June Mountain backcountry.

    Photo: Everett Phillips

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    Weak faceted grains within the old snow continue to show the potential for persistent slab activity. Recent avalanche activity and other reported signs of instability show that the snowpack is still sensitive following last week's storm. Incremental loading over the next 24 hours with snowfall and wind will increase the stress on older weak layers and may lead to an increase in avalanche activity. Persistent slab avalanches can be triggered from low-angle terrain below or adjacent to avalanche starting zones. Persistent slab avalanches often break in unexpected ways and could involve the full depth of the snowpack. Careful route selection and evaluation are important strategies to manage this problem. When in doubt, choose travel routes with lower slope angles and no overhead hazards to limit your exposure.

     

     

    (1/9/2024)

    Shooting crack breaking into presistent weak layers deep in the snowpack on a North aspect near treeline in the June Mountain backcountry.

    Photo: Everett Phillips

Avalanche Discussion

Precipitation is expected to begin this afternoon. Avalanche hazard will likely peak overnight before snowfall tapers off and winds die down early Thursday morning. If the storm tracks late we may not see a significant increase in hazard during daylight hours today. The accuracy of the forecast depends on the timing and amount of forecasted snowfall over the next 24 hours, which is uncertain. More snow is expected in the north of the forecast zone, and specifically in the Mammoth Lakes area. Be sure to verify that observed snowfall tracks with the forecast, and be ready to reassess the hazard if necessary.

Over the past week, two remotely triggered avalanches were observed in the Mammoth area. Both avalanches were small in size. However, they broke on different weak layers within the old snow. The first failed on a weak layer in the midpack on a feature susceptible to wind loading, while the second broke on basal facets near the ground in a more sheltered area. Continued signs of instability since the last avalanche and loading over the next 24 hours mean that similar avalanche activity will likely continue.

There are currently three layers of concern in the snowpack: (1) interfaces between wind-deposited snow and this morning's snow surface; (2) a mid-snowpack layer of facets buried in late December; and (3) advanced facets and depth hoar buried near the ground. Any of these layers may become the primary layer of concern depending on aspect, elevation, and loading patterns within the storm. Digging and probing as you travel through terrain will help to track the presence and depth of these layers.

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