Bollettino valanghe

Eastside Region

Eastern Sierra Avalanche Center
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Pericolo valanghe

Fri
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
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Non aggiornato — questo bollettino è scaduto 3 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Wind-deposited snow from the past 24 hours has stressed weak layers in the snowpack. The potential for large avalanches has expanded to more areas and avalanches could be easier to trigger.  Pull back from steep terrain until more information is available.

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–2.5

    Periods of heavy snowfall and extreme wind over the past 24 hours have increased the stress on older weak layers. In many areas, our weak snow has been waiting on the development of a heavier slab before becoming a problem. This storm has the potential to connect areas where we have seen instability already this season into a larger and more continuous avalanche problem.  Persistent slab avalanches can be triggered from low-angle terrain below or adjacent to avalanche starting zones. Once triggered, these 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 without exposure to overhead hazards.

    Photo: Steve Mace

  • Wind Slabs

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

    West and Southwest winds reached speeds up to 100 mph overnight. Winds are forecast to shift northwest today. Wind slabs are typically found on the sheltered side of terrain features that block or re-direct winds. Due to the extreme wind speeds unstable drifts may be found lower on slopes than usual and in unexpected places. Be suspect of steep terrain features that encourage drifting, such as the leeward sides of ridgelines, alpine couloirs, gully features, and cross-loaded depressions. Hollow-sounding drifts, shooting cracks through stiff or textured surface snow, and the snowpack collapsing under your weight are indications that you have found unstable wind slabs. 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.

    Be careful around ridgelines and other dropoffs in terrain where sensitive cornices may break further back than expected.

Avalanche Discussion

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. 

On 1/7/24, a large natural avalanche ran on White Wing Mountain following a loading event similar to last night.

There are currently three layers of concern in the snowpack: (1) Storm interfaces within or at the bottom of the most recent snow ; (2) a mid-snowpack layer of facets under a thin wind crust from the first week of the new year; 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.

Up to this point, the number of large avalanches has been limited by the patchy distribution of the overlying slab. Persistent slab activity is inherently difficult to forecast. The main uncertainty today is whether or not this storm created a consistent slab that connects previous pockets of instability into a larger problem.

 

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