Bollettino valanghe

Eastside Region

Eastern Sierra Avalanche Center
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Pericolo valanghe

Mon
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

Watch out for unstable slopes at higher elevations where dense drifts are layered over a weak base of loose sugary snow. The consequences of getting caught in an avalanche will be more severe with exposed and shallowly buried objects in the runout. Look for options with a wider margin for error while continuing to assess the thin and variable early-season snowpack.

Problemi valanghivi

  • Persistent Slabs

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

    In isolated areas, recent storms built a slab of wind-drifted snow over weak layers of facets and depth hoar at the base of the snowpack. The problem is most likely confined to terrain features prone to wind-loading which had continuous snow coverage in November and early December and have accumulated a strong slab since then. If triggered, the resulting avalanche could break in unexpected ways involving the full depth of the snowpack, making it more difficult to escape the slab and increasing the likelihood of collisions with rocks, trees, and shallowly buried objects in the runout. Persistent slabs are slow to gain strength and can remain a concern over an extended period. Common strategies for managing persistent slabs include setting terrain margins to avoid areas where the problem might exist and waiting to enter desired terrain until more information becomes available or stability improves.

  • Loose Dry

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–1.5

    Dry loose avalanches occur when weak surface snow tumbles down a steep slope and entrains additional snow below. The result is a long narrow debris fan extending from the trigger point into lower-angle terrain. Natural triggers could include clumps of snow falling from cliff bands, and trees shedding snow from branches. The avalanche's size depends on the amount of unconsolidated snow available in the avalanche track and the vertical fall of the steep terrain below the trigger point. These avalanches are expected to be mostly small today but could be consequential above terrain traps like cliffs. In areas where dry loose avalanches are most likely avoid confined or exposed terrain features with overhead hazards.

    (12/22/2023)

    Dry loose avalanche on a test slope, and on adjacent slopes in the bsackground.

    Photo: Everett Phillips

Avalanche Discussion

Recent avalanche activity:

Mt. Wood (reported12/22): A large avalanche started on a  north aspect above the treeline and reached its runout approx. 1000' lower. The type of avalanche and trigger are unknown.

Mammoth Lakes Basin (reported 12/23):  A large natural slab avalanche broke above the treeline on a north aspect and stopped low in the track. A second smaller avalanche ran in the adjacent start zone. Details of the second avalanche are unclear.

Multiple small natural dry loose avalanches have been reported in the northern part of the forecast area over the past 48 hours. These avalanches started on north-facing slopes near and above treeline around cliff bands and ran a short distance below the trigger point.

Snowpack details:

In the majority of drainages avalanche start zones with enough snow to be concerning are isolated. The exception is in the Mammoth Lakes area and on similar terrain features near and above-treeline closer to the Sierra crest. In these areas, the combined distribution of weak layers and an overlying wind-stiffened slab is more continuous. The difference in the spatial distribution of the persistent slab from one drainage to the next is a source of uncertainty in the forecast. In many areas the recent snow is only a few inches deep and is relatively loose, in other areas the slab is strong and more than a foot and a half deep. Whumpfs, shooting cracks, and full propagation in recent stability tests indicate that human triggering is possible in areas that have the wrong structure.

Non-avalanche hazards related to thin coverage and shallowly buried objects deserve special attention. Walking or skinning downhill may be a good option if the planned descent is less ridable than it appeared.

Regioni coperte