Bulletin d’avalanche

Eastside Region

Eastern Sierra Avalanche Center
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Danger d’avalanche

Sun
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 3 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Early-season conditions warrant a patient and cautious approach to avalanche terrain. Watch out for pockets of instability at higher elevations where dense drifts are layered over a weak base of loose sugary snow. Keep in mind the consequences of getting caught in an avalanche will be more severe with exposed and/or shallow buried objects in the runout.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    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 shallow buried objects in the runout.  Common strategies for managing persistent slabs involve setting wide margins to avoid entering terrain where the problem might exist, and waiting to enter desired terrain until more information becomes available or stability improves.

  • Loose Dry

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1

    Loose low-density surface snow on very steep slopes is vulnerable to sluffing if disturbed. Natural triggers may include spindrift, clumps of snow falling from cliff bands, and trees shedding snow from branches. Once loose snow begins to tumble it continually entrains snow on the slope below. Loose snow avalanches can be recognized by the long narrow debris fans extending from the trigger point into lower-angle terrain. These avalanches are expected to be mostly small today, but could be consequential above terrain traps like cliffs. If you venture into steep or extreme terrain, be aware of the possibility of sluffs and continually assess the probability of triggering. Avoid confined or exposed areas and terrain features where dry loose avalanches are most likely in overhead terrain.

    (2/24/2023)

    Touchy loose dry sluffs even on low-angled terrain.

Avalanche Discussion

Extended periods of clear dry weather in late November and early December led to advanced faceting of the lower snowpack. On northerly aspects, large striated depth hoar has been conspicuous at the bottom of the weak snow. This week’s unsettled weather added a moderate load on top of the faceted base. 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.

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. When dealing with higher uncertainty it is a good idea to give yourself wider terrain margins and to focus on continual observations to update your assessment of conditions throughout the day.

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

Régions couvertes