Bulletin d’avalanche

West Slopes Central

Northwest Avalanche Center
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Danger d’avalanche

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

Points saillants

Bypass steep terrain where the wind is drifting new snow and forming easy-to-trigger slabs. Expect higher avalanche danger in parts of West Central near Snoqualmie and Stevens Passes. A rise in snow levels on Friday will bring a  big change from cold conditions in the past week.

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–2

    Expect to see some active wind loading on Thursday, especially at upper elevations. Moderate to strong winds and plenty of snow available for transport is the tried and true recipe for fresh wind slabs. Wind slabs are almost always easiest to trigger during active loading and in the 12-24 hours after they form.

    Blowing and drifting snow should show you exactly which slopes are loading up and also where you are most likely to trigger an avalanche. Also, monitor changes in snow depth and stiffness and you gain elevation or change aspects. Look for higher wind speeds and more widespread wind slab formation in the eastern part of the forecast zone. 

    Use visual clues to help you identify wind loaded slopes. In this case, cornices often are a great indicator of a wind drifted slope beneath. 

    Photo: Joe Dellaporta

  • Storm Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2

    Wednesday’s storm dumped 8-12” of new snow across West Central, with even more in wind-loaded areas.

    The biggest question mark is how well the new snow is bonding to facets or surface hoar found in some areas before the storm. In some places, the old snow surface was incredibly weak. In many others, it was just dense wind slab or exposed crust. But there wasn’t a rhyme or reason to the pattern of distribution - basically where there was a weak snow surface and where there wasn't. 

    You can dig down and asses the bond of the new snow to the old snow surface. On the other hand, the distribution of weak snow was so random that you might find very different layering over a short distance. Either way, don’t ignore obvious signs of instability like whumping or shooting cracks. When in doubt steer towards lower angled, less consequential terrain.

    Loose dry avalanches could still pack a punch, especially in terrain that saw lighter winds and less slab formation. Have a plan for sluff management and use extra caution in terrain with trees or other terrain traps in the track or runout.

     

    (12/1/2023)

    Example of a D1.5 slab avalanche that failed on small faceted grains at the old-new snow interface. 

    Photo: Irene Henninger

Avalanche Discussion

As usual, there will likely be some significant variations in danger between different parts of the forecast zone. Slopes that see insignificant wind loading and don’t have facets below the new snow shouldn’t have much going on besides some sluff. But you could also find extensive wind loading and/or a weak snowpack structure in a neighboring valley - or even on the next slope. The actual avalanche danger is determined by the conditions wherever you are in the mountains. It’s up to you to observe and adjust your plan as necessary.  

White Chuck picked up 10" of new snow with up to 1.5" of precipitation recorded at various weather stations around West Central. Of the neighboring zones, the most avalanche activity was reported from Stevens Pass where very easy to trigger slabs were found. Some had wide propagation despite not being very deep. In West North reported avalanche activity was minimal despite the new snow and strong winds. Most who were out in the storm today found very small pockets of new snow fracturing 8-12” down. Despite their small size, they ran pretty far once they got going. On south aspects a sun crust under the new snow helped them run even farther.  Lots of dense, unreactive wind slab was found in exposed locations. In all forecast zones, observations were limited on Wednesday by very stormy conditions. 

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