Bulletin d’avalanche

West Slopes North

Northwest Avalanche Center
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Danger d’avalanche

Tue
Alpin
4 · Fort
Limite forestière
4 · Fort
Sous la limite
3 · Marqué
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Points saillants

Very dangerous avalanche conditions will develop at upper elevations Monday. Avalanches related to new snow and wind are concerning enough, and a deep persistent weak layer capable of producing very large avalanches still lingers. Travel in avalanche terrain is not recommended.

Problèmes avalancheux

  • Storm Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Very Likely
    Taille
    2

    Expect another 12in of new snow to accumulate over the next 24 hours bringing snow totals since Friday up to 2ft+. Large natural avalanches are very likely, especially in steep wind loaded terrain near and above treeline. Fresh slabs may be easy to trigger even below treeline. Carefully assess how new snow bonds to itself and old snow surfaces. Expect unstable conditions and look for red flags like shooting cracks, collapses, and recent avalanches. Avoid travel on or below slopes 35 degrees and steeper, especially convex rollovers and slopes below cornices. In areas sheltered from the wind, loose dry sluffs may run far and fast in steep terrain and could even trigger slab avalanches as they run downslope. Give the new snow time to settle and stablize and seek out simple, low angled terrain away from overhead hazard.

    (2/14/2021) A large natural avalanche (D2) in motion on Mt. Herman (N, 5000ft). Photo: Pete Durr

  • Deep Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    2–3

    Very large avalanches failing on a persistent weak layer buried 4-6ft deep may still be possible near and above treeline. The layer continues to become more difficult to trigger with time. Large triggers like cornice fall or motorized users are more likely to affect such a deeply buried weak layer, but just the weight of a person may still be enough. Above 4500ft, treat broad open slopes 35 degrees and steeper guilty until proven otherwise. 

    These conditions come with uncertainty and present a low likelihood/high consequence scenario. In the West North Zone, the last reported avalanche that failed on the deep persistent weak layer (mid-January crust/facets) occurred on February 3. Observations targeting this layer continue to show gradually improving stability, with signs of the weak layer healing (rounding) and more favorable snowpack test results (resistant and non-planar fractures and no propagation). Regardless, variability exists and additional time is needed to more accurately assess the sensitivity and distribution of the weak layer. Continue to build in a wide margin for error and approach terrain with a very cautious mindset.

    (2/11/2021) Natural slab avalanches that ran in early February along the east side of the 3 Fingers Mountain area in the West Central Zone.

Avalanche Discussion

The potential for natural avalanches will increase as heavy snow accumulates during the day Monday. Observers on Saturday and Sunday reported several natural and triggered avalanches and widespread shooting cracks. Avalanches occurred within new and wind drifted snow and at the old/new snow interface. In some areas, the recent snow may rest atop weak old snow surfaces with the potential for surprising and widely propagating avalanches. Avalanches related to new snow and wind are driving HIGH danger, and a lingering deep slab instability still weighs on our minds. These conditions require very cautious terrain and travel choices. Non-avalanche hazards also exist at this time including tree well and deep snow immersion. Find out more information about deep snow safety HERE.

Unfortunately, a snow biker triggered an avalanche on the deep persistent layer in the East Central zone on February 8 and was buried and killed. For more details and information, see the Accident Reports page and the post-incident investigation video below. 

During the ongoing Covid-19 pandemic, continue to follow the latest guidance from local, state, and federal health officials. Avoid unnecessary risks that could impact first responders and stress medical resources.  Wear a mask when appropriate, practice social distancing, and consider recreating closer to home.

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