Bulletin d’avalanche

West Slopes South

Northwest Avalanche Center
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Danger d’avalanche

Sun
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 5 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Even as the avalanche danger slowly drops, stay on your guard. The consequences of any deep slab event remain dire. Limit your exposure to very large avalanche paths including locations in the forest where they may run or stop.

Problèmes avalancheux

  • Deep Persistent Slabs

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

    Cooler weather should help the upper snowpack gain strength and somewhat reduce the likelihood of a deep slab avalanche. However, the underlying persistent weak layer remains largely unchanged. Three days of warm weather only increases our uncertainty regarding this buried layer. Very large and destructive avalanches from 2/26-3/1 near Crystal Mt. should cause us to focus more on the consequences of any slide rather than trying to guess “if” we can trigger it. Many of these slides broke trees and traveled far distances. Consider significantly limiting or even completely avoiding very large avalanche paths. This includes their runout areas well below treeline. 

    Making relevant observations regarding deep slabs can prove tricky. You may see numerous tracks on a slope, not experience any cracking, and conditions may “feel” stable. This should not be taken as a sign of stability. Even snow profiles and snowpack tests can struggle to provide reliable information when the layer sites 6-10’ below the surface. Make sure to talk openly with your group about the potential consequences of traveling in larger terrain as you make your choices for the day.

    (2/27/2021) This slide within Crystal Mountain Ski Resort was triggered by a single explosive during control work on Saturday (2/27) and is confirmed to have run on the Mid-January crust/facet combo. [5700' E aspect; Photo: Crystal Mountain Pro Patrol]

  • Wind Slabs

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

    You may find shallow wind slabs on the lee side of ridgelines and exposed features Saturday. These thin drifts formed from new snow and strong winds on Friday and could fail under your weight. Be suspicious of smooth slopes below cornices, at the base of cliffs, and in confined features like chutes where you’re more likely to encounter these wind slabs. While we expect these potential avalanches to remain small, they could harm you if they carry you over a cliff, into rocks, or serve as the trigger for the much more dangerous deep persistent slab.

    (12/27/2020) Example: Small natural Wind Slab in a rocky and confined feature. [NE 6500' Lakes Basin ]

Avalanche Discussion

Areas around the West-South zone could find 3-5” of new snow by Saturday afternoon. Much of this fell with strong winds which may allow you to encounter drifts 8-12” deep on lee slopes. The snowpack this week warmed significantly. As it cools this should help the new snow bond to the old surface. At mid and lower elevations this could result in a new breakable crust just below the surface creating challenging travel conditions. 

While several layers exist in the snowpack, the mid-January crust/facet layer continues to drive our forecast. The deep persistent layer sits 6-10’ below the surface in many areas. Snow profiles and avalanches confirmed this layer near Crystal, Mt Rainier, White Pass, and Mt St Helens over the past several weeks. Recent avalanches from the Mt Loop, Stevens Pass, Snoqualmie Pass, and Crystal areas demonstrate the power over these enormous avalanches. Many traveled to valley bottoms and overran common travel routes. Use a fresh set of eyes while evaluating your terrain selection in light of these uniquely large potential slides. Despite the lack of an avalanche problem below treeline, heightened conditions remain due to threats from above. Even as you travel in the forest, pay attention to places where your route may cross avalanche paths. Many slides ran near their historic runouts, broke trees, and traveled to valley bottoms during the past two weeks. 

For those traveling near Mt Rainier (Paradise/Tatoosh), the exceptionally stormy weather during nearly all of February severely limited observations in these areas. This leads us to a much higher degree of uncertainty. You can help us by submitting observations on our public site. 

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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