Bulletin d’avalanche

Stevens Pass

Northwest Avalanche Center
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Danger d’avalanche

Sat
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 6 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Dangerous conditions exist in the Stevens Pass area. Seasoned backcountry travelers continue to be surprised by widely propagating avalanches. An incoming storm with warming temperatures will create storm slabs as the afternoon progresses. Plan to avoid all slopes steeper than 30 degrees near and above treeline, and travel carefully around steep slopes at lower elevations. 

Problèmes avalancheux

  • Persistent Slabs

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

    Experienced travelers remotely triggered a persistent slab avalanche above Lanham Lakes on Thursday. This slide was on a northeast aspect at around 5,800ft, roughly 500ft wide, and 15-20" deep. It was suspected to have failed on weak snow just above the prominent 12/9 crust. On Wednesday on Susan Jane Peak near Stevens Pass seasoned backcountry travelers triggered a slide that was 16" deep, and roughly 110ft wide. This slide occurred in wind loaded terrain on a NE aspect at 5450’, and failed on 3mm buried surface hoar. That same party also reported finding buried surface hoar on westerly and southerly aspects a few inches above the prominent 12/09 crust. Information like this punctuates several observations from earlier in the week identifying buried surface hoar and facets in the upper snowpack. If you travel to mid and upper elevations, give any open slopes greater than 30 degrees a wide berth. Snowpits and snowpack tests can be useful to identify buried persistent weak layers, however, they can not prove their absence. Similarly, finding no instability on one slope doesn't mean you won't find it on another right next door. 

    (12/16/2020) This human triggered avalanche occurred on Susan Jane Peak near Stevens Pass. NE aspect 5450' SS-AS-D2-O

  • Storm Slabs

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

    Another incoming storm will begin to lay down a new blanket of snow on the pass. Temperatures are forecast to warm up as it comes in, making for an upside-down feeling snow surface. Steer around any open slopes greater than 35 degrees where you find more than 6” of new storm snow. You can use small slopes and hand pits to see which storm layers fail. Above 5,000ft, there is potential for any shallow avalanche in the surface layers to step down to the crust and become a much larger slide. Storm slabs may become easier to trigger during intense snowfall and warming temperatures in the afternoon. 

    Storm slab instability on a small convexity below treeline.

Avalanche Discussion

Roughly 15"-30"(40-80cm) of snow now rest above the prominent 12/09 crust. Above 5,000ft folks have remotely triggered wide avalanches in some drainages, but not in others. The snowpack in the area has become complicated and variable, and we just don't understand it. As the storm pattern continues, our uncertainty grows, as does the size of the avalanches we are seeing. 

In some areas, and especially at lower elevations, it appears that recent snow is bonding well. Rain and wet heavy snow could produce a few small and isolated loose wet avalanches by the afternoon. The snowpack at these lower elevations has been through and a lot recently, with warm weather and several rounds of rain. As a result, wet avalanche activity should be limited to only a few very steep and rocky slopes. Instead, anticipate areas of thin coverage, open creeks, and shallowly buried obstacles that will require your attention to navigate safely. 

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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