Bulletin d’avalanche

East Slopes North

Northwest Avalanche Center
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Danger d’avalanche

Sat
Alpin
2 · Modéré
Limite forestière
2 · Modéré
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

An uptick in easterly winds may have created shallow wind slabs in areas exposed to valley winds, and near cols and passes. You could trigger these reactive slabs on recently drifted features and on slopes steeper than 35 degrees. Chances for triggering large and surprising avalanches are declining, but it' still possible, especially in the far western and eastern part of the zone and in areas where wind has stiffened snow surfaces. 

Problèmes avalancheux

  • Wind Slabs

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

    On Thursday, easterly winds began to form reactive, shallow slabs in nearby zones. You are likely to find these slabs in areas where wind funnels through, such as in exposed east-west oriented valley locations, and near cols and passes. Feel for slightly thicker snow, and look for signs of active wind transport such as small plumes near ridgelines. If you find long shooting cracks in drifted snow, stay off of steep slopes nearby. Any avalanche in the new snow could step down to deeper weaker layers and result in a very large and avalanche. Consider the terrain you're connected to and avoid steep, open wind loaded slopes. Outside of wind-affected areas, you could trigger small sluffs in the weak snow surfaces.

    (2/11/2021) Long shooting cracks were observed in freshly drifted snow in wind exposed terrain on Thursday.

  • Persistent Slabs

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

    You may still be able to trigger wide and surprising avalanches on faceted grains over a crust, or buried surface hoar. Audible whumphing collapses, cracks shooting through the snow, and widely propagating avalanches are all warning signs. You could trigger these slides on convex rolls and unsupported features, especially areas that have been recently stiffened by wind. While the likelihood of these avalanches is dropping through time and quieter weather, the consequences remain high. If in doubt, opt for lower angle terrain and consider sticking to slopes less than 35 degrees.

    Persistent slabs have not been a uniform, widespread problem in the zone, although sporadic avalanches on a variety of weak layers have left us with a tricky pattern to decipher. Heres what we do know: We suspect a very large avalanche involved facets over a crust (mid-January crust) near Rainy Pass on the 8th (see discussion). This layer can be found about 2-3ft below the snow surface, and in the far western and far eastern edges of the zone.

    Layers of buried surface hoar from January 30th can still be found in the top 3 ft of the snowpack near Washington Pass. These layers caused both human triggered, and natural avalanches around February 2nd. 

    (2/9/2021) A very large avalanche at 6,400ft, ENE near Rainy Pass. This avalanche may have run on 8th on a deeply buried persistent weak layer. You can see many snowmobile tracks in the area. Photo: Jeff Ward

Avalanche Discussion

An avalanche fatality occurred on February 8th in the Knox Creek drainage in the neighboring East Central zone (See the short video in the media gallery below). Our condolences go to the friends and family of those involved. Similar snowpack conditions may exist near the western boundary of this zone.

On February 9th, a very large avalanche was reported at 6,400ft, ENE near Rainy Pass (see photo below). It is suspected to have run on the mid-January crust. An observer on Whistler Mountain reported sudden and propagating test results on a west aspect at 6,000ft as well as on a NW aspect at 6,600ft on this same layer. These facets over a crust can be found down about 2.5ft at 6,600ft in that area. This weak layer appears to be more shallowly buried and more problematic west of Washington Pass, as well as in the far eastern mountains of the Twisp River and Chewuch drainages. 

Observers on Delancey Ridge found tests indicating propagation on the Jan 30th interface of facets and surface hoar at 6050ft on a southeast aspect. On Sunday, observers dug a profile on a SW aspect at 7,200ft and found reactive storm interfaces down 1 foot and a layer of rounding facets (likely Jan 30th) about 2 feet below the surface. On February 3rd, we believe that a significant portion of Portley Bowl avalanched naturally following a ski triggered slab on a smaller part of the bowl on the 2nd. Another skier had a close call, also on February 2nd. We suspect buried surface hoar as the culprit for these two avalanches last week.

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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