Bollettino valanghe

East Slopes North

Northwest Avalanche Center
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Pericolo valanghe

Tue
Alta quota
3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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In sintesi

Light snow with ongoing westerly wind will maintain dangerous avalanche conditions at upper elevations on Monday. It will be easy to trigger avalanches above treeline where the wind has drifted stiff slabs of snow. You may be able to trigger large and surprising avalanches on older weak layers of snow especially in the eastern part of the zone.

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–2

    Continued snowfall and strong westerly winds will transport snow and form slabs on leeward slopes. Wind slabs will be bigger and easier to trigger above treeline. Feel for wind stiffened or hollow sounding snow as you enter open areas and change aspects. Be especially careful of cross loaded gully features and cornices. Watch for shooting cracks or whumphing collapses as a clear sign of instability.  Wind slabs could also step down to other weak snow layers in the top 2ft of the snowpack.

    (2/6/2021) A skier intentionally triggered a small wind slab avalanche on a SW aspect at 74000ft near Washington Pass on Saturday.

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2–3

    You could trigger wide and surprising avalanches on older, faceted grains or buried surface hoar 2-3ft below the surface. Audible whumphing collapses, cracks shooting through the snow, and widely propagating avalanches are all warning signs. You could trigger slides on convex rolls and unsupported features. If in doubt, opt for lower angle terrain and consider sticking to slopes less than 35 degrees.

    Observers reported avalanches and propagating test results on surface hoar near Washington Pass and especially in the Cutthroat drainage. Surface hoar tends to survive in open, sheltered, and shaded terrain, but may be difficult to identify. Use snowpits and snowpack tests to look for weak layers and carefully evaluate the snowpack. Further east, you may be able to trigger avalanches on older, weak facets above or between crusts from mid-January. Both of these weak layers can be found 2-3 feet below the surface. When the snowpack is this complex, keep your terrain choices simple.

    (2/3/2021) A ski triggered avalanche was reported in Portley Bowl on 2/2. The next day an observer took the bottom photograph at 10:50AM. The majority of the rest of the slopes in the bowl had avalanched, with no ski tracks observed in or out of the slides. These may have been solar triggered, and suspected to have failed on a buried surface hoar layer from 1/30.

Avalanche Discussion

On Sunday, observers on Washington Pass reported over 1 foot (30-40cm) of storm snow since the 5th and wind transport at ridgetops. On a SW aspect at 7,200ft, they found sudden test results at the recent storm interface down 1 foot and a layer of rounding facets about 2 feet below the surface. 

On Saturday, a skier intentionally triggered a small wind slab on a cross-loaded southwest slope at 7400ft near Washington Pass. On Friday, a large ski triggered avalanche was reported near treeline along with several small wind slabs in steep terrain below cliff features in Cutthroat drainage. Professionals reported shooting cracks and propagation from steeper terrain, as well. On Wednesday, we believe that a significant portion of Portley Bowl avalanched naturally following a ski triggered slab on a smaller part of the bowl on Tuesday. Another skier had a close call on Tuesday near an area often thought of as 'safe' ski terrain. We suspect buried surface hoar as the culprit for these past two avalanches.

Outside of wind-affected areas, you may find about 2-3ft of snow above the Jan 30th interface in the Washington Pass area. Another potential weak layer are small facets within, or above a crust from mid-January. This prominent crust can be found about 3ft below the surface at mid and lower elevations. Further east in the zone, the weakened crust from mid-January has been a concern, even below treeline. On February 1st, observers in the Twisp drainage reported collapsing at 4,800ft on a northeast aspect and snowpack tests show potential for avalanches to propagate on the mid-January crust facet layer. We have higher uncertainty with the shallower and weaker snowpack in the eastern part of the zone. If you travel to areas such as the Chelan-Sawtooth Mountains, Twisp River, and Chewuch drainages, take time to dig in the snow and look for older weak layers. 

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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