Bollettino valanghe

East Slopes North

Northwest Avalanche Center
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Pericolo valanghe

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

Dangerous avalanche conditions remain after a wet and windy storm brought widespread, very large avalanches. You can trigger deadly and destructive avalanches near and above treeline where the wind drifted deep slabs. You may still be able to trigger avalanches below treeline where you find either entirely dry snow or wet, weak snow.

Problemi valanghivi

  • Wind Slabs

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

    You can trigger large avalanches on leeward, wind-exposed slopes. Strong southwest winds transported snow and formed thick slabs on Tuesday night into Wednesday. Signs of wind slab instabilities are shooting cracks, whumphing collapses, and recent slab avalanches on leeward and cross-loaded slopes. stay off of slopes steeper than 35 degrees where you find deeply drifted snow, wind features, and overhead cornices. Cornices may be large and easy to trigger. If you do get up onto a ridgeline, give these a wide berth as they could break and trigger very large avalanches below. 

    From January 11th to 13th a major storm limited travel and observations at upper elevations. Very large and destructive avalanches ran through entire avalanche paths. With uncertainty around the likelihood of avalanches at upper elevations, it's best to approach avalanche terrain cautiously and make conservative terrain choices. 

    Example of intense wind loading off a ridgeline.

  • Storm Slabs

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

    You may be able to trigger storm slab avalanches in the recent snow above the rain line. The snow stayed relatively dry in the upper reaches of the below treeline band or above about 5,500ft. Use test slopes and snowpack tests to look for interfaces where the snow slides easily. Stay off of slopes steeper than 35 degrees where you find signs of instability. Avoid convex roll-overs and unsupported features, like pillows,  which will be easier to trigger than planar, supported slopes.

    At low elevations, you may find an entirely weak, wet snowpack. You may be able to trigger avalanches where you sink into wet snow up to the top of your boots. Deeper weak layers exist in areas with a shallow snowpack such as the Winthrop foothills, and the lowlands around Twisp. In these areas, you could trigger a persistent slab avalanche on old snow near the ground. These would be difficult to manage, could be triggered from a distance.

    (1/12/2021) Numerous crowns from natural storm slab avalanches on Rock Mountain near Stevens Pass. Photo: Josh Hirshberg

Avalanche Discussion

A major avalanche cycle occurred on the night of the 12th. Observers reported avalanches up to D3.5 in size (big enough to break mature timber and bury cars) including an avalanche that hit a closed portion of Highway 20, below Delancey Ridge. Observations and travel have been limited by the intensity of the storm and the amount of recent snow, rain, and wind. All of this leaves much uncertainty, especially around what's happening above the valleys.

From limited weather station data, Harts Pass received 2.7" in water equivalent, half of which likely fell as snow after the rain event at 6500ft. The Washington Pass weather station gained 2 feet of height of snow, even with the rain event, and similarly logged at least 1 foot after the shift back to snow on Tuesday night.

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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