Boletín de aludes

West Slopes Central

Northwest Avalanche Center
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Peligro de aludes

Wed
Alta montaña
2 · Moderado
Límite del bosque
2 · Moderado
Bajo el límite
1 · Débil
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Desactualizado: este boletín caducó 5 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

The potential for human-triggered avalanches in wind drifted snow lingers at upper elevations. Old layers in the snowpack are gaining strength but the deep slab problem still poses a low-likelihood/high consequence scenario. A conservative approach to terrain is recommended. 

Problemas de aludes

  • Wind Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1

    Several triggered wind slabs up to size D2 were reported Monday in the West North Zone, including a close call on Shuksan Arm where a skier was caught and carried. On Saturday a backcountry skier triggered a wind slab avalanche in the Whitehorse Mountain area near Darrington.  It was about 10 inches deep, on a north-facing slope at about 4800ft.  Thankfully no one was caught. You can read the observation here. 

    Avalanches in wind drifted snow could break on a variety of mid-storm interfaces or on a crust that formed up to 6000ft on March 4. Continue to approach upper elevation terrain cautiously. You are most likely to trigger a wind slab in northerly terrain steeper than 35 degrees, especially on convex or unsupported slopes. Use extra caution around ridgelines or any slopes where cornices exist. These unpredictable features are large and overbuilt and can break further back than you may expect or fall onto slopes below acting as huge triggers. Pay attention as you transition into wind-affected terrain, and look and feel for sudden changes in snow surface texture and hardness. Seek out terrain less than 35 degrees where you find recent avalanches, cracking, collapsing, or evidence of wind drifted snow. 

    (3/8/2021) A skier-triggered avalanche on Shuksan Arm that broke 1-3ft deep, 200ft wide, and ran 300 vertical feet. The skier dropped the cliff above the slope and triggered the avalanche upon landing. They were caught and carried but remained on the surface with no injuries. Photo: Pete Durr

  • Deep Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Unlikely
    Tamaño
    3

    Deep persistent slab avalanches continue to linger in our recent memories, and we are not yet ready to remove it from our list of avalanche problems.  The last known avalanche to fail on that layer was March 1 in the Stevens Pass and Snoqualmie zones.  These avalanches were around 10 feet or deeper. Evidence of deep slab avalanches probably occurring on that layer in the West Central zone was documented last week.  The weight of just one person may not be enough to cause an avalanche on this layer, but cornice fall or smaller wind slab avalanches could add the needed weight to trigger an avalanche on it.  

    Your greatest risk may well be travelling in low elevation terrain on the valley floor, where a distant large slope may avalanche, hitting you down at the bottom of the path.  If you are travelling through sparsely treed terrain or an open slope with one of those large paths above you, be aware of the unlikely consequence of an avalanche running down and hitting you.  For this reason, exercise caution and stay alert, possibly considering avoiding runouts of very large paths when you're traveling in the far reaches of avalanche terrain. 

    (2/24/2021) Very large avalanches have the possibility of running all the way down to the valley bottom. This is in the Barclay Lake area of the West Central zone, taken 2/24.

Avalanche Discussion

At least a foot of snow fell since March 4, and a few slab avalanches were reported in the West Central zone since. Several adjacent zones are in a prolonged period of unstable conditions with ongoing natural and human-triggered avalanches failing on the deep persistent layer, but no new ones have been noted in the last week. We suspect that conditions in the West Central zone are similar, especially in the southern and eastern portions of the zone. Reports of the most recent deep slab avalanches include ones that closed Highway 20 near Newhalem for nearly a week and very large natural avalanches from last week on Benchmark and Fortune Mountains just north of Stevens Pass in the West Central Zone. The avalanche near Mt. Misch north of Green Mountain also appears to have failed on the Mid-January PWL.  Last Monday, WSDOT teams triggered an 8ft deep D3 avalanche on the southeast face of Rock Mountain east of Stevens Pass. Last Sunday, a natural D3.5 occurred in upper elevation north-facing terrain in the Chiwaukum Range on the eastern edge of the Stevens Pass zone, and a natural D3.5 failed on Chair Peak (Source Lake) in the Snoqualmie Pass zone. 

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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