Boletín de aludes

West Slopes North

Northwest Avalanche Center
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Peligro de aludes

Thu
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

You can still trigger a large avalanche in wind drifted snow at upper elevations. Pay attention as you transition into wind-affected terrain. Avoid unsupported slopes 35 degrees and steeper with evidence of wind loading and minimize your exposure to cornices. 

Problemas de aludes

  • Wind Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1–2

    You are most likely to trigger a lingering wind slab above 4500ft on northerly convex slopes steeper than 35 degrees and in areas below cornices or rock bands. Several human-triggered wind slabs and a few close calls occurred over the past several days. Fortunately, there were no full burials or injuries. Much of the recent avalanche activity occurred above treeline, but observers on Tuesday reported shooting cracks and reactive wind drifts near treeline on Mt. Herman. Avalanches in wind drifted snow could break 6in-3ft deep 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, and give cornices a wide berth. 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. Seek out terrain less than 35 degrees where you find recent avalanches, cracking, collapsing, or evidence of wind drifted snow. 

    (3/8/2021) A snowboarder-triggered avalanche on Shuksan Arm that broke 1-3ft deep, 200ft wide, and ran 300 vertical feet. The snowboarder 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

Avalanche Discussion

From March 4-7, over 2ft of new snow accumulated above 4000ft throughout the West North Zone. Dry sunny weather arrived Monday with increasing clouds Tuesday. The strong March sunshine affected cold snow surfaces since the storm ended with widespread loose wet activity and melt-freeze crusts on solar aspects. Deposits of wind-blown snow are now a few days old, but still reactive to human traffic. Several triggered wind slab avalanches were reported in the past 48hrs in upper elevation terrain on Shuksan Arm, Table Mountain, and on the south side of Mt. Baker. Most were small and predictable, but some broke deeper, wider, and grew large. Mostly cloudy skies Wednesday are expected to limit additional wet loose activity, while a full day of sunshine is set for Thursday.

There are dormant layers (mid-January PWL) in our snowpack buried over 10ft deep that produced a widespread avalanche cycle in the Hwy 542 and Hwy 20 corridors in early February, and isolated but very large natural avalanches as recently as February 25. In other parts of the region (West Central, Stevens, Snoqualmie) very large and widespread natural cycles on the mid-January layer occurred in late February, and as recently as March 1. With quiet weather, no new deep slab avalanches for 1-2+ weeks, and consistent and favorable results in snow profiles and tests, there is enough evidence and confidence to remove the problem from the forecasts in adjacent zones for now. Regardless, we will continue to track the Mid-January PWL throughout the region for the remainder of the season. We will not soon forget the very large deep slab avalanches from February, and recognize these types of avalanches are challenging to accurately assess and forecast. It is not out of the question for large triggers like cornice fall to still cause a deep slab avalanche, and a significant weather event may even be enough to reactivate the layer and bring the problem back to life at some point again this season.

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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