Bollettino valanghe

West Slopes South

Northwest Avalanche Center
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Pericolo valanghe

Fri
Alta quota
3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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Non aggiornato — questo bollettino è scaduto 6 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Stormy weather returns to the West South zone Thursday and will increase the avalanche danger throughout the day. With complex and overlapping avalanche problems, keep your terrain choices simple, steer around steep open slopes especially when you see blowing snow or encounter fresh cornices.

Problemi valanghivi

  • Wind Slabs

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

    Look for strong easterly winds to combine with a few inches of new snow to build wind slabs throughout the day. You could encounter wind loaded slopes on some unusual aspects. Blowing snow will serve as your simplest clue of increasing danger. Take note of the direction of the wind and anticipate which slopes could harbor wind slabs. If you notice drifts or fresh cornices, steer around nearby steep slopes and seek out less wind-affected terrain.

    The recent cold and clear weather allowed weak snow to develop on some surfaces. If these new slabs form over these weak grains, they could trigger easily and break widely. Since you may encounter these slabs in some unusual spots and they may trigger easily, give this common problem a wider berth.

    (2/9/2021) This widely breaking slab was observed at Paradise on Monday (2/9). [6500' E aspect. Photo: Porter Michaels]

  • Persistent Slabs

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

    It honestly feels like we’ve been talking about persistent slabs in the West South forever. But, there’s a good reason for that. Avalanche observations continue to trickle in from areas around the zone. Most recently natural large avalanches were observed in the Sourdough Range (White River area) and Coal Creek Ridge (Goat Rock Wilderness). This layer truly continues to “persist” in the snowpack. As we head into another round of storm, consider how much confidence you have in the snowpack. This seems to be shaping up to be a classic low-likelihood/high-consequence scenario. You could travel all day with any obvious evidence of unstable snow, however, this should not overshadow the lengthy list of avalanches, well-documented snowpack structure, and sudden snowpack test results. The simplest way to manage this challenging problem is to choose simple terrain where you can avoid large open startzones.

    (2/6/2021) This slab was intentionally triggered by a small cornice drop on Saturday (2/6) in the Crystal backcountry. It propagated widely and is believed to have run on the Mid-January crust/facet combo. [E aspect, 6300'. Photo: Jeremy Allyn]

Avalanche Discussion

Look for the avalanche danger to increase throughout the day as slabs grow deeper. Locations near Mt Adams and Mt St Helens could see dangerous avalanche conditions at all elevations by the end of the day. 

While you are most likely to encounter new wind slabs on Thursday, it’s the deeper persistent issue that continues to demand our attention. Avalanche activity seems to have quieted down in the West South zone over the past few days, but don’t let that fool you. The lengthy list of natural, human triggered, remotely triggered, and explosive triggered avalanches is compelling. Additionally, a similar snowpack structure exists across much of the Washington Cascades. In other zones, the same weak layers resulted in notable near-misses and an avalanche fatality. (See the short video in the media gallery below for more information) Nothing in the recent weather nor snowpack observations suggests this layer significantly gained strength. 

You can track this layer by digging in the snowpack. You may find a reactive interface about 2’ below the snow surface. This layer (2/1) continues to demonstrate the ability to propagate a failure and create avalanches. Slightly deeper, a layer of facets over the mid-January crust has been the culprit in most recent slides. It doesn’t really matter which layer you trigger, both can produce large and dangerous avalanches that break widely across the terrain. You can help us track these persistent weak layers by submitting snow profiles and avalanche observations to our public page. 

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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