Bollettino valanghe

Snoqualmie Pass

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

Overlapping and complex avalanche problems should cause you to think twice and truly consider your confidence in the snowpack. Strong winds may from new slabs in some unusual locations throughout the day. However, it’s the deeper hidden weak layers that implore your to continue to keep it simple and avoid large open avalanche paths.

Problemi valanghivi

  • Wind Slabs

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

    Easterly winds should increase during the day Thursday and easily transport some of the light fluffy snow. Look for new slabs to build on unusual aspects throughout the day. As they grow deeper the hazard will increase. Blowing snow should clearly highlight the slopes you want to avoid. Watch for plumes along the ridgelines and snow blowing off trees. If you suspect the wind is loading a slope, steer around it, and look for less wind-affected areas. These slabs may form on very loose and unconsolidated snow. This could allow them to trigger very easily, entrain additional snow, and grow larger than there thin depth might suggest. 

    (2/9/2021) This human triggered wind slab occurred on Mt Snoqualmie on Monday, Feb 8th in the upper portion of the Phantom Slide path. SW, 5800'

  • Deep Persistent Slabs

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

    Nearly every day since Friday we’ve received direct evidence of deep persistent slabs lurking in the snowpack around Snoqualmie Pass. On Wednesday snow professionals in the Denny Creek drainage observed a new very large natural avalanche. This growing list of slides should give you pause. Deep persistent slabs can be troubling. You can travel all day and never see obvious signs of instability. It can feel “stable” or “safe,” all the while these weak snow layers lay in wait. Don’t let these “feelings” overshadow the more compelling observation: recent avalanches. While triggering these sleeping giants may prove difficult, any avalanche could grow very large and deadly. Failing cornices, small wind slab avalanches, or shallow spots in the snowpack could all serve as triggers for these deeper layers. Let this low-likelihood/high consequence problem shape your terrain selection by keeping it simple and avoiding large open avalanche start zones and moving quickly out of tracks and runout zones.

    (2/10/2021) This recent natural deep slab avalanche was observed from the Denny Trees area on Wednesday. It may have been triggered by a piece of cornice from above. Low Mountain, NE, 5000' HS-N-D2.5/R2-O, Photo: Jonathon Spitzer

Avalanche Discussion

This next storm won’t bring much in the way of snow to the Pass Thursday. Instead, the strong easterly winds should headline the event. Expect the avalanche danger to increase throughout the day as slabs grow deeper. While you may be more likely to encounter a new wind drift, it’s the deep persistent slabs that weigh heavily on our minds. These weak layers produced very large and unique avalanches at Alpental during control work, a significant near-miss at Stevens Pass, a fatal avalanche in nearby Salmon La Sac (See the short video in the media gallery below for more information), and a natural avalanche in Denny Creek. All of these locations share a very common theme, the same weak snowpack structure. Snow profiles dug Monday in the Alpental Valley and at the site of the recent fatal avalanche look nearly identical despite being in different forecasting zones, at different elevations, and on different aspects. This only confirms the generally widespread nature of this layer.

If you dig you may find two layers of concern in the snowpack. A thin crust (2/1) with small facets sits about 4’ below the snow surface. A second, much thicker mid-January crust rest about 5’ below the surface. It doesn’t really matter which of these layers you trigger, either would produce a very deep slide. If the recent avalanches aren’t compelling enough, these layers continue to consistently produce sudden failures in snowpack tests. We believe these layers to be most prevalent at mid-elevations, but could be present on any open slope above 4000’. Nothing in the recent weather nor in snow profiles suggest these weak layers have healed. 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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