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

Take a step back to lower-angle terrain is the best way to avoid avalanches with the current weak snowpack. Westerly winds will form stiff slabs that could overload the weak snowpack, resulting in large, widely breaking avalanches. Stay off and out from under wind-loaded slopes at middle and upper elevations, and pay attention to warning signs like cracking and collapsing.

Problemi valanghivi

  • Persistent Slabs

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

    7-10" of snow fell over weak faceted snow and buried surface hoar. Moderate to strong winds will continue to put additional strain on these weak layers as large wind slabs form on lee aspects below ridgelines. In these wind-loaded areas, avalanches may break widely across terrain features or be triggered remotely. You'll want to steer clear of steep lee slopes near ridgelines, where these avalanches are likely to be much larger. On sheltered slopes, these avalanches may act more like your average storm slab, but they may be reactive for longer than you expect.

    Long shooting cracks, whumping collapses, or natural avalanches are key indicators of dangerous avalanche conditions. Small, steep test slopes or convex features can help gauge the reactivity of the upper snowpack. But given snowpack variability, be cautious about extrapolating your results too broadly.  Instead, seek out low-angle terrain protected from the wind while the snowpack adjusts and we gain more information on how the buried persistent weak layers are handling the new load. 

    Natural slab avalanche with wide propagation in the Hairpin Valley. We suspect this avalanche failed within weak snow over the 2/7 crust. SS-R2-D1.5

Avalanche Discussion

Ongoing westerly winds will dominate the weather at least through Thursday. This wind event is so significant that the drifted slabs are adding stress to the weak, underlying snowpack. Stay diligent with your observations and give yourself greater margins of safety to avoid getting surprised. 

Since the weekend, a widespread cycle of natural and triggered avalanches in the Washington Pass area has been reported. A decent share of these are confirmed to have run within weak, faceted snow over the 2/7 crust. While most avalanches were small, we have limited information from wind-loaded terrain. Over the weekend, there was a report of a widely propagating, but shallow wind slab avalanche near Heather Pass. While we don't have much info on that incident, the propagation is a clear warning sign of persistent slab avalanches. There is uncertainty about the extent of avalanches these persistent weak layers will produce, but it's best to treat them with suspicion until they prove otherwise. 

On all but southerly aspects, there's 8-12" of dry, weak snow over the upper crust (February 7th). The layer immediately above the crust is very weak with larger faceted grains and surface hoar. The January 28th crust is thicker and now 12-16" below the surface.  On southerly aspects, there are more crusts and less dry, weak snow on the surface. Further west of Washington Pass, the crust may be thicker and deeper with more dry snow between them. Farther east, in places like the Chewuch and Twisp River drainages, the snowpack is shallower and weaker. Here, the crusts have become faceted on shady aspects, with the upper 2/7 crust barely discernible in some northerly terrain.

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