Bollettino valanghe

East Slopes North

Northwest Avalanche Center
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Pericolo valanghe

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Alta quota
2 · Moderato
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

Wind slabs are possible on steep slopes near ridgetop, especially in the western half of the zone where more snow may have accumulated. Tread cautiously and give these newly created slabs time to heal. Large persistent slab avalanches are becoming less likely, though still possible on isolated terrain features such as unsupported and rocky, convex slopes. Stay off of slopes steeper than 35 degrees if you find signs of instability such as shooting cracks or whumphing collapses.   

Problemi valanghivi

  • Persistent Slabs

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

    Persistent slab avalanches are dangerous and may surprise even the most experienced backcountry travelers. You may be able to trigger them from long distances away and from below. Any shallow windslab in the new snow could also step down and trigger a much deeper monster. Keep your distance from large slopes that are steeper than 35 degrees, especially unsupported and convex slopes. Avoid hanging out, and grouping up in potentially consequential runout zones. This may include sparsley treed slopes that could act as deadly strainers. While it is becoming less likely to trigger a persistent slab with time since the last major loading event, they still may be possible on Sunday. It's a good idea to give these well documented weak layers more time before venturing into avalanche terrain.

    The layer of concern is a thin weak layer of small facets (and in places surface hoar) buried about 2-4 feet below the surface. Near the top of Washington Pass, this weak layer will be sitting on an icy crust. Further east in the zone, the crust becomes thin and is breaking down into a mix of melt forms and weak grains. On Wednesday, observers reported whumpfing collapses and tests that indicated the potential to trigger avalanches on this weak layer, especially above 5,600ft. At lower elevations, there's less concern for triggering this layer.

    (12/23/2020) Persistent slab that likely ran on the Early December Crust. West facing slope in the trees at 6,800ft near Silverstar Mountain.

  • Wind Slabs

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

    Moderate winds with some 3-6" of new snow have created new windslabs in the high country. Freshly formed wind slabs could be reactive to human triggering on Sunday. Look for indicators such as shooting cracks and stiff snow over weak snow. Use test slopes to check how well these slabs are bonding. Avoid wind exposed slopes steeper than 35 degrees if you find signs of instability or fresh avalanches. Slabs will be deeper and widest in higher elevation exposed terrain, and near ridgelines. You can avoid getting avalanched by seeking out sheltered slopes and terrain less than 35 degrees in angle.   

    (1/10/2020) Example wind slab near Cutthroat Pass, photo from last season.

Avalanche Discussion

Just a few inches of snow was recorded at the Washington Pass station, but the higher peaks near and west of there probably received closer to 8" of new snow on Saturday. Previous surfaces were a mix of decomposed grains and wind packed particles in some areas, and weak snow over slick bedsurfaces in others. Observers have noted that many slopes ran during the widespread avalanche cycle of December 21st. In some cases, this has allowed folks to cautiously step out and travel onto slopes safely, if they know for certain that it already slid. This new snow may obscure evidence of where these slides have run, creating continued uncertainty, along with the potential to trigger a large persistent slab if it hasn't. There is also uncertainty about how the new snow and windslabs are bonding to recent surfaces above treeline. 

Observers have noted evidence of a widespread avalanche cycle in the Highway 20 area and the Twisp River drainage on December 21st. An observer reported a handful of large avalanches near Silverstar and Varden creek drainages. This includes slab avalanches on a northeast aspect at 6,400ft just west of Silverstar Creek, a west aspect at 6,800ft on Gardner Mountain, and a northeast aspect at 8,000ft on Silverstar Mountain. Size ranged from D2 to D3. Many avalanches were visible on southeast through southwest aspects of Delancey Ridge and Pine Creek above 7000ft. Some crowns propagated hundreds of feet with debris running over 2000ft into the valley below.

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