Avalanche forecast

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Northwest Avalanche Center
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Avalanche danger

Sun
Alpine
3 · Considerable
Treeline
2 · Moderate
Below
2 · Moderate
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Highlights

Expect dangerous conditions at upper elevations with actively forming wind slabs. Very large persistent slab avalanches remain possible. Stay off of slopes steeper than 35 degrees if you find more than 8" of new snow and signs of instability such as shooting cracks or whumphing collapses. Wind slabs will be thickest in upper elevation terrain and in areas open to wind exposure.  

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    Some 5-10" of new snow is expected with the storm into Saturday afternoon. Strong winds with cold temps may create reactive windslabs during the course of the day. This new snow is stacking up over generally weak and uncohesive surfaces. Freshly formed wind slabs could be surprisingly reactive, and may only need a persons weight to trigger one. Look for blowing snow, spindrifting off large cliffs, and instability such as shooting cracks. Use test slopes to check how well the new snow is bonding. Avoid steep open slopes if you find signs of instability or fresh avalanches. Slabs will be deeper and widest in higher elevation, wind exposed terrain, and near ridgelines. You can avoid dangerous slopes by seeking out sheltered slopes and terrain less than 35 degrees in angle.  

    (12/25/2020) Blowing snow can help show you which slopes may harbor windslabs.

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    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 any slope 35 degrees and steeper. Make sure you're not standing below or adjacent to steeper slopes. While these had some time to heal, another storm load may increase the likelihood of triggering on Saturday.  In any case, we still have a lot of uncertainty regarding triggerability. It's a good idea to give these weak layer more time before venturing into avalanche terrain.

    You can find 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 persistent slab avalanches. However, it's possible that you could trigger an avalanche that starts in terrain well above where you travel.  

    (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.

Avalanche Discussion

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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