Avalanche forecast

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Northwest Avalanche Center
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Avalanche danger

Mon
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Highlights

Heavy snow, sleet and rain have likely caused a natural avalanche cycle for Santa to watch out for. Dangerous conditions will continue on Christmas Day while the snowpack slowly adjusts to the warming trend. This could be a day to play with the toys, but not on or below steep slopes.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1.5–2.5

    The heavy weight of the new snow will make it easier to trigger large and surprising avalanches.  With a primed snowpack and plenty of weak snowpack structure in the zone, the only sure bet is avoidance of steep slopes. Stay out from underneath steep slopes, as you could trigger a slide from a distance. Keep an eye out for signs of instability such as recent avalanches, loud whumphing collapses, shooting cracks. The problem is a weak layer of facets in the lower half to third of the snowpack and often under one or two crusts. See Willie's video below for a great example of what we're dealing with.

    Persistent slabs may be most likely where the snowpack is fairly shallow, but even in an area with a thicker snowpack in the zone such as near Washington Pass, they aren't  out of the question. Some recent avalanches were suspected to have involved the December 8th surface hoar, about 2 feet below the surface. Older observations indicate that persistent slab avalanches on weak layers in the lower snowpack could be possible near valley bottoms and in isolated areas in the alpine, where the facets near the ground (mid-November) are still weak and may be close to getting overloaded.

    Example widely propagating persistent slab near Harts Pass in March 2022.

    Photo: Drew Lovell

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1.5–2

    Expect everything from upside down storm snow to rain on snow. You could experience storm slab avalanches, and wet avalanches- both slab and loose. Choose well supported terrain, and avoid steep slopes if you find signs of instability like shooting cracks or observe recent avalanches. Storm slabs that are 6" deep may not be hugely problematic, but the concern lies in potential for deeper weak layers to be triggered. Avalanches could be large if they break into snow that fell since the 18th or earlier. 

    (2/5/2022)

    An example of a skier triggered avalanche near Maple Pass last season.

    Photo: NWAC public obs

Avalanche Discussion

Christmas will be a dangerous day in the mountains as we likely will be coming out of a natural avalanche cycle. Saturday's storm dropped about 1" of water equivalent at Washington Pass, and 1.2" at Harts Pass. Both areas received about 9" of heavy, dense snow. A mix of sleet and freezing rain was reported from near the Rendezvous Hut. Observers reported lots of 6-12" storm slabs east of Washington Pass, and very weak snowpack structure near the ground. Warming temperatures at higher elevations was causing the snow surface to become wet and sticky. 

The mountains close to the Cascade crest picked up close to 29" of snow through the 20th. On Tuesday, observers near Cutthroat Lake reported two larger natural slab avalanches likely due to new snow and wind loading. At least one of these may have run on older weak layers around 7,200ft, ENE in Swamp Creek (see photo below).  Observers continue to report lots of collapsing and cracks shooting through the snow in the eastern foothills. On the 18th, an observer triggered a small avalanche on an unsupported northerly feature below treeline in the Silverstar drainage. The slide ran on the mid-November facets. The December 18th interface holds weak grains of small facets, surface hoar, and crusts on southerly aspects. You may be able to find buried surface hoar from December 8th about 2.5 feet below the snow surface.

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