Avalanche forecast

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Northwest Avalanche Center
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Avalanche danger

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

A significant winter storm will test buried weak layers in the snowpack.  Avoid wind-loaded avalanche terrain at upper elevations.  As the new snow piles up in sheltered areas, you may encounter reactive shallow storm slabs or fast-running loose dry avalanches.  Take stock of changing conditions and bundle up!

Avalanche problems

  • Storm Slabs

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

    A storm system will collide with an arctic air mass entrenched on the east side of the Cascades late tonight and Tuesday, bringing strong alpine winds and low-density snow to the area.  The East North zone is on the northern periphery of some of the higher snowfall forecasts, injecting more than normal uncertainty.  Storm totals of 10-18" by Tuesday afternoon near Washington Pass with tapering amounts further east are in the realm of possibilities. This will bring a smorgasbord of storm driven/new snow avalanche problems. 

    The wind will easily blow the new snow onto lee slopes, increasing the chances of natural or human-triggered wind slab avalanches below ridgelines.  It's in these areas that slab avalanches could become large enough to injure, bury or kill a person.  It's an easy call on Tuesday to avoid steep open slopes where you see blowing snow and avoid traveling below slopes with sharp cornices.  As you transition to elevations with firmer wind-pressed snow, seek out slopes less than 35 degrees. 

    In more wind-sheltered areas, soft slabs avalanches will likely be shallow and may break easily within storm layers or on surface buried 12/18 (thin sun crusts on steeper southerlies, a fresh crop of surface hoar in sheltered areas).  If you see shooting cracks or signs of recent avalanches, avoid convex rollovers and unsupported terrain features.  Use hand pits or small test slopes to assess how the new snow is bonding to the old snow surface.  Last but not least on the avalanche spectrum, you're likely to trigger dry loose avalanches in very steep terrain. These loose avalanches or sluffs, have a lot of cold dry snow to entrain and may run fast enough to knock you off your feet.  Don't let one catch you by surprise and use caution around terrain traps. 

    (12/20/2021)

    A storm slab avalanche on a steep rollover from last season in the Hairpin valley. 

    Photo: Adam Butterfield

  • Persistent Slabs

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

    This problem lives in the lower elevations east of Washington Pass, including terrain near and east of Mazama.  Especially in the foothills, we often don't think of the potential for large and surprising avalanches.  However, several observations of cracking and collapsing in these eastern mountain environs culminated in a large 500 ft wide avalanche this past week.  In these locations, you may find a layer of weak sugary facets in the lower portion of the snowpack, often under 1 or 2 crusts.

    If you see shooting cracks, feel sudden collapses, or hear whumpfs from the snowpack, these are clear signals to avoid traveling on or under any nearby steep slopes.  Plan your route with a wide buffer between you and avalanche terrain,  persistent slabs can be triggered from a distance. 

    (12/14/2022)

    Large avalanche crown running across the top of an east-facing slope in Finley Canyon, southeast of Twisp.

    Photo: Public Obs

Avalanche Discussion

3-6" of snow accumulated Saturday night and Sunday with very cold temperatures and moderate westerly ridgeline winds.  On Sunday, professionals in the Silverstar area found a few inches of fresh snow and no wind effect up to 6400'. However, they experienced frequent collapses at lower elevations with very small slabs releasing on buried weak layers below 5000' and on unsupported terrain features (see photos).  Prior to the new snow, avalanche professionals this week noted surface hoar in sheltered terrain.  In exposed areas, the wind knocked these weak grains over.  A thin breakable sun crust was observed on steep southerly slopes.  These recently buried surfaces (12/18) may become important layers as we head into an increasingly active weather pattern.  

In the WA Pass areas, two other notable layers remain in the snowpack. Buried SH (12/8), often found about ~12 inches below the surface in wind-sheltered terrain continues to linger, but this layer has not been an issue thus far. While you may still find the mid-November facets and crusts in the lower portion of the snowpack, professionals have not reported any recent significant snowpack test results.

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