Avalanche forecast

West Slopes South

Northwest Avalanche Center
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Avalanche danger

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

Recent large avalanches and a known buried weak layer are two red flags that should have your attention heading into Sunday. It may not be your normal post-storm day in the Cascade Mountains, so give them time to adjust to the new snowfall. Take a good look at the snowpack structure and default to slopes under 35 degrees when in doubt. 

Avalanche problems

  • Storm Slabs

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

    Saturday was a busy day with plenty of avalanches around the West South zone. We anticipate that to continue into Sunday as temperatures fluctuate and we wrap up this storm. You could find that slabs are still quite reactive a day later, which isn't always the case in these parts. When looking for a potential slab problem, you can use uptrack tests, hand shears, or test slopes to help supplement your more formalized snowpit tests. If you find heavy snow over weak snow, blocks failing easily, or cracking of the snow surface as you travel, you should avoid open slopes over 35 degrees. 

    To complicate matters, a layer of buried surface hoar is likely present in many locations throughout the zone (see discussion for more details). If slabs are breaking on these persistent grain types, they can act in surprising ways such as propagating far distances or wrapping around terrain features. If you dig and see a thin gray line in the snow around the depth you are getting failures in your tests, you’ve likely found the issue and should take extra care.

    (12/10/2022)

    A skier triggered slab in the White Pass backcountry believed to have failed on the 12/8 surface hoar layer. This avalanche was 150 feet wide and it's crown was as deep as 2.5 feet. [NNE 6700' HS-ASc-D2-R2-O]

    Photo: White Pass Pro Patrol

Avalanche Discussion

The West South was right in the bullseye for this storm and Saturday’s totals did not disappoint. Many areas of the zone received around 15” of snow on Friday night, with some NWAC weather stations recording winds up into the 70 mph range. An unlikely winner for the storm was Mount St. Helens, who outpaced Rainier, Adams, and areas closer to the crest. 

We are lacking information about avalanches in many areas of the zone still, but Crystal Mountain Pro Patrol reported a very active day with their control work on Saturday. Numerous large avalanches and a few that grew very large were triggered with explosives. Crown depths were anywhere from 1-2.5 feet deep and some slides were able to run full path. Down at White Pass, we received word of a human-triggered avalanche that is believed to have failed on the 12/8 surface hoar. The avalanche was a hard slab close to 2.5 feet deep at its thickest and 150 feet wide. Other observations from Saturday indicate that the wind was a player as expected, with larger drifts near the base or cliffs or ridgelines and wind-pressed surfaces found in more open wind-prone areas.

On Friday, NWAC forecasters and avalanche professionals were able to identify buried surface hoar at Mt. Rainier and in the Crystal Mountain backcountry. This layer (12/8) has also been identified in the Mt. Hood, Olympics, Snoqualmie Pass, and Stevens Pass zones. You can expect to find this layer 2-3 feet below the surface and you may see a thin gray line. Snowpack tests showed propagation on this layer prior to the bulk of this storm’s loading. Before committing to terrain on Sunday, take a look for this weak snow and the slab qualities above it. 

With so much recent snow, early-season hazards are diminishing, but you should still look for holes and open creeks. And keep your partner in sight, it could be a struggle to get up if they stumble. 

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