Bulletin d’avalanche

West Slopes South

Northwest Avalanche Center
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Danger d’avalanche

Sun
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 3 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

If you are thinking of heading to a volcano, be prepared for significant snowfall and potentially High avalanche danger, especially at Mount St. Helens. Throughout the rest of the zone, stormy weather should start to taper but you may still be able to trigger avalanches on steep slopes over 35 degrees. If the snow isn’t light and fluffy, you have a higher chance of triggering a slab avalanche.

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–2

    Regardless of your location in the zone, you’re most likely to trigger a slab avalanche in wind-loaded terrain. With plenty of low-density snow available for transport, new shallow slabs may be forming throughout the day or you could still encounter lingering slabs from earlier on in the storm. Increasing winds on Saturday should have an easterly component to them, meaning you could see some atypical loading patterns including slabs forming on different aspects or slopes being more cross-loaded than you are used to. Use visual clues such as blowing snow, textured surfaces, new cornices, or drifts behind objects to determine wind-loaded aspects. Once you know which way the wind was blowing, look for smooth “pillow-like” piles of snow that appear “fat” or feel stiffer compared to the surrounding snow to identify Wind Slabs. Using small test slopes or uptrack tests as you travel can give you clues about how wind-drifted snow is reacting to your weight as a traveler. Closer to the crest or at lower elevations, you’ll likely need to manage Wind Slabs on more specific terrain features such as confined gullies and convex rollovers. As you reach the alpine, the distribution may expand to more open slopes over 35 degrees where the possibility of triggering an avalanche increases.

    (2/28/2023)

    A shallow skier-triggered slab avalanche in a steep confined gully. This slab was intentionally triggered by Crystal Mountain Patrol on 2/28.

    Photo: Crystal Mountain Pro Patrol

  • Loose Dry

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–1.5

    In more wind-sheltered areas, plenty of low-density snow is available to produce avalanches in steep terrain. Some of these avalanches may technically fail as soft slabs but behave more loose since the recent storm snow is light and fluffy. If you find a pocket that has more cohesion, be aware that slab avalanches are still possible. Regardless of how you want to categorize them, they will be easier to trigger on steep slopes over 35 degrees and could grow in size as they move downhill and fan out. You should consider avoiding large open slopes at upper elevations until the snowpack has more time to settle. Sometimes referred to as “sluffs”, don’t become complacent and be tripped up by one of these avalanches near terrain traps or in extreme terrain.

    (3/3/2023)

    A natural small loose dry avalanche in very steep terrain. [E, 3800ft, L-N-D1-R1-S]

    Photo: Dallas Glass

Avalanche Discussion

It’s been a windy and snowy week around the West South and we’re not done quite yet. The volcanoes especially are going to continue to get pummeled on Saturday, with 1 to 2 feet of snow expected. Mount St. Helens is set to be the winner during this storm and receive the highest snow totals, but Adams and Paradise may not be far behind. There’s a chance if the snow totals pan out that very dangerous conditions (High) are reached at Mount St. Helens.

Any new snow will only add to the significant totals already experienced this week. Around the zone, 1.5-2 feet has fallen since Thursday (3/2 interface). This is on top of the roughly 1.5 feet received earlier in the week (2/25 interface). Both storms were generally cold and delivered light fluffy snow. All that to say, it’s getting pretty deep out there and you’ll have to manage new snow problems if you head into the mountains this weekend.

On Thursday, skiers near White Pass reported actively blowing snow and shallow Wind Slabs that were reactive to human triggers. In the Crystal Backcountry, avalanche professionals reported triggering multiple small (~6 inch) soft slab avalanches near rocky terrain and confined gullies. On Friday, NWAC forecasters in Cement Basin observed slight wind transport near ridgelines, some small but sensitive cornices, and plenty of low-density snow lacking much cohesion. They were able to find subtle interstorm layers, however. The sun came out mid-day and helped snow “slab up” some at lower elevations on slopes that see direct sunshine. 

On Wednesday, skies cleared between our two big snow producers, and widespread weak surface hoar was reported. The sun also made an appearance, resulting in a thin sun crust on slopes that see direct sunshine. We do not have any reports of these surfaces coming into play for avalanche activity since being buried, but you should be aware of them as you perform your slope scale assessments. We expect the majority of avalanche activity to occur above the 3/2 interface but isolated areas could fail as deeply as the 2/25 given the right trigger and the right piece of terrain. 

Régions couvertes