Bulletin d’avalanche

West Slopes South

Northwest Avalanche Center
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Danger d’avalanche

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

Points saillants

Dangerous avalanche conditions exist in the West South zone due to the size of potential slides failing deep in the snowpack. Persistent slabs are almost impossible to predict and take time to heal, so avoid large open slopes where they're more likely. The storm snow could fail on wind-loaded features or be loose enough to trap you if you fall. 

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1.5–3

    This is the problem that should be shaping your travel plans.  The poor snowpack structure and recent avalanche activity to support a persistent slab problem are well documented in the zone. In some areas, you could consider this a deep persistent slab. In others, the EFC is still shallow enough to label it as persistent. The distribution and likelihood of triggering are a bit difficult to pin down, but most avalanches have occurred above 5500ft. Slides this past week have failed on a variety of aspects (NW-SE). They have been erratic and unpredictable, making it almost impossible to identify which slopes are primed and ready to release. What's not hard to predict is the size. They will be more than large enough to bury and kill you. Your best management tool will be to avoid large open slopes and their runouts where one of these wide, deep, and deadly avalanches could occur.

    (3/1/2024)

    A deep avalanche near Bear Gap in the Crystal Mountain backcountry. [N 6200']

    Photo: Andy Harrington

  • Wind Slabs

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

    Winds have been blowing from all directions the last few days, meaning slabs could exist on various aspects. You could find a wind-loaded pocket on both sides of the same ridge.  New shallow slabs may develop during the day, or older ones could be hidden under the surface. So, pay attention to changes in the density and depth of the snow as you travel. Firmer or thicker areas are likely wind-loaded and should be avoided. Visual clues such as fresh drifts, cornices, and textured surfaces can give you clues to local wind patterns.  Since the shifting winds and fresh snow may make these slabs challenging to locate, your best bet is to identify common features where wind slabs like to form and avoid them. They'll be more likely near ridgelines, along the base of cliffs, on convex rollovers, and at the tops/sides of chutes.

    In wind-sheltered areas, the low-density snow could result in dry loose avalanches on steep slopes. Don't get tripped up by one of these sluffs and pushed into a terrain trap.

    (3/2/2024)

    A deep (up to 4 foot crown) wind slab avalanche triggered by explosives at Crystal Mountain. It's on a fairly convex and unsupported slope. 

    Photo: Crystal Mountain Pro Patrol

Avalanche Discussion

The word right now is persistent. Persistent grains deep in the snowpack. Persistent weather patterns impacting the zone. Persistent avalanche forecasting. That is to say, not much is expected to change on Monday. More low-density snow, an uptick in wind speeds, and shifting winds are forecast. Sun breaks could occur between convective snow showers. If they're long enough, expect increased avalanche activity on slopes that see direct sunshine. Higher snowfall totals and wind speeds are expected on the volcanoes.

The mountains have received multiple feet of snowfall over the past week, almost doubling our seasonal snowpack. The constant and, at times, intense loading has stressed weak layers, now buried deeply in our snowpack. Natural and human-triggered slides have occurred on the Early February Crust (EFC) and other layers just above it near Crystal Mountain. Snowpack tests at White Pass and Paradise have resulted in sudden and clean breaks on these layers. In neighboring zones, avalanches on these interfaces have been triggered remotely and even resulted in a fatality on Friday in the nearby East South. The USGS has passed along several observations of very large avalanches occurring on all of the volcanoes in our zone, including Mount St. Helens.

The takehome point is that these weak layers have been waking up as stormy weather has increased the load over them. Persistent weak layers don't heal quickly and will be with us for some time. You'll need to exercise patience by avoiding large open slopes where these slides could be very large and deadly. Just because a slope has tracks doesn't mean it's safe. All it takes is one person to find the "sweet spot" to trigger one of these beasts. 

The potential for deep snow immersion (SIS) has increased as low-density snow has fallen. A SIS fatality occurred on Friday further north where similar storm accumulations exist. Keep your partners in sight at all times, and be sure they don't tip into a tree well or get stuck in a spot they can't climb out of. 

Régions couvertes