Bulletin d’avalanche

West Slopes North

Northwest Avalanche Center
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Danger d’avalanche

Fri
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 4 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Higher avalanche danger exists in upper elevation terrain where wind transported snow forms unstable new slabs. Monitor the wind loading pattern in the terrain you travel and steer around steep, freshly loaded slopes. You might see rollerballs or other signs of instability related to warmer temperatures and sun on Thursday.

Problèmes avalancheux

  • Wind Slabs

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

    Active wind transport above treeline was reported on Wednesday. Reactive slabs could exist where the wind is strong enough to transport abundant new snow. Fresh slabs could form near treeline if wind speeds are stronger than expected. Look for signs of active or recent wind transport like blowing snow and wind scoured snow surfaces. New wind slabs might be easier to trigger where they form over slick crusts. Even very small avalanches can have high consequences in extreme terrain or step down and trigger a larger avalanche. Anticipate stronger wind speeds - and increasing avalanche danger - the farther north and east you are in the zone, 

    Example of a small wind slab on a southerly aspect formed during northerly winds. New wind slabs often bond poorly to slick sun crusts below. Photo: Adam Ü

Avalanche Discussion

Thursday you could see signs of instability like trees shedding snow, rollerballs, or even a small loose wet avalanche caused by rising freezing levels and sunny skies. Solar slopes have had a few days of previous sun exposure but could still produce a small avalanche in some areas. Keep an eye on sun-exposed terrain overhead.  

The snowpack near treeline is fattening up and beginning to fill in terrain features. Wind affected surfaces are commonly found above treeline but in many areas, a few inches of light snow hides old, unreactive wind slabs. Low elevations still have ground cover poking up through the snowpack.

Surface hoar and facets are found at the snow surface in most shaded terrain while sun-warmed slopes are topped with a crust. 2-3’ of settled, right side up new snow from last weekend sits on a layer of buried surface hoar. However, the distribution of this layer varies greatly across the zone and no new avalanches were reported. Test results show lessening reactivity. That being said, the best policy is to not completely dismiss this layer, especially in just the right terrain feature (steep, unsupported, and sheltered enough to protect surface hoar when it was at the surface, but open enough it could actually develop). Farther south and east in the Cascades, a thinner slab sits on a more evenly distributed layer of surface hoar and has reached “problem” status.

Crust-facet layering that produced a surprising human-triggered avalanche in the Baker Backcountry 10 days ago is 4-6’ deep in most areas. Heavy loads like a smaller avalanche or landing (or not landing) a big drop might produce a large avalanche in any of these weaker layers that still exist in the snowpack.

Régions couvertes