Bulletin d’avalanche

West Slopes North

Northwest Avalanche Center
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Danger d’avalanche

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

Points saillants

Factor in multiple avalanche types in your plan for the day. Wet avalanches on sun-exposed slopes are possible in the morning. Slab avalanches at higher elevations are an all-day concern but are more likely in the afternoon when winds are stronger. Pay attention to changing conditions and travel in simpler terrain to reduce uncertainty.

Problèmes avalancheux

  • Loose Wet

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

    Wednesday should start clear and sunny with increasing clouds midday. Though freezing levels will start low, temperatures will rise through the day and the April sun doesn’t need much time to affect the snow surface. Wet loose avalanches are possible if new snow warms, becomes wet, and weakens on sun-exposed slopes or at low elevations. Minimize travel on and below slopes where you observe wet loose avalanches or other signs of instability like pinwheels or rollerballs. If skies quickly cloud over in the morning then wet loose avalanches are less likely.

     

    Example of small wet loose avalanche initiating from steep treed terrain.

  • Wind Slabs

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

    Weather stations across West North recorded 8-16” of new snow at treeline. Increasing winds will redistribute abundant new snow and build wind slabs on leeward slopes. Alpine terrain will have more wind slab development in the afternoon but expect to see some at mid-elevations as well. In addition to new wind slabs, you can trigger day-old slabs, some of which formed over buried surface hoar.  One remotely triggered avalanche was reported on Tuesday from northerly, near treeline terrain. Though intact surface hoar layers are likely spotty in distribution, they are present and could be the weak layer in slab avalanches in the coming days. Remember that avalanches that fail in surface hoar layers can release above you, when you are in the middle of the slope or surprise you when you least expect it.

    Blowing snow will be the best indicator of active wind loading in the surrounding terrain.  Also note the pattern of wind scoured versus loaded slopes to help identify where wind slabs are building. Track the change in snow depth as you move in and out of wind affected terrain and look for stiff slabs overlying weaker snow below. Hand pits and test slopes can help you gauge the bond of shallow wind slabs, but assessing a deeper layer of surface hoar often requires snowpack tests. When in doubt just navigate around steep features and slopes where you suspect wind loading or that simply have deep new snow.

    Example of wind scouring snow from windward slopes (the other side of the ridge) and transporting it onto leeward slopes.

Avalanche Discussion

Despite significant snowfall, no natural avalanches were reported with improving visibility Tuesday. Observers did trigger shallow storm slabs about 6” deep in wind sheltered terrain. Of greater concern is a small but deeper slab (~2’) that released in a layer of preserved surface hoar. Though the avalanche occurred on a small terrain feature, it was remotely triggered from below. The lack of observed natural avalanche activity during the storm shows buried surface hoar is probably not widespread - but the jury is still out with a lack of observations from other parts of the zone. If you were to find buried surface hoar it would be on northerly, mid and upper elevation slopes where it managed to survive sun and warmer temperatures. In a snowpit you would find surface hoar below the most recent snow but above a melt-freeze crust from early April. Clouds limited direct sun but warm temperatures affected the snow on all aspects to at least 4500'. With clear skies and cooling overnight expect lower elevations - and southerly slopes in some parts of West North - to sprout a crust Wednesday morning.

 

 

Régions couvertes