Bulletin d’avalanche

West Slopes Central

Northwest Avalanche Center
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Danger d’avalanche

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

Points saillants

Strong winds are forecast to pummel the Central Cascades. Reduce the chance of triggering an avalanche by avoiding steep slopes where you witness active wind loading or that hold smooth, fresh-looking drifts. If you start seeing natural avalanche activity take a bigger step off and away from wind loaded slopes.

Problèmes avalancheux

  • Wind Slabs

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

    Wind slab formation Tuesday will be dependent on a few things. One is the amount of snow available for transport. The slopes that will provide most of the snow have some amount of crust on them. That could limit what can get blown onto leeward slopes.

    Southeasterly winds also often come with an unusual pattern. Some mountains and ridges will see sustained strong winds while other areas might remain calm. There will likely be stronger winds farther east in the zone but be ready to see them anywhere. 

    Finally, there is surface hoar and faceted snow at the surface on northerly slopes. Though fresh wind slabs might be fewer in number and size, what you do find might be more reactive than normal.

    Blowing snow will be the main observation to identify areas of active wind loading. Fresh drifts of smooth snow are wind slab. Test slopes and hand shears will help gauge slab reactivity and depth. You may see slabs form farther downslope or on different aspects than usual.

    Example of a small, human triggered wind slab in alpine terrain.

    Photo: Zach Bunch

Avalanche Discussion

In general spring-like conditions prevailed on the west slopes over the weekend. Despite colder-than-average temperatures, the sun was just strong enough to stiffen and crust up the snow surface on solar aspects. Some shaded slopes also picked up a soft crust when low clouds combined with sun overhead created greenhouse-like conditions. A combination of wet loose, dry loose, and slab avalanches to D2 was reported from nearly all west slope forecast zones.

Most noteworthy is a large human triggered avalanche in the Tatoosh Range near Rainier. The crown appears to be 1’-1.5’ deep with wide propagation. Without detailed information about the weak layer and bed surface it’s hard to discern the exact setup that produced this avalanche. It could be an isolated area where a thin layer of faceted snow survived and was buried during the latest storm. Warming conditions and low clouds may also have played some role in increasing reactivity. No other human triggered avalanches quite matched this one in size, but there was enough slab avalanche activity through the weekend to see the new snow took longer than normal to stabilize.

The factor that has most folks stepping back from bigger terrain is a layer of weak, faceted grains. They might be found below the most recent snow but above a crust that formed on all aspects about a week ago. Though no avalanches were directly linked to this layer, reports of whumphing (in West North) and a slightly unusual pattern of avalanche activity warrant a bit more caution, especially in terrain with cliffs, rocks, or other terrain traps in the runout. 

 

Régions couvertes