Bulletin d’avalanche

West Slopes Central

Northwest Avalanche Center
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Danger d’avalanche

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

Points saillants

Upper-elevation slopes holding wind drifted snow are where you are most likely to trigger a slab avalanche. Use lower angle, supported terrain to navigate around steep, wind loaded start zones. You may find avalanche danger a touch higher farther south in the forecast zone near and south of Hwy 2 where there was heavier snowfall and stronger winds.

Problèmes avalancheux

  • Wind Slabs

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

    Anywhere from 6 -12” of new snow fell in West Central, often accompanied by moderate to strong winds. Moderate snowfall and winds are forecast to continue throughout the night and will thicken up existing slabs. Wind slabs initially formed on a variety of surfaces including slick crust, old settled snow, or a thin layer of faceted snow or surface hoar overlying crust. At lower elevations, the bond to the underlying crust was reported to improve by the end of the day, but it could be another story as you gain elevation.

    Keep an eye out for the typical signs of wind transport like scoured ridgelines, fresh drifts and cornices, and slabby, dense-looking snow to figure out where wind slabs lurk. Monitor changes in new snow depth and stiffness as you climb higher and change aspects. Evaluate the uppermost layers with hand pits, test slopes, and other informal tests. Some steep slopes at mid and upper elevations will likely produce avalanches while others won't. It’s up to you to figure it out. Or just stick to lower-angle slopes for another day and let fresh slabs bond before taking on bigger terrain.

    Example of smooth wind slab on the left and scoured snow on the right.

Avalanche Discussion

Anticipate small loose snow avalanches in areas where unconsolidated new snow has a poor bond to the slick crust below. You can definitely get some snow moving with you down the hill if descending steep terrain. If we see any sun on Saturday, steep cliffy areas will shed snow and start loose snow avalanches on slopes below. Keep an eye on what is above you, especially during periods of warming or sun.

On Saturday, the storm will be winding down, but the wind will be a primary factor in maintaining avalanche danger. Light snowfall and westerly gusts of 20-25mph could continue through the morning. By the end of Friday,  6-7" of snow accumulated at Stevens Pass. Stevens Pass Patrol reported that avalanches were easy to trigger in steep start zones and especially on leeward slopes. They also said that the old snow surface was still wet and unfrozen below 4,000ft.

NWAC Forecasters in the Alpental Valley found that the transition from rain to snow started around 3700'.  Initial impressions are that a moderate to poor bond exists with the underlying crust. The Summit at Snoqualmie Pro Patrol agreed with that assessment based on their observations from the top of Alpental. They also noted a few pockets of natural activity in wind-loaded terrain near the base of cliffs.

Observers in West North reported shallow slabs that were easily triggered on wind loaded slopes at mid and upper elevations. Signs of instability and avalanche danger generally increased when climbing into upper elevations where the wind had created more of a slab. Warming also made the snow more slabby and moist snow was found at the surface at and below 5000’.

 

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