Bulletin d’avalanche

Snoqualmie Pass

Northwest Avalanche Center
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Danger d’avalanche

Wed
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 5 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

While the new snow may be enticing, it brought an abrupt change to the snowpack. Avalanches may trigger easily especially at mid and upper elevations and on wind-loaded slopes. Give the snowpack time to adjust by choosing lower angled and less wind-affected terrain.

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1.5–2

    Moderate to strong winds prevailed throughout this storm, easily blowing snow and likely forming firmer and thicker drifts. Some of these harder slabs may act differently than the softer wind slabs we frequently see at Snoqualmie Pass. They could break widely and fail above you. Give them a wide berth. Look for clues like textured surfaces and drifts to help anticipate which slopes and features could hold wind slabs. Then seek out lower angled or less wind-affected snow. In some locations, these wind slabs may sit on older weaker snow or a firm and icy crust. Combining harder wind slabs with these possible weak layers makes for a potentially dangerous setup. Even though the storm is ending, these wind slabs are still fresh and will need time to gain strength.

    (1/2/2022) Example: a shooting crack in a firm wind pillow at Snoqualmie Pass earlier this year. 

    Photo: Robin Pendery

  • Storm Slabs

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

    Near pass level, we have good observations that suggest the new snow bonded well to the old surface. However, as you ascend in elevation our uncertainty grows. You may find avalanches can fail at the base of the new snow or on layers created during the storm. A variety of firm crusts and weak snow grains were observed on the surface over the past week. “Variable” was the word most often used (see discussion for more details). So, while you can use small slopes and hand-pits to test the new snow, they may poorly predict how any particular slope will react. At mid-elevations, ease into the terrain by choosing lower-angle and smaller less consequential slopes to gather lots of information. It may be worth approaching the mountains a bit more conservatively until we all see how these potential weak layers fared during the storm.

    (1/29/2022)

    The thin and breakable crust capping facets in the facet-crust sandwich. These conditions were often seen on NW-NE aspects. 

    Photo: Dallas Glass

Avalanche Discussion

While this wasn’t a huge snow producer for Snoqualmie Pass, it was an abrupt change to the snowpack and a return to winter for the mountains. As of Monday afternoon, about 8” of snow accumulated at Pass level. Higher in the terrain, much more snow fell. The top of Alpental recorded 16-18”. Snow showers should decrease Monday night, but several more inches of snow could still fall at the pass before Tuesday afternoon. 

This new snow fell on a variety of snow surfaces. Numerous observations sent to NWAC reported icy crusts, near-surface facets, large surface hoar, and facet-crust sandwiches. An avalanche professional traveling in the Alpental Valley Sunday afternoon reported freezing rain generally destroyed the weak surfaces below 4000 ft. This was corroborated by observations on Monday morning that the new snow bonded well to the old surface below the same elevation. However, these weak grains may still exist higher in the terrain and east of the Pass. Near-surface facets and facet-crust sandwiches were found on all aspects, even in gladed treed terrain. Firmer melt-freeze crusts were most frequently seen on steeper SE-SW aspects where the sun and clear nights caused a prolonged freeze-thaw process last week. While more spotty, surface hoar was frequently observed near the top of the recent inversion layer (3700-4500’). While these shallow weak layers will be relatively easy to observe, their variability will make decision-making uncertain. 

Fortunately, below the most recent surface, the snowpack is strong and well settled. We are not tracking any weak layers below the 1/30 interface. Any avalanche concerts should remain near or above the base of the new snow.

Régions couvertes