Bulletin d’avalanche

East Slopes Central

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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Points saillants

You can trigger large avalanches on weak snow layers buried by the recent snow or on older layers from Mid-November. While uncertainty exists around the distribution of these avalanches, the most likely place to trigger one is on north facing terrain at upper elevations, particularly in areas where the wind has drifted the snow deeper. Before committing to steep terrain, use your probe or pole to find potentially wind drifted locations and small, inconsequential test slopes to see how the snow reacts. You should stick to lower angle slopes if you feel the snowpack collapse or the test slope produces a mini avalanche. 

Problèmes avalancheux

  • Persistent Slabs

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

    During the storm from 12/8-12/9, avalanches ran on the mid-November facets at Mission Ridge, Crystal Mountain. We suspect that a couple of observed avalanches in the East North zone also involved older weak snow layers. These avalanches ran naturally, during control work, and by skiers in backcountry terrain. Avalanches involving weak persistent snow layers are tricky and tend to stick around for a while. The snowpack will seem to adjust over time to a new snow load, but it may only take an unlucky someone to find the right spot to trigger one of these avalanches. This uncertainty should make you take a step back and closely evaluate slopes that may harbor this weak snow. 

    Northerly aspects at higher elevations, where the wind has drifted the snow into deeper and harder slabs, are the most likely locations for triggering these deeply buried weak layers. Be particularly suspect of steep open terrain below ridges, areas with stiff wind drifted snow, steep rolls in the middle of slopes, and near rocks or cliffs where you are more likely to trigger these avalanches. Due to a calm finish to the storm and a recent dusting of new snow, some areas with wind drifted snow could be hard to identify by surface texture. Instead, use your probe or ski pole to identify areas where the snowpack is deeper than average or stiffer than the snow in sheltered areas. These are likely areas of wind loading. Dig down and identify sugary, faceted layers or feathery surface hoar layers buried in the snowpack. You can also use small test slopes to see if you can get a slab of snow to move. Stay on lower angle slopes if you find the overlaying snow slides easily on these layers, experience whumping collapses, or observe cracks shooting out from your equipment.

    (12/10/2022)

    3ft deep persistent slab avalanche in a heavily wind loaded area at Mission Ridge

    Photo: Drew Carlton

Avalanche Discussion

Many locations in the East Central zone have a weak snowpack structure with facets formed in mid-November buried near the ground. Most avalanches on these deeply buried facets are from northern aspects in wind-loaded terrain near and around Blewett Pass and Mission Ridge. Observations from the Teanaway and Salmon La Sac areas may indicate a decreased likelihood of triggering these avalanches on this layer in these drainages. While we are gaining confidence that areas with wind drifted snow are more likely places to trigger these avalanches, we still have some uncertainty. Therefore, it is still critical to observe the snowpack where you are traveling and scale back your terrain choices if you see any red flag signs of instability or find weak snow layers buried beneath stiffer slabs of snow. 

The sun has been warming southern aspects, and now most sun exposed terrain has a melt freeze crust on the surface. Morning valley clouds may burn off in the afternoon or not reach higher elevations. If you find yourself under blue skies, note that snow on steep, southern aspects may get heavy and wet. You could see roller balls or wet snow sliding off steep terrain near rocks or cliffs. Stay out from under cliffs or rock faces that begin to shed snow. 

If you are out and about in the mountains, you can help us by submitting an observation. 

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