Bulletin d’avalanche

West Slopes North

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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Périmé — ce bulletin a expiré 4 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Older slabs linger while fresh wind transport increases the risk of triggering a small slab. Steer around areas of active wind loading, particularly steep or unsupported slopes near ridgelines. Near and below treeline, a small loose avalanche could knock you off your feet on a slope approaching 40 degrees.

Problèmes avalancheux

  • Wind Slabs

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

    A windy system Sunday-Monday started warm and ended cold, but winds affected most of the storm snow near and above treeline. This should help limit the amount of snow available for transport. An average of 8-10" of settled storm snow sits above a thick and supportable crust in the terrain up to at least 6000 ft. In some areas the snow has been scoured down to the crust, but in other areas winds through 2/13 formed deeper drifts. Most human-triggered avalanches since the storm have been triggered near ridgelines and below wind lips or failing cornices (see photo gallery).  While these drifts are becoming stubborn to trigger, unsupported rollovers give them added life and an avalanche would be large enough to bury you.  Use terrain selection to limit your risk of triggering one of these avalanches. With increasing winds on Thursday, modest amounts of wind transport will create fresh, shallow slabs layered above older wind-compacted layers. These will be somewhat more reactive, but less consequential. These are of more concern in very steep or extreme terrain features, but continue to steer around them on any slope steeper than 35 degrees.

    It's been several days since the last reported avalanche on a deeper weak layer but it's still worth thinking about as you travel through the backcountry.  Infrequent avalanches have occurred on a subtle weak layer over a crust 2-4' down in wind-loaded terrain near and above treeline. Most of the avalanche activity on this layer occurred Wednesday, 2/8, but an avalanche was reported late in the day Saturday 2/11 in Canyon Creek, remotely triggered by a snowmobile. The large avalanche occurred on a 5400' north-facing slope and was 2 feet deep. Some large natural activity during recent storms produced similarly sized avalanches on steep lee slopes in the vicinity of lake Ann (size D2-D2.5). However, with the 2/12 crust extending to over 6000 ft, the potential for humans to trigger these layers would be limited to above-treeline terrain. 2/12 which extends above 6000 ft. NWAC forecasters didn't find anything concerning in test results at 5500 ft (E, W) aspects on the south side of Mt. Baker on Wednesday. 

    (2/15/2023)

    Around 8" of wind-affected snow sits above the 2/12 crust (which exists up to at least 6000 ft).  Mt. Baker south side.

    Photo: Irene Henninger

Avalanche Discussion

Light winds, cool weather, and sunny skies allowed wind slabs to continue a gradual healing process while keeping most snow surfaces cool. With increasing winds on Tuesday, avalanche danger may increase slightly in the afternoon.

Besides a few wind slab avalanches triggered early this week,  small dry loose avalanches were reported on steep shady terrain, and widespread rollerballs and small wet loose avalanches (up to D1.5) were observed on steep sunny aspects on Tuesday. These aspects now have a thin crust. Surface hoar now sits on the snow surface in some areas as clear skies allow snow surfaces to weaken. This weakening creates the potential for dry loose avalanches on Wednesday and steep terrain near and below treeline. Snow surfaces deserve attention as we start heading back into an active weather pattern later this week into next week. 

Régions couvertes