Bulletin d’avalanche

East Slopes North

Northwest Avalanche Center
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Danger d’avalanche

Sun
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

Strong sun and very warm temperatures are expected to destabilize the snowpack. The likelihood of wet loose avalanches, large cornice falls, and dangerous persistent slab avalanches may increase with daytime warming. Time your travels to avoid steep slopes in the sun once they become manky, and avoid steep shaded slopes that are covered by stiff old slabs. Put a large buffer between you and terrain connected to large open slopes,  especially those at upper elevations with overhanging cornices. 

Problèmes avalancheux

  • Persistent Slabs

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

    Widespread whumphing collapses and a number of large natural and human-triggered avalanches have occurred on facet/crust layers from late January since last weekend. This instability is sticking around for longer than normal, and professionals are avoiding many steep slopes. The majority of instabilities and avalanches have occurred on shaded slopes at upper elevations, although observations from Thursday found the same facet/crust combination on southerly aspects. The warm temperatures may also increase chances of triggering these slides on all aspects as the snow surface softens. Along with the warming, wet loose avalanches or cornice falls may also trigger larger and wider persistent slabs. Minimize your exposure to large avalanche paths and look for safe locations to stop or regroup. Choose simpler and lower angle terrain until this instability abates. 

    "Red flag" signs of the danger include whumpfing collapses, shooting cracks, and tests that propagate or fail suddenly.  You can dig down and find the culprit, the late-January weak layer, 1.5-2 feet below the surface.  In many areas, this layer is a combination of weak sugar-like facets sandwiched between two buried crusts.  Below 5,600ft you may find buried surface hoar at this interface.  Deeper and more consequential slabs may be found on lee slopes near the Cascade crest and near and above treeline where westerly winds have previously created thick slabs. In the eastern part of the zone, a similar weak upper snowpack structure exists but the slabs resting above the persistent weak layer may be thinner.  See or hear something?  Let us know.  

    (2/4/2022)

    A party remotely triggered avalanche (R4-D1.5) from 600 feet away in the Hart's Pass area. This avalanche sympathetically triggered another small slide nearby. East, 6,800ft.

    Photo: Josh Cole

  • Loose Wet

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

    Snow surfaces are undergoing rapid change each day. Strong sun and warm temperatures are melting it into wet/sticky snow after just a few hours of direct sun. Expect wet snow on southerly aspects and moist, sticky snow over drier snow east and west aspects. As the day warms you may increasingly find areas where you could trigger rollerballs and loose wet avalanches. Some of these could be large if the terrain feature is large such as at higher elevations. If you observe wet, gloppy snow and rollerballs, expect you could initiate wet loose avalanches on very steep slopes. Loose avalanches could also trigger much larger and wider persistent slabs in some areas. 

    Keep in mind overhead hazards or other areas where wet avalanches may hold increased consequences. Cornices may fail sporadically with warm temperatures. While large blocks of hard snow crashing down a slope are a hazard in their own right,  they may also trigger large avalanches on the slopes below. Give them a wide berth on ridgelines during this warm spell, and stay out from underneath them. 

     

    (2/10/2022)

    Example wet loose avalanches.

    Photo: Matt Primomo

Avalanche Discussion

Small wet loose avalanches were easy to trigger on steep SE aspects at upper elevations on Friday. On Thursday, observers noted a cornice fall that triggered a large wind slab avalanche in rocky north facing terrain. Professionals have been finding facets sandwiched between crusts as the culprit for persistent slab instabilites. Widespread whumpfing, rolling collapses, and remote triggering of avalanches have been fairly common over the last week. There's considerable uncertainty about how the very mild temperatures and strong sun will affect these persistent slabs, but now is not a good time to step out into bigger terrain.

You'll likely encounter variable snow surfaces and crusts of varying thickness after a cool overnight and days of W-NW winds that buffetted exposed windward slopes. Snow surfaces moistened on all aspects Wednesday as temperatures rose above freezing at all elevations and clouds increased.  While snow surfaces have changed over the last week, snowpack test results continue to show sudden results on northerly aspects on the late January crust/persistent weak layer throughout the Highway 20 corridor including at Harts Pass,  Silverstar area, Hairpin Valley and on Kangaroo Ridge,  A party of four triggered a large persistent slab near Maple Pass and three were partially buried last weekend. Thankfully, all finished the day uninjured. A fatality occurred on the Duffey Lake Highway in Canada on a similar persistent weak layer the same day, and a Special Public Avalanche Bulletin is in effect through the weekend.

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