Bulletin d’avalanche

East Slopes North

Northwest Avalanche Center
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Danger d’avalanche

Sat
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

Choose simple terrain to avoid triggering a persistent slab avalanche.  Put a large buffer between you and terrain connected to large open slopes,  especially those at upper elevations with large overhanging cornices.  Telltale signs of an unstable snowpack include whumpfs or collapses even in lower angle terrain. 

Problèmes avalancheux

  • Persistent Slabs

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

    You can still trigger a surprising and dangerous persistent slab avalanche Friday.  Observations from Thursday found the same facet/crust combination extending around the compass to southerly aspects with collapses occurring on this layer.  Smaller natural avalanches or cornice fall may also trigger avalanches during this mild and sunny stretch of weather.

    While you can trigger these avalanches on steep slopes, you may also trigger them in unusual places far away from typical start zones.  Deeper and more consequential slabs are more likely to be found on lee slopes near the Cascade crest and near and above treeline where westerly winds have drifted snow over the last week.  Minimize your exposure to large avalanche paths and look for safe locations to stop or regroup.  Choose simpler and less wind-loaded terrain until conditions change. 

    "Red flag" signs of the danger include whumpfing collapses, cracks shooting through the snow, 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.  In the eastern part of the zone, a similar weak upper snowpack structure exists but the slabs resting above the persistent weak layer are likely 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
    1–1.5

    After a cool overnight,  temperatures will once again climb above freezing under mostly sunny skies.  With winds forecast to be lighter on Friday than they have been for much of this past week,  snow surfaces may soften on an increasing range of aspects and elevations.  

    If you see pinwheels or rollerballs, the snow feels wet and heavy,  or see new loose wet avalanches, change aspects or move to lower angle terrain.  Be mindful of overhead hazards and terrain traps that can increase the consequences of being caught in a small avalanche. 

    As the mild and sunny weather continues, limit time below slopes with large overhanging cornices (see photo below) and give them a wide berth if traveling along ridgelines.  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. 

    (1/21/2022)

    Loose wet avalanches (likely from Jan 20th) on steep sun-exposed slopes near the top of Washington Pass.

    Photo: Drew Lovell

Avalanche Discussion

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 in the Hairpin Valley and on Kangaroo Ridge. 

Professionals found the same facets sandwiched between crusts on solar aspects near Hart's Pass Thursday and experienced collapsing on this layer. Widespread whumpfing and one large rolling collapse were noted by professionals in lower angle terrain near Washington Pass Tuesday.  NWAC forecasters on Monday found a touchy snowpack in the Silverstar area at low, mid, and upper elevations.  On Sunday, guides near Hart's Pass reported numerous natural persistent slab avalanches on leeward sides of ridges, large collapses, (traveling up to 600ft). 

Notable avalanche observations in the last week:  On Saturday 2/5, a party of backcountry travelers triggered a 3-foot deep slab avalanche near Maple Pass on an east aspect at 6,600ft. Three of the four riders were involved with the avalanche and one was carried 400 vertical feet, but unharmed. See the photo below. On Friday 2/4, observers remotely triggered a slab avalanche (size R4-D1.5) from approximately 600ft away on an east aspect near 6,800ft in the Hart's Pass areas. This slide triggered another (sympathetic) small slab avalanche. 

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