Bulletin d’avalanche

Flathead Range & Glacier NP

Flathead Avalanche Center
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Danger d’avalanche

Thu
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 6 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Complacency is the enemy! Dangerous avalanches can still break 1 to 4 feet thick on weak layers of surface hoar and sugary facets. They are most sensitive on north to east aspects above about 5,500 feet. Though scattered collapses and stability tests on south-facing slopes show that you can't write off those aspects as safe. Manage that uncertainty with careful terrain selection. Aim for low-angled slopes free of trigger points like breakovers where you'll find safer powder to ride.  

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    2–3

    Three weeks, and counting, of accidents, rider-triggered, and natural avalanches. The majority of recent slides are failing on weak layers of surface hoar and sugary facets buried 1 to 4 feet deep in the snowpack, atop the mid-January crust. More avalanches have failed where these layers were better preserved: on north to east to southeast aspects near and above about 5,500 feet. Don't write off southerly slopes as safe and get suckered into steeper terrain. Large collapses and stability tests indicate that these layers can still fail on those aspects as well, though the evidence is less widespread. The problem is uncertainty. The solution is terrain. Choose slopes less than about 30 degrees in steepness. Look for runs with consistent slope angles. Avoid convex breakovers and areas where recent winds have stiffened the snow near the surface.

    (2/15/2021) Northeast aspect, around 6,300 feet, 35 degree slope. Triggered from below.

Avalanche Discussion

Observers at upper elevations in the Whitefish Range, and in John Stevens Canyon, reported whumpfing on south aspects on Tuesday. Weak layers around the mid-January crust collapsed under their weight on low-angled ridgelines. Stability tests also continued to show that these layers can propagate a fracture across the slope in some areas. Riders triggered two persistent slab avalanches in the Whitefish Range on Monday. One large avalanche, on a southeast-facing slope, was intentionally triggered with a ski cut. The other, on northeast facing terrain, caught and buried a skier in neck-deep debris. Fortunately she was uninjured. Cam triggered a larger persistent slab with a small cornice drop in the Swan Range on Sunday.

Are you spotting the pattern? Weak layers of surface hoar and facets can still fail, resulting in dangerously large avalanches. These layers are widespread above about 5,500 feet across the forecast zone. They have been more reactive on north and east aspects where surface hoar buried in late January was better preserved. They are less predictable on southerly slopes where sun and wind may have damaged the offending crystals in some areas. Snow professionals, myself included, are still choosing low-angled terrain to manage that uncertainty and you should too. 

Up to 12 inches of loose, unconsolidated snow can sluff in very steep terrain. It seems like an afterthought to the danger of persistent slabs, but it's hard to avoid getting caught by moving snow in confined chutes. Or if they overtake you from above. Terrain traps like cliff bands and trees, or gullies where debris can pile up deeply, increase the consequences of a large loose dry avalanche. It's not out of the question for a big sluff to trigger the buried persistent weak layers. That's reason enough to pay attention as loose snow starts to slide.

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