Bulletin d’avalanche

Flathead Range & Glacier NP

Flathead Avalanche Center
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Danger d’avalanche

Fri
Alpin
2 · Modéré
Limite forestière
1 · Faible
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

You can trigger loose, fan shaped avalanches where you find 8" or more of cohesionless snow at the surface. Below ridgelines and in the tops of chutes where recent winds have stiffed the surface, you may be able to trigger small wind slabs. Above about 7,000' you may be able to trigger an avalanche that breaks 1 to 2 feet deep on buried weak layers. Treat slopes with breakovers and rocky outcroppings with suspicion. 

Problèmes avalancheux

  • Loose Dry

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

    Loose dry avalanches can carry you downhill on very steep slopes where you find more than 8 inches of loose, sugary snow. These sluffs can run long distance where they overrun the slick 1/13 rain crust. That combination of factors is more prevalent on the easterly sides of our mountain ranges, and at upper elevations. If you travel in that type of terrain, avoid confined chutes where you can't escape moving snow. Avoid steep slopes with terrain traps below. Small sluffs can be dangerous if they push you over a cliff or pile up deep debris in a gully.

    Sluffing in steep terrain.

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    1

    On leeward slopes near the crest of the Swan Range and the Continental Divide, south winds may have drifted a few inches of snow below saddles and into the tops of chutes. In most cases they will be small and growing stubborn. You may be able to trigger them where for formed atop weak, sugary facets. They'll be denser than the snow in the trees or further down slope. Even small avalanches that knock you over on slopes above terrain traps can have magnified consequences.

    (1/26/2021) Cracking near the edges of isolated wind slabs. Marias Pass area.

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    2

    High elevation slopes that area not covered by the 1/13 crust? Steep slopes with rocky outcrops or rollovers? Grey stripes of buried surface hoar and faceted crusts about 1 to 2 feet down in your snow pit? If the answer is "yes" to any of these questions, you've found the terrain where persistent slab avalanches remain a concern. The crust reaches to about 6,500-7,000 feet depending on where you go. Where it's supportable, you and your machine will have a tough time collapsing the buried weak layers. The safest slopes are planar with a deeper, uniform snowpack and those free of breakovers and rocky areas.

    (1/25/2021) Slopes that are not covered by the 1/13 crust, that did not slide during the last avalanche cycle, are still suspect for the persistent slab problem.

Avalanche Discussion

The snowpack has been faceting over the past week creating loose, sandy riding conditions. That's been especially true at higher elevations where it has been coldest. On Tuesday, a party on the east side of Mt. Grant watched icefall trigger a large loose dry avalanche  which started around 8,000 feet and ran down to mid-elevations. The same day, another team climbing in the southern part of Glacier noted a deeply faceted snowpack on shady slopes above 8,000 feet. The windward slopes of our region have been scoured down to the 1/13 crust, or they have trace amounts of loose snow atop old wind board.

Wednesday brough a small change to the otherwise calm weather pattern of late. Winds picked up a bit and a few inches of snow accumulated here and there. Mt. Aeneas and Lake McDonald were windiest with light winds and gusts in the 20s elsewhere. The Noisy Basin, Tunnel Ridge, and Flattop Mountain weather stations got the most new snow - 3 to 5 inches in total. 

There are weak layers of surface hoar and faceted crusts buried 1 to 2 feet below the surface. The last avalanches failing on these layers ran on the 13th before the snow surface refroze into a stout crust. Since then there have been no reported collapses or slab avalanches. The most recent feedback we have from these layers is from stability tests which show that they can still propagate a fracture across the slope. So where can you initiate the fracture that results in a large avalanche? On steep slopes without the 1/13 crust, which reaches to about 7,000 feet, you may still be able to collapse one of these weak layers. Or in areas with a shallower snow cover or with trigger points such as rocky outcrops and convex breakovers. I'll still be giving that kind of terrain a healthy margin for error and plan my day with planar, well-supported slopes in mind.

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