Bulletin d’avalanche

Flathead Range & Glacier NP

Flathead Avalanche Center
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Danger d’avalanche

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

Points saillants

Buried weak layers remain a concern. If triggered, these avalanches are likely very large and unsurvivable. Keep in mind that as you gain elevation, ridgetop terrain is affected by winds and may be reactive to the weight of your sled or boards.

Problèmes avalancheux

  • Persistent Slabs

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

    A layer of weak, faceted snow buried 2-4 feet was the failure layer in multiple large avalanches during Wednesday and Thursday's snowfall and intense wind events. This layer is several inches above the hard, early February ice and crust on shady, mid- and upper-elevation slopes. This weak snow has been challenging to identify in snowpits and does not always produce concerning results in stability tests. 

    This week's cool temperatures will slow the strengthening of this weak layer. You may not experience any signs of instability like collapsing or cracking before triggering a slide. 

    Choose terrain free of convexities to increase your safety margin. Steer around thin areas, such as those scoured or peppered with rocks, in favor of a deeper, more consistent snowpack. Cracking beneath your boards or machine indicates an unstable slab, and a sign to move to lower-angle terrain. 

    (3/2/2024)

    The fracture line of a very large avalanche on Nyack Mountain. 

    Photo: Mark Dundas

  • Wind Slabs

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

    This problem consists of two distinct generations of wind-drifted snow. The older slabs formed during Wednesday and Thursday's strong to extreme southwest winds. It consists of hard slabs that may be several feet thick. These features may be masked by recent snowfall, sound hollow like a drum, support you and your machine, and offer no signs of instability until they fail. You are more likely to trigger them from thinner spots in the slabs. Triggering a hard slab avalanche poses the risk of the slide stepping down to a deeper buried weak layer.

    More recent slabs formed over the weekend in the new snow. These should be easier to identify by soft, fresh drifts, dunes, rippled snow surfaces, and snow pillows. You may notice cracking beneath your boards or machine, indicating you have found an unstable slab. Let this direct you to lower-angle terrain. 

    Today's snowfall will replenish fetches and be available for transport, especially above 6,500 feet. Blowing snow and cornices point in the direction of danger.

    (3/3/2024)

    Stubborn, older cornices and wind drifts were found on Sunday in central GNP.

    Photo: Sarah Williams

Avalanche Discussion

Reports of natural avalanches have continued to come in from the past week. Many of these avalanches are large to very large and have the characteristics of a persistent slab problem. Crowns and fracture lines are several feet thick; many of these slides are more than 100 feet wide. These slides likely failed during the intense wind-loading event on Wednesday night or Thursday morning. They likely failed on a layer of weak faceted snow above the early February crust. We expect to continue to catalog similar avalanches when we have clearing skies and visibility. 

Incremental snow accumulations and light winds are expected today. As many observers reported on Sunday, there have been few signs of instability during their tours. Two observers in the northern Whitefish Range had no propagating results in their snowpack tests but were alarmed to see a large avalanche with a crown of 3 feet (Red Meadow observation, Link observation). In the Flathead Range, multiple crowns of very large avalanches have been observed (photo 1, photo 2, photo 3), with all of these slides starting at upper elevations.

Observers completing a snow survey in Glacier National Park submitted photos of debris that impacted the Going-To-the-Sun-Road from the Red Rock Slide path (photo 1, photo 2). The debris appears to be wet loose from the Leap Day rain event. Red Rock is just upstream from Avalanche Creek and is a dramatic path that drops steeply for 3484 feet off the shoulder of Mt Cannon (photo). This path hits the road yearly and occasionally dams up the adjacent McDonald Creek.

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