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 2 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

The primary concern is avalanches failing 2 to 4 feet deep on a persistent weak layer. Winds drifted snow into thicker, denser slabs above 5000 feet. Manage both problems by selecting lower-angled slopes on aspects that face NW-N-NE-SE.

Expect a Wet Loose avalanche problem IF the sun stays out longer than anticipated. Rollerballs and gloppy snow are signs of an impending Wet Loose avalanche cycle. Move to shaded slopes if you see this beginning. 

Problèmes avalancheux

  • Persistent Slabs

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

    A weak layer of facets near the early February Crust is the culprit of many large natural and human-triggered avalanches in the last few weeks. Exercise caution on shaded slopes near and above 6500 feet in elevation. If triggered, slabs can break 2 to 4 feet deep and will be difficult to escape. The most conservative option is to stick to lower-angled slopes in areas where the problem exists. Avoid steep rollovers and rocky outcroppings if you choose to enter steeper terrain on shaded slopes. 

    (2/29/2024)

    Crown of a large hard slab avalanche that failed on the night of February 28 on the north face of Devil's Hump.

    Photo: Adam Clark

  • Wind Slabs

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

    Winds transported snow into fresh slabs on leeward-facing aspects. Wind slabs range from thin to thick and new to old. The more sensitive of the two will be thin slabs that formed in the last 24 hours. Drifted snow should be easy to identify by its dune shape or textured appearance.

    Wind Slabs formed on slopes that harbor weak layers buried deep in the snowpack. It is possible that triggering a small Wind Slab could step down to a dangerous Persistent Slab avalanche. Avoid this problem by sticking to slopes less than 30 degrees, or by choosing aspects that face south or west.

    (3/4/2024)

    A rider triggered this soft slab east of WMR on Monday, 3/4. The slab involved snow cross-loaded onto the mid-elevation, east-facing slope by southerly winds. The rider escaped unhurt.

    Photo: Flathead Avalanche Center

Avalanche Discussion

Avalanche activity in the last week has ranged from small to large (D1-D2), with a few very large (D3) avalanches. Most reported avalanches were Storm/Wind Slabs on slopes above 5000 feet. The larger avalanches are Persistent Slab avalanches failing 2 to 4 feet deep on a layer of facets above a hard crust buried in early February. Avalanche activity on this layer is most common above 6500 feet on aspects that face N-NE-SE.

Instabilities in the new and drifted snow should be relatively short-lived. Winds were transporting snow yesterday. Expect to see signs of cracking with the possibility of triggering smaller slabs on leeward-facing aspects. These new thin slabs rest over thick, hard slabs from the past week of moderate to strong winds. Although those are gaining strength, I’d still be wary of jumping onto steep slopes that face NW-NE-SE.

Persistent weak layers buried near the early February Crust remain untrustworthy. Snowpit tests continue to show mixed results of propagating and non-propagating. In some areas, this layer is buried at a depth where a human is less likely to trigger. On the flip side, if triggered, the resulting avalanche may be unsurvivable. Common trigger points are convex rollovers and thin parts of the slab. Unfortunately, it is sometimes impossible to identify where those thin parts are. Rocky outcroppings or areas scoured by the wind are the usual suspects.

The weather forecast suggests periods of clear skies. IF the sun stays out for longer than anticipated, or if you travel to the Whitefish Range which is supposed to see fewer clouds, then expect a loose snow avalanche problem on aspects getting hit by the warm sun. Sluffs, rollerballs, and gloppy snow are signs of a Wet Loose avalanche problem. Move to lower-angle slopes or slopes shaded by the sun if you see this problem beginning. 

 

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