Bollettino valanghe

Swan Range

Flathead Avalanche Center
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Pericolo valanghe

Thu
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 4 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

On Saturday, a snowbiker triggered a two foot thick slab avalanche in terrain sheltered from the wind. Recent wind loading from moderate and strong winds may steer you into this same kind of terrain. Stick to slopes less than 35 degrees to avoid this problem. Though not impossible, there is less of a chance to trigger a slab on windward and sunny aspects. 

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    On Monday, a rider found shooting cracks and touchy cornices in the northern Whitefish Range. Wind speeds since then continued to drift snow into a more cohesive slab. Slabs of drifted snow may range from 6 inches to a foot thick, which would be enough to knock a human off their feet. Wind slabs are more common on the downwind side of ridges and cross-loaded gullies. Terrain on the windward side, or terrain sheltered from the wind are safer options for managing this problem.

    (1/23/2023)

    Shooting cracks in wind drifted snow. 

    Photo: Sara B

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    A fragile layer of surface hoar is responsible for remotely triggered avalanches in the past two weeks, and one as recent as four days ago. This rider triggered a 2-foot slab from a convexity on an east-facing slope. All other reported avalanches failed on north and east aspects. Rob did find buried surface hoar on both south and west aspects in the last week, which tells us we can’t write off those aspects as safe. Observations and recent weather history tell us that this layer is harder to find on those aspects.

    (1/21/2023)

    This small, but potentially consequential avalanche was triggered remotely by a snowbiker. The rest of the slope cracked but did not slide.

Avalanche Discussion

For the past three days, remote weather stations in all ranges were at speeds capable of transporting snow. The question is, where was there enough snow to transport it into a slab?

Generally, riders found anywhere from 2 to 6 inches of snow above crusts at the middle elevation band (5000-6500 ft). Ultimately, it is not a lot of snow to form slabs large enough to be a concern. You may find some small and isolated pockets of wind-drifted snow at those elevations. The exception is the Swan Range, where more snow fell, and wind speeds averaged 15 to 18 mph for all three days. Expect to find slabs of drifted snow at middle and upper elevations there.

Above 6500 feet, more snow fell, and crusts are thinner or nonexistent. This will give the wind more fuel to form slabs. Slabs of drifted snow may be thicker and larger the higher you climb. These drifts likely rest on weak layers of facets and surface hoar, which can produce a dangerously large avalanche.

There is a trick to dancing around wind slabs with a persistent slab problem of buried surface hoar. Typically, the travel advice for managing wind slabs is to turn to terrain sheltered from the wind. However, terrain sheltered from the wind can spell trouble, especially in the Swan Range. Managing our slope angle and the terrain above us is the only way to effectively avoid this problem.

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