Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

Thu
Alta quota
2 · Moderato
Limite del bosco
1 · Debole
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 5 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

At upper elevations, be alert for soft slabs of drifted or wind-stiffened snow on steep slopes. These can produce avalanches large enough to hurt you or your partners where they're a foot or so thick, or above gullies, cliffs, or trees. On shaded slopes with no supportable crust below the new snow, you may still be able to trigger slabs that break on weak old snow buried one to two feet deep. Avoiding slopes steeper than about 35 degrees is a straightforward way to enjoy the fresh snow while minimizing your exposure to these hazards.

Problemi valanghivi

  • Wind Slabs

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

    Beware of soft, freshly-formed drifts on slopes steeper than about 35 degrees. These will be sensitive to the weight of a person or snowmachine today. The debris may run surprisingly fast and far where it sits on slick melt-freeze crusts. Triggered slides may be dangerous where the slabs are more than about a foot thick or above terrain traps. A shift to northerly winds at the end of Tuesday's snowfall may have deposited drifts on southerly slopes as well as the usual suspects - northerly and easterly slopes. 

    (2/15/2022)

    Small wind slab that failed on a northeast aspect at upper elevations

    Photo: Cam Johnson

  • Persistent Slabs

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

    On isolated northerly slopes, you can still trigger a slide that breaks one to two feet deep on old, collapsible weak layers. The most likely candidates are below ridgelines, have convex profiles, and don't have hard melt-freeze crusts below the new snow. They may be sheltered from winds by dense trees nearby. Rely on an inclinometer to stay off and out from under slopes like these that are more than 35 degrees in steepness. If you're descending and the slope abruptly steepens in front of you, stop and re-assess. 

Avalanche Discussion

Three to eight inches of low density snow have accumulated across the forecast region in the past 36 hours. Light southwesterly winds with moderate gusts accompanied the snowfall, peaking after sunset Tuesday when snowfall was most intense. They've since died down and veered northerly.

Not much snow and not much wind - but enough to form fresh drifts or stiffen the new snow into slabs in upper elevation terrain. Look for these in terrain that tends to collect blowing snow - the tops of chutes and couloirs, the shaded sides of gullies, and downwind of passes and saddles on ridges. With the wind shift to the north, we may see slabs continue to develop on southerly slopes today. 

On steep, sheltered, northerly slopes at mid- and upper elevations, buried weak layers... well, they don't quite lurk, since they're not producing triggered slides or audible whumpfs. Maybe linger, since they are still propagating in some large column tests (obs here and here). These layers consist of rounding facets or surface hoar buried by slabs of settled snow one to two feet thick. On many slopes, a grey stripe of surface hoar is obvious in a snowpit wall. It's hard to eliminate this structure as a concern, particularly on slopes steeper than about 35 degrees. The most reliable way to manage this hazard is to avoid likely trigger points in terrain where it's... linger-ier. Picture convex openings on otherwise treed slopes well below ridgelines. 

Tuesday's snow is low-density and generally cohesionless, and on many slopes, it's sitting on hard, slick melt-freeze crusts. It'll likely sluff easily with turns on steep slopes. These sluffs will mostly be harmless. Mostly. Don't let them carry you into something harder than you, like rocks or trees.

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