Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

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

In sintesi

Pay close attention to changing conditions. As you gain elevation, avoid steep slopes with stiff, hollow wind slabs that can break in unpredictable ways. Where the surface snow becomes saturated, watch for wet loose avalanches in very steep terrain. On sheltered slopes, with no supportable crust below the recent snow, avoid collapsing persistent weak layers where the slope steepens abruptly to more than 35 degrees. The danger will likely rise into tomorrow as a Pacific storm begins to affect our area by nightfall.

Problemi valanghivi

  • Wind Slabs

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

    A skier intentionally triggered two small wind slabs Friday on a north-facing ridge. Most drifts you’ll find today are leftovers. They’ll be hard and sound hollow. Fresh drifts could form as snow showers develop this afternoon. Expect slabs, old and new, on the downwind side of ridges and saddles or the sidewalls of gullies. You’ll notice rounded, etched snow that’s firmer than in sheltered terrain. Hard slabs can catch you by surprise and break above you. Pay close attention to how much you sink into the snow. All of a sudden, you’re riding on a firm, hollow surface? Stiff snow that cracks around you? Dial it back a bit and look for better riding in sheltered terrain.

    (2/18/2022)

    Riders intentionally triggered this small wind slab on a north-facing test slope at 6400 feet.

    Photo: Alex

  • Loose Wet

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

    Riders saw rollerballs below 5000 feet Friday. No refreeze overnight has likely increased the threat of wet loose avalanches at lower elevations. Freezing levels Saturday could climb to 6000 feet, wetting dry snow at mid-elevations for the first time. Warm air, clouds, and new precipitation starting as rain mean that you could find wet snow on all aspects. If you see rollerballs or small natural point releases, avoid being on or under very steep slopes. Wet debris is hard to escape and is more dangerous above terrain traps.

    (2/13/2022)

    Rollerballs in Whitefish Range

    Photo: Mark Ambre

  • 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 recent snow. They may be sheltered from winds by dense trees nearby. Use an inclinometer to stay off and out from under slopes like these that are more than 35 degrees.

    (2/16/2022)

    One out of many: a single stability test propagated under moderate force on a layer of weak facets about a foot below the surface. This propagating result was at 6650' on a due south 29* slope. Spatial variability abides!

    Photo: Zach Miller

Avalanche Discussion

Warm and windy weather precedes an approaching Pacific cold front, bringing more snow, more wind, and falling temperatures. An arctic front is on its heels. Be prepared for changing conditions throughout the weekend.

Friday, one out of three observers skiing in the Middle Fork was able to trigger wind slabs on a north-facing ridge. The other two found thin, stiff slabs that were not reactive to stomping. They all noted strong ridgetop winds, but only some blowing snow. Old, hard slabs at upper elevations developed Wednesday and Thursday. They may not be so easy to trigger today, but their stiffness makes them less predictable. They can break above you or your machine and need your careful attention.

Riders Friday also noticed wet, heavy snow and rollerballs below 5000 feet. Weather stations show above freezing temperatures at that elevation throughout the night. No refreeze can increase the threat of wet loose avalanches. Freezing levels Saturday could climb to 6000 feet, wetting dry snow at mid-elevations for the first time. 

Low-density snow that fell early in the week drifted onto many aspects. Ridges have a variety of firm wind-affected surfaces. Under the recent snow, the old surface was a hard crust that extends into the alpine in the Swan and Flathead zones. Most of the reported avalanches from mid-week slid on that crust. It’s spottier on northerly slopes and in the Whitefish Range, where surface hoar is still causing some stability tests to propagate. Weak faceted snow still rests one to two feet below the surface in the Flathead zone. These persistent weak layers are mainly a concern on slopes without a supportable crust above them in the snowpack.

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