Bulletin d’avalanche

Flathead Range & Glacier NP

Flathead Avalanche Center
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Danger d’avalanche

Fri
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 4 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Steer around drifts of recent snow found along ridgelines and below saddles. Cracking in the snow surface is a good reason to seek out areas that are less steep or better sheltered from the wind. Decrease your risk by selecting terrain that lessens the consequences of a potential slide.

Problèmes avalancheux

  • Wind Slabs

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

    You can find slabs of wind-drifted snow 1 to 3 feet thick along ridge lines, below saddles, or downwind of cross-loaded features. Today, Southwest winds will taper to threshold speeds for drifting snow and allow recently formed wind slabs some time to heal. This respite will be shortlived as winds are anticipated to pick up tomorrow (Friday). 

    Yesterday (Wednesday), riders found light-blowing snow and freshly formed drifts.  The sensitivity of these slabs ranged from stubborn to reactive.  Cam was in the Middle Fork corridor, where he initiated shooting cracks and some very small slabs.  Another party in the Essex area backed off of their line when they found the entry choked off by sensitive wind drifts. They moved to low-angle terrain until they found   Guy and Rob, however, jumped on wind-loaded test slopes in the Swan Lake area with only minimal cracking in the wind-stiffened surface.  WMR patrol found slabs ranging from 6-12" inches during avalanche control work.  Observers throughout the advisory area continue to spot natural slab avalanches that likely failed in wind-drifted snow, a few of which were big enough to bury a person.  

    Dune-shaped featured or rippled surfaces are indications of wind-drifted snow. Cracking around your machine or boards is a clue that you've found an unstable slab. These slabs will be difficult to manage if they exceed 8" thick and where the slope angle is 35 degrees steep or more.  Choose windward terrain or a slope sheltered from the wind for a safer experience.

     

    (2/8/2023)

    Multiiple generations of wind-drifted snow on this cornice. 

    Photo: Cam

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–3

    Observers continue to find layers of buried weak snow that are reactive in snowpit tests. This layer is most prevalent on shady aspects 1-2 feet below the surface in the Flathead Range and southern Glacier Park and 2-4 feet below the surface in the Swan Range. This recent snowfall and wind may test buried facets and surface hoar.

    A surface slide has the potential to step down to this layer, triggering a much larger slide. Continue to choose lower-angle terrain free of overhead hazards and convexities. Slopes with clean runouts are an excellent way to build a margin of safety into your day.

Avalanche Discussion

Today is shaping up to be a great day in the mountains. That is if the sun doesn’t spoil the new snow. Only a week ago, we witnessed how quickly a little solar input can rework the snow surface. During last Thursday’s clear conditions, riders on sunny slopes lamented as blower power deteriorated into moist, gloppy snow, then rapidly refroze into a breakable crust. Moist snow produced roller balls and small loose wet slides on steep slopes. Where the snow remained dry, the sun initiated loose dry sluffs. 

It’s looking like we can expect some high clouds today, but it’s hard to say if it will help or hurt. Although cloud cover provides some shading from the sun’s short-wave radiation, thin and high clouds can also trap long-wave radiation. This effect, called “green housing,”  increases ambient temperatures and can cause more intense and widespread warming of the snow surface. Also, remember, with each day, the sun gets a little higher, and its effect on the snow surface becomes more pronounced.

If the sun does break out early today, anticipate a fate similar to last week. Rollerballs and pinwheels should usher you to shadier slopes. The riding will be more enjoyable there, anyway. Recall, however, that buried weak layers are lurking on aspects NW through NE to SE. Dig down to see if these layers are present, and if you find them choose a lesser slope angle and avoid traveling above terrain traps.

Come one, come all! Flathead Avalanche Friends Fest, a four-day festival from Feb 8-11, has a full schedule with mini-clinics, a keynote speaker, après, and a Big-A$$ Raffle. A little something for everyone. We hope you can join us to celebrate our community, learn about snow, and support our safety mission. Learn more here.

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