Bulletin d’avalanche

Swan Range

Flathead Avalanche Center
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Danger d’avalanche

Fri
Alpin
1 · Faible
Limite forestière
1 · Faible
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

Enjoy generally stable avalanche conditions. If your decisions take you to steep terrain, only expose one rider at a time. Further reduce your chances of triggering an avalanche by avoiding steep rollovers and rocky terrain features. Move to shadier aspects IF the sun begins to affect the snow surface.

The avalanche danger may rise to MODERATE in the afternoon if there are fewer clouds than forecasted. 

Problèmes avalancheux

  • Persistent Slabs

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

    Weak layers buried 1 to 4 feet deep continue to show signs they are unreactive. Since the new year, we have received no reports of instability on buried weak layers, and the last natural persistent slab avalanches likely failed on December 30th during the post-Christmas storm. Triggering a large avalanche on these layers is unlikely, not impossible. 

    Travel advice:

    • Only expose one rider at a time in steep or consequential terrain.
    • Avoid common trigger points (i.e. steep rollovers or thin and rocky terrain)
    • Choose slopes with long, clean runouts free of surface obstacles. 
    • If you want to avoid the problem altogether, choose lower-angled slopes.

    The weather forecast is calling for partly cloudy skies. If you experience sustained sunshine, move to shadier aspects once the sun affects the snow surface. There is uncertainty about how shallow buried weak layers on sunny aspects will handle sustained sunshine.

    (1/4/2025)

    A profile from the flank of a large slab avalanche that failed during the post-Christmas snow events. The 32-inch thick snow slab failed in facets resting on a 5cm thick and knife-hard crust formed around December 18. 

    Photo: Mark Dundas

Avalanche Discussion

I’ll start with a common saying: “Low danger does not mean no danger.” That does carry weight today. Some components of low danger that I think are worth noting are: small avalanches in isolated areas or extreme terrain and natural and human-triggered avalanches unlikely. How does that fit into today?

Slabs of wind-drifted snow may linger on exposed ridgelines in the alpine (extreme terrain). Look for subtle cues of wind-textured snow and dune-shaped drifts. These should be small and unlikely to trigger in most locations. The exception to this is in alpine terrain with bigger fetches and more exposure to south and westerly winds. 

Observations agree that it is unlikely to trigger a slab avalanche failing on weak layers buried deep in the snowpack. The last reported human-triggered persistent slab occurred on December 29th. Snowpit tests show stable results since the turn of the New Year. We have received no reports of instability (shooting cracks or whumpfs) since the New Year. All this generally points in a positive direction and supports the low likelihood of triggering.

The weather forecast today is calling for some sunshine. Move to shady aspects IF the sun stays out for a prolonged period of time. There is uncertainty about solar-induced triggering of persistent slabs. Specifically on the slopes getting hit by the sun that have weak layers buried less than 2 feet below the surface. The easy way to manage this is by moving to shady aspects once you feel the sun affect he snow surface. Most areas have a surface crust or only a few inches of snow on a crust on southerlies, so I expect any sluffing in the new snow to be minimal.

Régions couvertes