Bulletin d’avalanche

Flathead Range & Glacier NP

Flathead Avalanche Center
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Danger d’avalanche

Thu
Alpin
3 · Marqué
Limite forestière
2 · Modéré
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

Triggering slabs that involve recent snow remains a danger on steep slopes. That danger will increase rapidly IF winds start blowing snow and creating reactive slabs. Be alert for signs that the recent snow is consolidating the storm snow into slabs, like cracking or recent slides. Any triggered or natural avalanches will entrain significant amounts of debris and run long distances.

Problèmes avalancheux

  • Storm Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1.5–3

    Avalanches releasing in recent or freshly deposited snow remain a hazard around the compass. The particular flavor of hazard, however, may be terrain-specific. At upper elevations, the hazard involves dense slabs of wind-stiffened or drifted snow. These will be confined to the terrain most exposed to winds or that tends to channel winds and increase wind speeds. If wind speeds increase, they'll become more widespread.

    On steep slopes where the recent snow has settled into a cohesive slab, it's possible to trigger slides that break 18 to 36 inches deep. These can entrain substantial amounts of snow as they move downslope. These can occur where the snow seems too soft to slide, so be alert for subtle signs that the low-density snow is becoming more cohesive. These include short cracks near your boards or machine. You can test for this while you move, by doing slope cuts on short, inconsequential slopes and roadcuts, and by stomping on switchback corners and the uphill sides of switchbacks. 

    Lastly, on very steep slopes where the new snow remains loose and cohesionless, you can trigger long-running sluffs. These can be dangerous in confined terrain like chutes and couloirs, where it's difficult to escape the debris. And above terrain traps, where even small sluffs can lead to trauma or deep burials. 

    (12/29/2024)

    Looking up the debris field after an accident in which a skier was caught, carried, and partially buried. The soft slab avalanche involved new snow. 

  • Persistent Slabs

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

    The hazard of triggering slabs that release on old, weak snow lingers. On sunny slopes, the primary concern is facets above a melt-freeze crust. On shaded slopes, it's surface hoar buried in early December, or facets buried around Dec. 18th. With recent snowfall, these layers are now buried deeply enough that they're less likely to be collapsed by the weight of a rider or snowmachine on many slopes. Warm temperatures prior to the storms have also strengthened these layers.

    They remain a hazard in terrain where the snow depth is variable, such as convex slopes and slopes scoured and drifted by winds near ridgelines and summits. Weak layers can take longer to strengthen in this terrain, and spots of thin snow cover that can be trigger points. This includes steep, Debris from avalanches involving new snow can trigger these layers as it runs downslope. Avoid terrain with these characteristics and the runouts of steep start zones if you see blowing snow. 

    (12/30/2024)

    A rider triggered avalanche that failed on facets buried in mid-December. 

    Photo: Cam Johnson

Avalanche Discussion

Rare conditions. But before any gushing about the riding, it's critical to note that you can still trigger dangerous avalanches. The storm snow may be more sensitive to the weight of a snowmachine or rider today, as it consolidates into soft slabs. Or gets stiffened by winds and drifted into slabs. Slope-specific evaluation of conditions can help you enjoy the new snow and the rest of the new year.

Most of Tuesday's observations highlighted deep, cohesionless powder. A few also noted that the snow seemed to be settling into slabs as the day progressed, and you can see that at weather stations, which recorded 2 to 5 inch drops in snow height. Slabs involving new snow can be surprisingly soft, so look for the subtle signs that indicate the new snow is consolidating and can act like a slab - cracking and test slopes or hand pits that release in chunks rather than loose powder. While loose dry avalanches become less likely as the snow settles, they remain a hazard in very steep, confined terrain.

Lastly, we've only had one recent report of a triggered slide that broke on old snow. This involved facets over a melt-freeze crust on a southerly aspect in the southern Whitefish Range. A similar sturcture has been noted in John F. Stevens Canyon. The surface hoar buried on Dec. 7th remains a concern as well. I'd be leery of any steep terrain where digging revealed a distinct grey stripe beneath the recent snow.

Ok, the gushing. Remarkable coverage for this time of year. The November AR and recent storms have laid down even some old-growth alder. And the snowpack is generally right-side up, with few reactive weak layers. And the riding... bottomless, low-density snow that's unaffected by wind or sun. Happy New Year, indeed. 

Downloaded the app yet? 

You can now access our region's forecasts, observations, and weather data directly from the Avy App (IOS; Android). Many features are accessible offline, so you can doublecheck danger ratings and travel advice on the drive to the trailhead or in the backcountry. 

Events and Education:

Want to start your New Year on the right foot? Sign up for a Partner Rescue Clinic for motorized Riders on January 4. Check out our calendar for other upcoming events. 

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