Bulletin d’avalanche

Whitefish Range

Flathead Avalanche Center
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Danger d’avalanche

Wed
Alpin
2 · Modéré
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

Triggering a slab of recently wind-drifted snow remains possible near and below ridgelines, on all aspects at upper elevations. This hazard is isolated at mid-elevations. Larger slabs could also fail on older weak layers buried 1 to 2 feet deep. Lower your exposure today by riding in sheltered terrain, where the snow is least affected by the wind. Let cracking in the surface snow direct you to lower-angle slopes.

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2.5

    The primary avalanche hazard remains the possibility of triggering deposits of wind-drifted snow 1 to 2 feet thick at upper elevations. These may be found on all aspects, thanks to the variable wind directions the past three days. The most recent generation will be on westerly aspects; these will be the most sensitive to a rider. Isolated slabs may remain sensitive to a rider at mid and low elevations, thanks to easterly winds accompanying the Arctic air as it flowed east from the Divide. 

    Look for fresh deposits of windblown snow near ridgelines and summits, and downwind of saddles and other terrain that funnels westerly winds. These will have a different texture than snow on undisturbed slopes - more like cake flour - and you are likely to see cracks or chunks break off if you undercut a sled or ski track. Linear cracks shooting away from you are a clear sign of an unstable slab. Older slabs may sound or feel hollow and have a very hard, polished surface. Triggered wind slabs could step down to older, buried weak layers, resulting in larger, more dangerous slides.

    (2/8/2025)

    A natural cornice fall triggered this small wind slab with potentially nasty consequences on a north slope at 5500 feet. Both the cornice and slab developed during easterly winds.

    Photo: Micheal Reavis

  • Persistent Slabs

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

    Very weak snow is buried 18 to 24 inches deep - close enough to the surface for a rider to trigger a large slab on isolated slopes. Stay alert for signs that the slabs over these weak layers are sensitive to your weight or the weight of a snowmachine - shooting cracks, whumpfing collapses, and triggered slides on inconsequential test slopes. 

    Recent signs of instability have mostly been reported from southerly slopes. These include large column tests propagating on a layer of facets above a hard crust (Red Meadow, WMR Sidecountry) and surprising collapses. On shaded slopes, a layer of weak facets and buried surface hoar appears to be widespread but has not yet produced many reports of instability.

    Terrain choices can limit your chances of a nasty surprise. Keep a close eye on slope angles and terrain configuration. Planar slopes with clean runouts that are closer to 30 degrees offer the lowest exposure. Be wary of convex slopes that have variable snow depths due to wind scouring and drifting. 

    (2/9/2025)

    The profile of these two slopes differs slightly - the left is more planar; the right is convex. Both are above trees, though the left is further above the trees, with a lower-angled slope.  

    Photo: Flathead Avalanche Center

Avalanche Discussion

Brrrr! This morning's temperatures are among the lowest of the winter. While the cold doesn't immediately affect the snowpack and avalanche danger, it can magnify the consequences of a mishap or accident of any kind in the mountains today. Consider carrying extra gear and supplies if you're out, in case someone in your party is immobilized.

Groundhog Day was a week ago, but like weatherman Phil Connors, we seem stuck in a repeating loop. Ours includes Arctic air, gusty winds from atypical directions, and slabs of wind-drifted snow. The most recent iteration of this loop involved 6 to 12 or so hours of moderate, easterly winds Monday. With plenty of low-density snow available for transport, I expect some soft slabs of drifted snow to exist on westerly aspects at upper elevations this morning. These may be sensitive to the weight of a snowmachine or rider on steep slopes. They can break 1 to 2 feet deep and involve enough debris to bury or injure a rider, particularly on slopes above trees, gullies, and rocks. 

Today's winds look to be light and northerly before veering east and southeast. They're unlikely to be strong enough to drift significant amounts of snow in most locations. However, it doesn't take much to form reactive slabs, as a near-miss in southern Glacier National Park on Saturday demonstrates. 

Keep in mind that Dry January left the region with some remarkably weak snow surfaces. The snow that's accumulated since 1/31 has buried these with 1 to 2 feet of soft snow. So far, that snow appears to be too cold and unconsolidated to be a slab, though the weekend's storm snow has settled notably the past few days.  As people get out, however, evidence is accumulating that slabs are sensitive to a person's weight on isolated slopes or the load is approaching a tipping point. Factor in that uncertainty as you enjoy the great riding, particularly on southerly slopes in the Whitefish Range. 

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