Bollettino valanghe

Whitefish Range

Flathead Avalanche Center
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Pericolo valanghe

Tue
Alta quota
2 · Moderato
Limite del bosco
1 · Debole
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 2 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

You can trigger slabs of wind-drifted snow near and below ridgelines at upper elevations. These are isolated at mid elevations, though they will be more widespread if winds are stronger than forecast. Slopes sheltered from the wind offer great riding with the least exposure to this hazard today. An isolated hazard of triggering a large slide that breaks up to 2 feet deep on old snow exists. 

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2.5

    It remains possible to trigger slabs of drifted snow 1 to 2 feet deep on steep, upper elevation slopes. Freshly formed slabs will be the most sensitive to a rider's weight, as shown by several rider-triggered slabs in the Swan Range Sunday and an unintentionally triggered slab in southern Glacier National Park on Saturday. Some older, harder slabs may remain unstable as well.

    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. Avoid traveling under steep start zones where you see blowing snow.

    For the most part, recently formed wind slabs will be found on typically leeward slopes. However, with winds shifting to the north and east overnight, expect to find a few on typically windward slopes as well. Triggered wind slabs could step down to older, buried weak layers, resulting in larger, more dangerous slides.

    If winds are stronger than forecast today, this hazard may become widespread at mid elevations, particularly downwind of terrain features that funnel northerly and easterly winds.

    (2/7/2025) Photo: Adam Clark

  • Persistent Slabs

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

    With great riding in low-density snow, it's easy to forget that very weak snow is buried 18 to 24 inches deep - close enough for a rider to trigger a large slab on some steep slopes. So far, evidence suggests this hazard is isolated. Stay alert for signs that 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) Fracture propagated along a layer of facets above a crust 55cm from the surface on a SE aspect. Photo: Erich Peitzsch

Avalanche Discussion

Despite the Moderate and Low danger I've listed for today, there's some uncertainty about conditions that's worth factoring into your decisions.

First, wind slabs at mid-elevations: based on recent wind speeds and the weekend's obs, these appear to be isolated. I wouldn't be surprised, however, if I was out today and found a slope well below ridgelines that did harbor a recently formed slab, thanks to the swirling gusts of the past few days. And there's the potential for these to develop on usually windward slopes today, if winds are stronger than forecast.

Second, Dry January* left the region with some remarkably weak snow surfaces. The snow since 1/31 has buried these with 1 to 2 feet of soft snow that so far appears to be too cold and unconsolidated to be a slab on shaded aspects, or doesn't contain enough water weight to break down crusts on solar aspects. 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 in as you enjoy the great riding, particularly on southerly slopes in the Whitefish Range. 

 

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