Lawinenlagebericht

Flathead Range & Glacier NP

Flathead Avalanche Center
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Lawinengefahr

Thu
Hochgebirge
2 · Mässig
Waldgrenze
1 · Gering
Unterhalb
1 · Gering
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Kernpunkte

Remain leery of steep, upper-elevation slopes in the northerly/ shaded half of the compass with more than about 8 inches of new or drifted snow. In this terrain, you can trigger slabs large enough to bury or injure a person. Shooting cracks and whumpfing collapses are signs that you can trigger these slabs from lower angled slopes below or adjacent to steep slopes.

Lawinenprobleme

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1–2

     While the weekend's new and drifted snow is settling, it sits on a weak layer of surface hoar and facets on northwest through northeast to southeast aspects. This structure poses a lingering avalanche hazard on and below steep slopes in the shaded or northerly half of the compass. The most straightforward way of avoiding this hazard is to avoid this terrain.

    It's not a hazard on every steep slope, however. While it's not unreasonable to try picking out the ones without this hazard, you do up your risk. Slopes with less than about 8 inches of new snow are unlikely to produce a slab large enough to bury or injure you, unless you're above a terrain trap. Slopes in leeward terrain with dense or hard slabs may produce slabs that break above you. And of course, terrain where you experience shooting cracks or whumpfing collapses are clearly unstable. 

    These conditions are more widespread at upper elevations across the range, and at mid and upper elevations in the Lake McDonald area. 

    (12/10/2024) Note the shallow ski penetration. An example of a slope where the new snow is less than about 8 inches deep and not stiffened or drifted by the wind. 

Avalanche Discussion

Today's one of those days where the avalanche hazard doesn't fit so neatly into our problem classification scheme. There's new snow sitting on a weak layer of surface hoar/facets or melt-freeze crusts across the region. That's straightforward - a Persistent Slab avalanche problem on steep, shaded slopes in areas that the weekends storm favored with 8 inches or more of snow, like the northern half of the Whitefish Range, the Apgars, and the Lake McDonald area. 

And there's higher, wind-exposed terrain where winds drifted lesser amounts of new snow into dense slabs on steep, leeward slopes. That's Wind Slab avalanche problem that can be avoided by avoiding terrain like that in upper elevations in the Swan and Flathead Ranges. This far out from the storm, though, most of those slabs should have stabilized. 

It's the in-betweens that are problematic, for which I want to squeeze in a new category - "Persistent Wind Slabs." These would be slabs of drifted snow sitting on weak facets and thus, remain unstable. They linger at upper elevations in the usual haunts for wind slabs - the leeward sides of ridges, downwind of saddles, and crossloaded into gullies well below ridges. They're likely, though, to exhibit the same unpredictability of Persistent Slabs - releasing remotely and/ or breaking across terrain features in multiple start zones. 

And then there are slabs where the effects of the wind are more subtle. Maybe not enough drifting to form a dense slab that's clearly harder than the snow on nearby slopes. But enough to stiffen the recent snow and make it more sensitive to triggering by the weight of a person or machine. It's a subtle distinction with perhaps big consequences. 

You can avoid these taxanomic questions by avoiding steep slopes with more than about 8 inches of recent or difted snow that's sitting on a stripe of surface hoar and facets. If you don't, know that you're upping the uncertainty and risk. 

Did you hear about the app? 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 win debates about the avalanche problems right in the backcountry or submit an obs while driving home. 

Registration is open for this season's avalanche courses. Get some learning done!  There's an Avalanche Awareness class tonight, Wednesday, December 11th, at 6:00 pm at the Stonefly Lounge in Coram.

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