Lawinenlagebericht

Flathead Range & Glacier NP

Flathead Avalanche Center
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Lawinengefahr

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

Steep mid and upper elevation slopes harbor a serious avalanche hazard: slabs releasing on weak snow buried up to two feet deep. Audible collapses (whumpfs) and shooting cracks are unmistakable signs of this danger. Stick to low-angled slopes or terrain that's unlikely to harbor the hazard. 

Lawinenprobleme

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1.5–2.5

    Beware of steep, convex, shaded slopes around 6000 feet, where you can trigger dangerous avalanches that break 1 to 2 feet deep on weak facets above a hard, slick crust. While recent observations indicate that's the bulls-eye terrain for this structure, it likely lurks on similar terrain throughout the mid and upper elevation bands. The most straightforward way of reducing your risk of triggering a large, dangerous avalanche is to stick to slopes less than 30 degrees that aren't exposed to avalanches running from start zones above you. 

    (3/17/2022)

    The crown of a recent soft slab avalanche is partially filled in from new and blowing snow. This avalanche is on a southeast aspect above 6000 feet.

    Photo: Pete Costain

  • Loose Wet

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

    You can trigger sluffs of wet snow on very steep slopes where you find more than about 6 inches of wet, loose snow. These conditions are most likely on mid-elevation slopes late in the day or during periods of sustained sun. Snow falling from trees or cliffs can also trigger wet snow avalanches. Rollerballs and pinwheels of wet snow are clues that the snow is getting wet enough to release. You can avoid this hazard by timing your travel to move across or under steep, sunny slopes while they remain frozen. 

    (3/15/2022)

    Small wet loose avalanche. Similar slides are possible today.

    Photo: Blase Reardon

Avalanche Discussion

The primary avalanche concern today and through the weekend is triggering slabs 1 to 2 feet thick that break on a layer of weak facets that formed above a slick, hard crust left by the early March Atmospheric River event. Riders have reported natural avalanches, remotely-triggered avalanches, and audible collapses (whumpfs) failing on this structure in the past few days, as well as propagation in some snowpack tests. Several of these triggered slides would have had graves consequences for any rider caught in the debris. It's a serious hazard: a weak structure close enough to the surface for a rider to trigger and triggered slides large enough that injury or burial is likely. That combination has me avoiding the terrain that likely harbors it. 

While observations show the structure is relatively consistent throughout the forecast region, reports of avalanches and whumpfs have been concentrated on mid-elevation, north- through east-facing slopes between about 5800 and 6400 feet. That could be sampling bias - that's the most accessible terrain with reliably good riding. It also may be where the layer of weak facets is most developed. I could posit an explanation why; it'd involve a lot of terms like "exaggerated temperature gradient" and "latent heat release" and "melt-layer recrystallization." It's also nothing I'd bet my life on. I'm simply avoiding shaded slopes steeper than about 33 degrees in that bulls-eye elevation band. I'm unlikely to commit to similar terrain above that without a lot more information, even slopes higher than the rain/ snow line in early March. The potential consequences coupled with the uncertainty weigh in my descisions more than the likelihood of triggering.

We've dropped Wind Slabs from the problem list. Observers did not report any blowing snow on Thursday, and the snow surface on windward slopes up to about 7000 feet consists of melt-freeze grains that aren't easily transported. Above that elevation, I'd be alert for older wind slabs on leeward slopes. The BNSF forecast team in John F. Stevens reported two recent natural avalanches in this terrain the look to have involved slabs of drifted snow. 

Our final Community Chat is next Tuesday, March 22. In these events, Host Sara Boilen debriefs near-misses. This time she turns the attention to a close call she had earlier this season. Sign up for this virtual, free event here.

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