Lawinenlagebericht

Flathead Range & Glacier NP

Flathead Avalanche Center
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Lawinengefahr

Fri
Hochgebirge
1 · Gering
Waldgrenze
1 · Gering
Unterhalb
1 · Gering
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Kernpunkte

You're unlikely to trigger an avalanche on most slopes. Deeply-buried weak layers are strengthening. You are unlikely to collapse these layers and trigger a slide on slopes where you find supportable ice lenses or melt-freeze crusts 4-20 inches below the snow surface. On very steep, northerly slopes where the near-surface snow is cohesionless, you can trigger loose dry snow avalanches. These can be dangerous if they carry you into or over a terrain trap. 

Lawinenprobleme

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Unlikely
    Größe
    2–3

    On isolated upper-elevation slopes, the hazard of triggering large, hard slab avalanches lingers. These can break on weak layers of snow buried in late November and mid December. On most terrain below about 7400 feet, stout melt-freeze crusts or ice lenses near the snow surface prevent a rider's weight from collapsing these layers. The exceptions are slopes where the crusts or ice are thin and weak and where the snowpack is generally shallow. If your climb or descent has these characteristics, it'd be worthwhile to dig and test the snow structure. Ride one at a time so you can minimize the consequences of a surprise. 

    (1/4/2023)

    Rob investigating the crown of a very large natural avalanche that appears to have released on weak snow buried in mid December. Link Mountain.

  • Loose Dry

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1–1.5

     Northerly slopes steeper than about 40 degrees where the near-surface snow is cohesionless and not capped by recently-formed melt-freeze or rime crusts - that's the description of the terrain where you can find great riding and where you can trigger dry snow avalanches. On slopes with continuously steep pitches, these can entrain enough snow to carry a rider. They can be dangerous if they carry you over or into a terrain trap - cliffs, talus, trees, and gullies. Pick lines that aren't above features like these. Slope cuts can be an effective test for this hazard. They can also get you into trouble if you're too low, too slow, and have nowhere to go. 

    (1/4/2023)

    A skier-triggered avalanche involving cohesionless snow above Marion Lake in the Flathead Range. These will become more widespread as the near-surface snow facets. 

    Photo: Paul Fotter

Avalanche Discussion

"A crust, or not a crust, that is the question." Hamlet was a backcountry rider, and in our current conditions, that is how he'd start his soliloquy. 

Today, he'd be asking whether a hard, supportable crust exists 4 to 20 inches below the snow surface. And whether a breakable crust was present at the snow surface. In both cases, a "not a crust" answer would mean the potential for triggering avalanches that would or could be dangerous.

On most slopes below about 7400 feet, storms at Christmas deposited ice lenses or refrozen melt-freeze snow that are now covered by 4 to 20 inches of snow. These very hard layers drastically reduce the chances a person could collapse weak snow buried in mid December or late November. You can find those layers roughly mid-snowpack and near the ground, respectively.

The mid-December layer has shown up recently as a thin (1 cm) thick stripe of rounding facets and surface hoar that sometimes propagates with hard force in extended column tests (here and here). It was likely the failure plane for several very large avalanches that released last week, including one Rob and Guy checked out near Link Mountain in the Whitefish Range Wednesday. The late-November layers present as 4-finger hard, rounding facets 12-20 inches above the ground, or as rounding depth hoar above an ice lens at the same depth. I find the latter structure more concerning.

Many riders are reporting that last week's snow is faceting in this week's cold, dry conditions. Nights with few clouds - like Wednesday night - are accelerating this process. The near-surface snow is becoming cohesionless, so the potential for triggering sluffs of loose, dry snow is growing. A party reported unintentionally triggering a slide involving snow like this Wednesday in the Flathead Range.

Thus Hamlet's second "A crust, or not a crust" question. Triggering loose snow avalanches isn't a hazard on slopes where the snow surface is capped by recently-formed sun or rime crusts. It can be, however, on very steep (40 degrees or more) slopes without a crust at the surface. That's mostly terrain that faces northwest through north to northeast above about 6500 feet. That's also the kind of terrain where many people explore during Low-danger conditions like now. If that's where you're headed in the next few days, pick lines that minimize your chances of getting caught and carried in a sluff that could carry you somewhere unpleasant.

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