Lawinenlagebericht

Whitefish Range

Flathead Avalanche Center
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Lawinengefahr

Fri
Hochgebirge
3 · Erheblich
Waldgrenze
2 · Mässig
Unterhalb
1 · Gering
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Veraltet — dieser Lagebericht ist 3 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

Dangerous avalanche conditions will develop at upper elevations if snow and wind arrive as forecast. Avoid steep slopes with slabs of new and drifted snow 8 inches or more thick. Triggered slides may entrain older wet snow, making them more dangerous. 

Lawinenprobleme

  • Storm Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Likely
    Größe
    1–2.5

    As new and drifted snow accumulate today, the likihood of triggering slabs that break at the new/ old snow interface will rise. So will the potential size of triggered slabs. These slabs can be dangerous where they're more than 8 inches thick. This hazard will be isolated at mid elevations, where the snow surface will be slower to freeze and snow accumulations will be lower. 

    Retreat to lower-angled slopes as snow accumulates during the day. Avoid slopes steeper than about 35 degrees if you encounter shooting cracks. You can test the sensitivity of slabs on small slopes with runouts free from terrain traps. 

    (12/23/2022)

    A slab of new snow that released on a short, inconsequential test slope.

    Photo: Mark Dundas

  • Persistent Slabs

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

    On isolated slopes at upper elevations, the hazard of triggering slabs that break below the new snow/ old snow interface lingers. The  likelihood will increase as new and drifted snow load a layer of surface hoar buried 12/1. You can trigger these slabs from low-angle terrain. Fractures can propagate long distances. This layer is roughly 8 inches below this morning's snow surface. 

    Whumpfing collapse and shooting cracks are clear signs of this hazard. Avoid steep start zones and the runouts below them. Dig into the snow and look for a grey strip of feathery crystals about 8 inches below today's new snow. Sudden collapses or propagation in Extended Column Tests are clear evidence that slabs can be reactive to your weight.

    (12/4/2023)

    This layer of surface hoar (buried 12/1) may become more sensitive to a rider's weight as it is loaded by new and drifted snow today.

    Photo: Cam Johnson

Avalanche Discussion

The big picture today is that the danger will rise as new and drifted snow accumulate, and that the layer of surface hoar buried 12/1 still poses a hazard. Beyond that, however, my confidence in today's forecast is low. Snowfall totals are uncertain and we only have two direct observations about the fate of the surface hoar after the rain and warm temperatures of the past few days. It's also not clear yet if today's snow will be falling on wet or refrozen snow surfaces.

Both of those observations are from the southern Whitefish Range, which received 1.3 inches of rain that soaked the snow above the surface hoar. The layer was present but degrading and did not propagate in Extended Column Tests. Rainfall was two to three times that further north in the Whitefish Range, and it's unlikely the surface hoar survived that deluge. The hazard of triggering a slab that breaks on the 12/1 persistent weak layer is isolated. A more widespread hazard will be triggering slabs of new and drifted snow. I expect these slabs to be more sensitive to a person's weight where they're building on refrozen snow surfaces. And I expect those above about 6500 feet in elevation. 

A similar Storm Slab hazard will be today's primary concern in the Swan and Flathead Ranges. Expect this hazard to be more widespread as the day progresses and as you climb higher in elevation. Complicating the picture there today is a lingering hazard of slides breaking deeper in the snowpack, below the old snow/ new snow interface. The  Swan and Flathead zones didn't get the same soaking as the Whitefish Range. So the 12/1 persistent weak layer is more likely to be preserved and remain sensitive to a rider's weight. Today's snowfall will add load to this layer. I'd be much more leery of steep start zones and the runouts below them in these ranges, and thus more conservative in my terrain choices.  

The above discussion is focused on terrain above about 6000 feet. Below that elevations, snow heights decrease rapidly. Avalanche hazards also diminish. Below about 5000 feet, snow cover is thin and patchy. 

Snowfall is forecast to continue tonight, though winds should diminish. If the snowfall arrives as forecast today and tonight, conditions tomorrow will remain dangerous. 

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