Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

Sun
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2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 5 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Adjust your tactics today as you change elevation and aspect. At mid-elevations, wet snow avalanches remain a hazard, despite a few inches of new snow. At upper elevations, keep the refrain from the past seven weeks in mind: thick, dangerous hard slabs linger on convex slopes with variable snow cover. Use your spidey-senses to alert you to easterly slopes with glide cracks and deformed snow cover. An investigative mindset might be a useful approach today.

Problemi valanghivi

  • Loose Wet

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

    Air temperatures at mid elevations have only dipped below freezing for a few hours in the past few days. That's left a thick layer of wet, cohesionless snow near the snow surface. You may find few inches of sticky new snow is now insulating the mush. Beware of very steep slopes - around 38 degrees - where the snow is not supportable. Triggered sluffs of sticky snow can gouge deep into the snowpack, entraining dangerous amounts of dense debris that's hard to escape. If you find difficult riding where you're falling into mushy wet snow, stick to low-angle slopes or move on to terrain where the snow supports your weight or the weight of your snowmachine. 

    (3/7/2021) Natural wet loose avalanche crossing the Marion Lake Trail.

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    2–3

    Choose terrain that minimizes your chances of triggering a hard slab that breaks on weak facets or surface hoar buried in late January. This hazard is isolated to slopes steeper than 35 degrees where the slabs have thin spots that allow a person's weight to collapse these persistent weak layers. Slopes likely to have these conditions are often scoured by crosswinds and have a convex shape. Snowpack tests can't reliably determine stability, so it's best to simply avoid slopes likely to harbor shallow trigger points. 

    (3/17/2021) The hard slab avalanche here shows the kind of terrain to avoid: a very steep, unsupported slope with numerous potential trigger points.

  • Glide Cracks

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

    Those brown frowns and jagged cracks where the ground is visible are glide cracks. The entire snowpack is slumping and slowly sliding down the hill on smooth ground underneath - like bear grass or bedrock. They give a rumpled look to the snow surface down slope. They can fail all at once resulting in very large and dangerous avalanches. Cracks are scattered on sunny aspects throughout the region, though outliers exist. If they're above you, don't dawdle and gawk. If you ride down towards a bumpy, rumpled surface, steer away; a  freestyle, mogul bashing run may go awry. 

    (3/19/2021) Glide crack on a NE aspect at 6400 feet.

Avalanche Discussion

Bush, mush, and whoosh. Clancy's darkly humorous description of the three elevation bands describes today's conditions in the Swan Range well. In the Whitefish and Flathead Range, we might modify it to bush, mush, and crush, because of the lingering potential for triggering unsurvivable hard slabs at upper elevations. Although this potential exists in the Swan Range as well, it's secondary to freshly-formed slabs of wind-blown snow. 

It can be a fool's errand to guess how deeply the snow surface has refrozen from the safety of your home office. But here we go: air temperatures dropped below freezing early Friday morning at upper elevations, and finally did the same at all mid elevation stations early this morning. Skies cleared across the region overnight, though some clouds lingered in the Ten Lakes and Essex areas. I expect the snow surface has a supportable crust above about 5500 feet, and a sometimes-supportable crust between about 5500 and 4000 feet. In the Swan Range, a few inches of new snow has complicated matters. It's insulating the mush at mid elevations, so the hazard of Loose Wet avalanches should be more acute there on very steep slopes. 

We've had two reports of glide cracks on northeast-facing slopes in the past few days, so I've expanded the rose for that hazard to include all easterly slopes. If you're traveling below steep slopes with an easterly aspect, look overhead for cracks - the brown frowns - and slopes where the snow surface is rumpled and bumpy, like a mogul field. Don't dawdle under these and tempt fate. The hazard exists in all zones, though it's been bumped from the Swan by the newly developed Wind Slab avalanche hazard.

We just don't have enough evidence to write off the hazard of triggering hard slabs on deeply buried weak layers. Triggering these slabs has grown harder as the slabs have grown thicker. And the weak layers themselves have grown less uniform as they're compressed by consolidating snow above them. However, it'd be awful to ride across a patch where the slab is thin and the layers remain intact; these trigger points could start the crack-propagation-release chain that results in large, unsurvivable avalanches. Best to avoid terrain likely to harbor these bomb triggers.

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