Bollettino valanghe

Whitefish Range

Flathead Avalanche Center
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Pericolo valanghe

Sat
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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Non aggiornato — questo bollettino è scaduto 6 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Dangerous avalanche conditions exists at middle and upper elevations. New and drifted snow is stressing persistent weak layers. You can trigger large and dangerous avalanches that break 3 feet deep and propagate wider than you would expect on slopes steeper than about 35 degrees. Stay out of trouble by choosing low-angled terrain that's sheltered from the wind and isn't exposed to steep slopes above.

Problemi valanghivi

  • Persistent Slabs

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

    Over the past week, riders have reported triggering avalanches on upper elevation slopes and on shaded, mid-elevation slopes.  As new and drifted snow have thickened slabs, the reported slides have grown wider, deeper, and more dangerous. The situation is even more serious now that new snow and wind have stiffened slabs and loaded layers of buried surface hoar and facets to their tipping points. Avalanches that fail in the storm snow are likely to step down and trigger these deeper layers.  Avalanches that start up high have a lot of snow they can entrain as they run down the slope.  The results can be deadly.

    (2/4/2021) Failed about a foot deep above the 1/13 crust

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    2

    Large drifts of new snow are developing below ridgelines and on the sidewalls of chutes and gullies. A triggered avalanche can step down to deeper weak layers, producing much more dangerous slide. Avoid steep slopes with textured, rounded drifts, particularly those that are too firm for your boards or machine to sink in. Take note of blowing snow and cornices. You'll find safer conditions on low-angled, sheltered terrain where the snow doesn't send cracks shooting out from your feet or machine. 

    (1/31/2021) Southern Whitefish Range

Avalanche Discussion

The forecast region has seen a spate of large and very large natural, human- and remotely triggered avalanches over the past week. People have reported near misses, and at least one person was caught, carried and slightly injured. These slides have occurred nearly every day, not just during storms. They have been breaking very wide and running fast and far down slope. It's clear that weak layers that formed in late January have been at their tipping point for days.

This hazard is growing even more serious as a potent storm lashes our area. As the day progressed on Thursday, natural, explosive triggered, and human-triggered avalanches grew larger and failed at the new/old interface, and on deeper buried weak layers. With more snow and winds today, the hazard will grow more widespread.

Since midday Thursday, the Noisy Basin SNOTEL has recorded 1.6" of new water. It may receive at least half again that much by midday Friday. The Flattop Mountain SNOTEL picked up 0.7" of new water and may get another 0.6" during the same time period. In the Whitefish Range, Stahl Peak reports 0.5" of snow water equivalent. These Snow Water Equivalents amount to 5-15 inches of new snow.  Westerly winds made slabs larger below leeward ridgelines.

Buried surface hoar and facets are widespread at middle and upper elevations across the region. They lie under soft slabs of recent and drifted snow atop the stout mid-January crust. Slabs are bigger, and thus, the stress on weak layers is greater, as you gain elevation into terrain where winds have drifted new and recent snow. That structure -  bed surface, sensitive weak layer, slab - is the recipe you learned about in any avalanche education you may have had. There is a lot of new loose snow at the surface, and avalanches will entrain large amounts of debris as they speed down the hill. The hazards are not limited to starting zones. Avalanches can run down to lower elevations.

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