Boletín de aludes

Swan Range

Flathead Avalanche Center
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Peligro de aludes

Thu
Alta montaña
2 · Moderado
Límite del bosque
2 · Moderado
Bajo el límite
1 · Débil
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Desactualizado: este boletín caducó 4 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

Weak layers buried roughly a foot below the surface remain untrustworthy. A remotely triggered slide occurred within the last week. Triggering an avalanche is possible on slopes where winds drifted snow into thicker, cohesive slabs. Steer towards south and west aspects where you are less likely to trigger an avalanche failing a foot thick.

Problemas de aludes

  • Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1–2

    We’ve transitioned to a Persistent Slab problem in the Swan Range. Persistent weak layers of buried surface hoar continue to show unstable signs in snowpit tests, and remotely triggered avalanches occurred six days after this layer was buried. Triggering an avalanche is proving to be more of a concern in areas where winds drifted snow into thicker slabs. You can significantly reduce the chances of triggering an avalanche by riding on solar aspects where the sun destroyed this fragile weak layer before it got buried. 

    (1/17/2023)

    Snowpit tests continue to untrustworthy signs. This weak layer of surface hoar propagated across the entire column with moderate force. 

  • Wind Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1–1.5

    Wind speeds cranked up yesterday (Tuesday) and blew what little snow there was to transport into thin slabs. Mt. Aeneas averaged 18 mph with gusts up to 32 mph out of the southwest. Observers noted drifting above 6000 feet in the Flathead Range, with no signs of instability. An observation from the northern Whitefish Range showed reactive slabs in wind-drifted snow. Thin new slabs of snow will continue to thicken a more dangerous slab above persistent weak layers. This is a sign to hedge your bets on slopes sheltered from the wind. You can also choose windward aspects for a safer option.

Avalanche Discussion

Dribs and drabs of snow are piling up above 6000 feet. Since January 9th, Noisy Basin in the Swan Range has picked up 3.1 inches of snow water equivalent (SWE). Stahl in the northern Whitefish Range was not reporting for a couple of days, but is still showing 1 inch of SWE. Flattop in Glacier National Park is showing 1.5 inches of SWE. This snow came after an extended dry spell at the beginning of January. Remember that distant memory of low danger and sunshine with 8 to 20 inches of powdery snow?

This brief overview of the last two weeks paints a picture of our upper snowpack today. The 4 to 20 inches of snow faceted on many shaded aspects at the beginning of January. During that time, photos of feathery surface hoar trickled in. The sun shined and warmed solar aspects, cooking the snow surface and forming a thin sun crust. This is what all the recent snow is resting on.

Observations from all three regions show a developing Persistent Slab avalanche problem. We made the transition to calling it that in the Swan Range, where enough snow piled up above these weak layers for it to be problematic. The BNSF Avalanche Safety Program found propagating pit results on faceted snow above the Christmas Crust, which is now buried roughly 22 inches below the surface. I observed a crown near Essex that I suspect failed on this layer during the recent rain event. A rider in the northern Whitefish Range triggered three small slabs in wind-drifted snow. These all failed on layers of faceted snow 5 to 10 inches below the surface.

Our primary concern right now for Persistent Slabs is in the Swan Range. With that said, persistent weak layers buried below the surface do exist in the Whitefish Range, Flathead Range, and Glacier National Park. If more snow piles up, or we continue to see signs of instability, this may become a problem in those ranges shortly.

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