Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

Thu
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto last year. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Natural and triggered wet snow avalanches will be possible on steep slopes receiving sustained sunlight. Some recently-formed slabs of drifted snow may remain sensitive on upper-elevation leeward and cross-loaded terrain. On isolated slopes, a person may still be able to trigger hard slabs that break several feet deep.

Problemi valanghivi

  • Loose Wet

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

    As warm, spring sunshine weakens the snow surface, the wet snow avalanche hazard rises. Areas that did not develop a thin crust the past two days will be quickest to start shedding. 

    Solar aspects will be most susceptible to this problem today with pinwheels and rollerballs being the precursor. Move to colder, shadier aspects or less steep terrain before the snow surface turns slushy. 

    Avoid crossing on or under very steep slopes in the heat of the day and plan your exit accordingly. Small slides can have high consequences above terrain traps and they may gouge deeper into the snowpack.

    (3/7/2025)

    Wet loose snow sluffs like these are possible today, particularly on slopes where Tuesday's sun didn't leave behind a thin melt-freeze crusts. 

    Photo: Rob Millspaugh

  • Wind Slabs

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

    Wind slabs may still be sensitive at upper elevations on slopes downwind of ridgelines, below cornices, and on the lee sides of cross-loaded slopes. You have found a concerning slab if the slab is greater than 8 inches thick or above a terrain trap. Signs of wind-affected snow should be obvious today - dunes, drifts, etched snow surfaces, and large cornices. 

    While Tuesday's winds were moderate with strong gusts, there was limited snow still available for transport. The slabs that formed this weekend likely didn't grow. That's giving them time to stabilize. That's a slow process in cold temperatures. If you choose to ride terrain where these slabs are likely, climb or descend lines that aren't above terrain traps.

    (3/18/2025) Northern Swan, 6700' SW Wind-sculpted drifts - clear sign that the leeward (looker's left in this image) slopes below have slabs of wind-deposited snow. Photo: Chris Gotschalk

  • Persistent Slabs

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

    On isolated slopes, avalanches that break on deeply buried weak layers remain possible. Thoughtful terrain choices can minimize your exposure to this unlikely but serious hazard. So can adjusting to changing conditions; move out from under steep start zones with active wind loading or getting direct sun.

    This hazard is probably confined to slopes where wind-scouring keeps slabs thinner and the weak layers are closer to the surface and slower to sinter. And it probably takes larger triggers to collapse these layers - falling cornices or the debris from avalanches involving surface snow. Nonetheless, it's a hazard worth factoring into  your objectives and travel.

    Choose terrain that has planar or concave profiles, deep and consistent snow cover, and minimal overhead hazards. Lots of terrain meets these conditions, and lots of that terrain has great snow right now. If you do choose terrain where this hazard is more likely, dig to see if the structure is present; sudden collapses and propagation in tests indicate that structure indeed exists.

    (3/17/2025)

    The crowns and debris of large natural avalanches that look to have failed March 15 or 16. The flanks of the slide in the center are thick enough to suggest the slide released on weak layers buried deep in the snowpack.

Avalanche Discussion

Under today's partly to sunny skies, the hazard of wet snow avalanches will rise, though temperatures will struggle to reach freezing at upper elevations. As we near the Equinox (early tomorrow morning), shortwave radiation gets more intense, with longer periods of insolation. And shortwave radiation is the primary driver in the phase change to wet snow. There, got my geek out for the week.

In  everyday terms - you may be wearing plenty of layers, but fresh snow may still be dropping from trees and rocks, fanning out as rollerballs, and entraining wet surface snow into sluffs. Triggering wet snow avalanches is possible in these conditions. While most will be small, they can be dangerous above terrain traps. The snow on shaded slopes has offered great riding and is likely to stay dry today.

The wind slabs deposited this weekend are stabilizing. Some, however, remain sensitive to a person's weight on leeward upper elevation slopes. Likely trigger spots are on their thinner edges, or near rocks lining chutes. A natural avalanche cycle Sunday and Monday showed that at upper elevations, the slabs could be surprisingly large and dangerous. At mid elevations, the isolated slabs that remain sensitive are mostly small.

As I noted Tuesday, we don't have enough evidence to write off the potential for triggering large to very large hard slabs that break 2 to 4 feet deep. Although an isolated hazard, it's worth accounting for in your objectives and terrain management. Stumbling onto a trigger point would have serious consequences. 

Late Tuesday, we received out 501st observation for the year. That's a lot of observations! Each has helped the forecast team write more relevant and accurate products. Each has helped other riders in the Flathead's backcountry community make choices that kept them safe yet find good riding. Thank you to everyone who's submitted. At this pace, we'll likely tally more observations than any other winter. Please, keep them coming. If submitting an observation is intimidating, keep in mind that every observation is valuable, and the form on the Avy app has far fewer boxes than the one on the webpage. 

 Check out Telefestivus at Blacktail Ski Area on Saturday, March 22, and Sunday, March 23. In conjunction with the event, Tamarack Brewing will host a fundraiser with a raffle and film for the Friends of the Flathead Avalanche Center from 5 to 8 PM Saturday, March 23. 

Regioni coperte