Bollettino valanghe

Swan Range

Flathead Avalanche Center
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Pericolo valanghe

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

In sintesi

The snowfall has stopped, but the hazard of triggering an avalanche large enough to bury you remains. Slides may break under 1-2 feet of recent snow or deeper on buried weak layers. These hazards are more widespread and more sensitive at upper elevations. Shooting cracks and whumpfing collapses are your sign to move away from steep slopes. 

Problemi valanghivi

  • Storm Slabs

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

    Snowfall over the past two days improved the riding but increased the chance of avalanches in the new and wind-blown snow. You can trigger avalanches large enough to bury or injure a person, especially on slopes with 8 inches or more of fresh or drifted snow. These slopes will be more widespread at upper elevations, with the largest and thickest slabs found on the leeward sides of ridgelines.

    Cracks shooting from your sled or board are clear signs these freshly-formed slabs are unstable. Utilize hand pits to see how the new snow is bonding to the underlying crust, and take advantage of small, safe terrain features to test this bonding with your skis or sled. 

    (3/6/2022)

    An example of a natural soft slab that on a steep slope near a ridge. Expect similar slabs today. 

    Photo: Mark Dundas

  • Persistent Slabs

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

    At upper elevations, you can still trigger an avalanche 2-3 feet deep on surface hoar buried 12/1. Or near the base of the snowpack, on facets above a hard crust. You're more likely to trigger these slabs where they've been recently loaded by new and drifted snow. 

    Dig into the snow and look for a grey strip of feathery crystals 2-3 feet below the snow surface, or a layer of soft, loose grains above a hard crust near the ground. Sudden collapses or propagation in Extended Column Tests are clear evidence that slabs can be reactive to your weight. Avoid steep start zones and the runouts below them.

    (12/4/2023) Northern Swan, 6300', N aspect Surface Hoar standing tall below 20cm of new snow

Avalanche Discussion

Yesterday’s snowfall favored the Northern Whitefish and Swan ranges. Stahl Peak in the Whitefish picked up 1.0” of SWE and 11” of snow.  Noisy Basin in the Swan is reporting 1.4” SWE, and I’d bet my $10 that there’s more than 12” of new snow there (the SNOTEL still isn’t reporting heights). Storm totals for the last 48 hours are at 2” SWE for these favored locations. Winds have been light to moderate, so anticipate thicker and more sensitive slabs downwind from saddles, ridges, and other prominent terrain features.

Weather station data and a handful of recent observations from the Whitefish Range, Flathead Range, and Glacier National Park suggest that the 12/1 surface hoar and deeper weak layers were potentially wiped out by the rain event earlier this week. However, with this small sample size, I’m not about to clear the persistent slab problem off the worksheet. If there are any holdouts, they’ll be hiding wherever the rain was least impactful, probably somewhere above 7500’. The Swan Range might be a different story. Cam and I found some intact buried surface hoar and facets that produced mixed results in our test pit.

Regardless of what zone you’re traveling in, consider doing some due diligence as you cross into upper elevations. Take a look under the snow surface and see if you can find the 12/7 crust. While you’re at it, feel free to dig a little deeper to see if there is any surface hoar or facet layers below or just a mass of refreezing wet snow. If you find no crust and/or a buried weak layer, fall back to simpler terrain.

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