Bulletin d’avalanche

Flathead Range & Glacier NP

Flathead Avalanche Center
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Danger d’avalanche

Mon
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 4 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Several days of exceptionally windy weather have ended, but riders can still trigger slabs of wind-drifted snow. These features will be thicker and more dangerous as you gain elevation. Softer and safer snow is found in locations sheltered from this week's blustery conditions. 

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2

    Powerful southwest winds over the past several days resulted in a patchwork of wind-affected snow. Winds diminished overnight but not before leaving several generations of slabs in its wake. Expect to encounter hard and soft slabs on leeward slopes open to the wind.

    Yesterday's observations noted gusty winds, drifting snow, large swaths of wind-affected snow, fresh cornices that broke easily under a rider's weight, and stubborn and reactive slabs.

    Slabs will increase in thickness and become more dangerous as you gain elevation. These features can be found near and below ridgelines, on the sidewalls of cross-loaded gullies, or around prominent rock outcroppings. A scalloped surface or dune shape formation may identify these wind features. Older, harder slabs may feel and sound hollow like a drum. Slabs are generally denser than the surrounding snow and may keep you or your machine from sinking into the snow.

    Cracking beneath you is a clue that you have found a slab. Cracks shooting away from you indicate instability directing you to either a lower-angle slope or terrain sheltered from the wind.

     

    (2/4/2023)

    A shooting crack in wind-affected snow.

    Photo: Kevin Oberholser

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    1–2

    Observers continue to find buried facets and surface hoar in snowpits. These weak layers are gaining strength slowly but continue to be a concern. This layer is 1-2 feet below the surface in the Flathead Range and Southern Glacier Park and 1-3 feet below the surface in the Swan Range. This layer has been found in most aspects but is most developed on NW-NE-SE slopes.

    The only way to identify this hazard is to dig down and see if this weak layer is present. Where you find it, choose lower-angle terrain free of overhead hazards and convexities. Small avalanches have the potential to step down to buried weak layers.   Slopes with clean runouts are an excellent way to build a margin of safety into your day.

    (2/4/2023)

    The current Persistent Slab problem involves facets beneath a crust formed in mid-January.

    Photo: Dundas

Avalanche Discussion

Winds diminished overnight and generally remain below the threshold for drifting snow until a weak Pacific system enters the Flathead later today. Unfortunately, most of the moisture with this disturbance is, you guessed it, heading well south of our area. The Swan will again take home the lion's share with 0.75 inches of SWE forecast for Noisy Basin by tomorrow morning. If the storm materializes, the danger may rise to Level 3 or CONSIDERABLE in the Swan. The Flathead and Whitefish Range should pick up several inches of fresh snow by Monday morning, but not enough to change the avalanche danger rating. 

The past few days have been unusually windy. South and southwest winds varied between moderate to extreme and wreaked havoc on slopes open to the wind. I went searching for wind-affected terrain Saturday and Sunday and found it. Areas open to the wind, both windward and leeward, were "messed up." The surface was scoured down to a hard layer in locations taking the brunt of the wind head-on. Snowfields, where the wind was moving across or up a slope, had a dense surface that kept my skis from sinking more than a few inches. On Sunday, leeward aspects held freshly formed slabs of graupel that cracked but did not propagate. Fresh cornices were easy to break off with the light pressure of my skis.

The Persistent Slab problem remains in the Flathead Range, southern Glacier Park, and the Swan Range. We were ready to remove this problem from the Flathead/GNP zone, but Rob and the BNSF snow safety team had propagating results in their snowpit tests Thursday. This was concerning enough for them to alter their descent plan. We want to downgrade this problem in the Swan from Possible to Unlikely because the weak layer is strengthening and is now buried to the extent that it is becoming difficult to human-trigger. We have decided to allow the dust to settle from our recent wind storm and reevaluate. The slopes that experienced wind-loading over the past few days are the same aspects harboring the most well-developed weak layer.

Weather trivia: On Friday, a new record for the coldest wind chill ever recorded in the U.S., minus 108 degrees Fahrenheit, was set at the summit of Mount Washington (6288') in New Hampshire. Ambient air temperatures dipped to minus 46 F with sustained winds of 96 mph and gusts to 108 mph.

Kalispell has recorded 77 consecutive days with snow on the ground, nearing the record of 87 days set in 1960.

Come one, come all! Flathead Avalanche Friends Fest, a four-day festival from Feb 8-11, has a full schedule with mini-clinics, a keynote speaker, après, and a Big-A$$ Raffle. A little something for everyone. We hope you can join us to celebrate our community, learn about snow, and support our safety mission. Learn more here.

Motorized Level 1 - Instructed by Eric Knoff and Six Points Avalanche Education from Tuesday, February 7, 2023, to Thursday, February 9, 2023. More details are here

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