Bollettino valanghe

Swan Range

Flathead Avalanche Center
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Pericolo valanghe

Wed
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2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

Varying wind directions in the last week have left slabs of drifted snow on many aspects. The most recent is a thin and reactive slab on north through east-facing slopes. Identify slabs of drifted snow by looking for dune shape drifts, or by feeling for hard over soft snow. Stick to wind-sheltered slopes for safer riding. Keep slope angles low on north through east-facing slopes to avoid getting caught in a 2 to 3-foot slab failing on a persistent weak layer.

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–3

    A weak layer of buried surface hoar rests 2 to 3 feet below the surface in wind-protected terrain. We received reports of human-triggered avalanches last Wednesday in the northern Swan Range. Although this layer is beginning to show signs of strengthening, continue to keep slope angles low on north through east-facing aspects. A triggered avalanche 2 to 3 feet thick can easily bury and injure a human. 

    (1/29/2023)

    Surface hoar is now buried 2 to 3 feet (70 to 90 cm) below the surface in terrain sheltered from the wind. This layer is highlighted by the black card in the snow. 

    Photo: Cam

  • Wind Slabs

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

    Observers on Sunday triggered small slabs of wind-drifted snow on various aspects above 5000 feet. Sarah and Guy observed isolated cracking in slabs of drifted snow that were 1 to 2 feet thick in the Swan Range yesterday (Monday). Weather stations in all three mountain ranges are turning speeds capable of transporting snow out of the south and southwest. Shooting cracks around your board or machine, pillowy-looking drifts, and hollow-feeling slabs are all signs you’ve found a wind slab. Terrain that is sheltered from the wind is your safest option for managing this problem.

    (1/29/2023)

    Triggered shooting cracks in wind-drifted snow. 

    Photo: Cam

Avalanche Discussion

Since wind slab is the primary problem to manage throughout the forecast area, here is a snapshot of recent snow totals and winds starting with January 26 and going to today.

  • Whitefish Range: 4 to 6 inches of recent snow.
    • Winds: SW averaging 11 to 17 mph -> N and NE averaging 12-18 mph -> SW averaging 9 to 12 mph
  • Swan Range: 10 to 16 inches of recent snow.
    • Winds: SW averaging 22 mph -> E averaging 35 mph -> SW averaging 25 mph
  • Flathead Range: 6 to 10 inches of recent snow
    • Winds: SW averaging 6 to 11 mph -> NE averaging 6 to 10 mph -> SW averaging 8 to 17 mph

How can we utilize this information today? Expect to find older, stubborn slabs of drifted snow on south and west-facing aspects. These formed from recent north and east winds. Yesterday, winds changed direction back to a westerly flow, forming a more sensitive slab on north-to-east facing aspects. New slabs of drifted snow will likely be thin and easy to manage. They should be easy to manage by identifying and riding around areas of active wind transport and pillowy-looking surfaces. I expect them to be relatively thin because winds stiffened the surface on most aspects, reducing the amount of snow available to transport.

A weak layer of surface hoar buried in the Swan Range is beginning to show signs of strengthening. Extended column tests since January 26th are showing propagation with hard force, or no results.  The last reported avalanche likely failing on this weak layer was 6 days ago (Wednesday). Although there are signs of improvement, we are still wary of slopes that face east and north. A hard slab 2 to 4 feet thick will be difficult to escape.

Buried surface hoar was found in the northern Whitefish Range and in parts of Flathead/Glacier National Park. People have felt collapses along ridgelines and on valley floors since January 28th in the Flathead Range. Although the structure is found, its distribution appears "spottier" and harder to find in those ranges. Although outside of our forecast area, avalanche professionals at Teton Pass triggered a slide on buried surface hoar above a crust on easterly aspects (observation). These occurred on January 21st. 

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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