Bollettino valanghe

Swan Range

Flathead Avalanche Center
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Pericolo valanghe

Sat
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

Choose terrain where a slide won't send you through rocks and trees, over a cliff, or into a gully. Steer around dune shaped pillows of snow or move to an area that is sheltered from, or scoured by wind. Cracking around your feet or machine is a sign that wind-drifted snow can avalanche on steep slopes.

Problemi valanghivi

  • Wind Slabs

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

    Southwest winds redistributed mid-week modest snowfall into thin slabs. Winds have tapered but may rise to the speed capable of transporting snow later today.  You can find drifted snow near ridgelines or prominent terrain features where the wind decelerates and deposits snow.

    Slabs will be more stubborn to trigger than the past couple of days. You can identify these features by denser snow which keeps you or your machine from sinking more than a few inches. Dune-shaped features or a scalloped surface indicate that the wind has been at work. 

    Windward terrain or areas sheltered from recent wind offer slopes without a wind slab problem. If you plan on riding steep leeward terrain, choose runouts free of surface obstacles which can magnify the consequences of a slide or terrain traps where the snow can pile up deep.  

     

    (2/7/2023)

    We observed three crowns that were likey wind slabs. These failed on a north facing aspect. One of the crowns is obscured by cloud cover. 

    Photo: Cam

  • Persistent Slabs

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

    A small collapse felt by a skier near the continental divide is the most recent sign of instability we received from weak layers formed in mid-January. This persistent slab problem hasn't been particularly talkative of late. To get to know it better, we must be active listeners and ask the right questions. Digging a pit is the best way to "break the ice," so to speak. Observers continue to find weak structures throughout the advisory area. This appears to be most prevalent on shaded aspects at middle and upper elevations in the Swan Range, Flathead Range, and Southern Glacier Park.  

    In the Swan Range, the primary concern is a layer of surface hoar capped by a hard crust formed during the MLK weekend. This layer is buried 2-4 feet below the snow surface and has repeatedly produced propagating results in Extended Column Tests and Propagation Saw Tests. In some areas, more recently buried surface hoar layers may or may not be more reactive.

    In the Flathead Range and Southern Glacier Park, this interface is buried 1-2 feet below the snow surface and consists of well-developed facets above and below the crust. Propagating results have been less frequent in this zone, but the structure certainly isn't confidence-inspiring. In a recent trip to the Middle Fork corridor, Cam described these layers as "sitting ducks waiting for a slab."

    Continue to choose lower-angle terrain free of overhead hazards and convexities where you find these layers. Slopes with clean runouts are an excellent way to build a margin of safety into your day. Keep in mind that even a small slide has the potential to step down to buried weak layers and trigger a much larger slide.

    (2/8/2023)

    Poor upper snowpack at middle elevations in the Flathead Range. These weak layers are sitting ducks waiting for a slab.

    Photo: Cam

Avalanche Discussion

Yesterday (Thursday), an observer scoped a recent glide avalanche along the crest of the Northern Swan Range. This avalanche failed naturally and was located directly adjacent to a glide crack that opened earlier this season. Riders reported many "brown frowns" early this season, possibly due to above-average autumn temperatures warming the ground surface before the season's first snowfall. Honestly, glide avalanches are a bit of a mystery. Fortunately, the Northern Rockies Mountain Science Center has been working to crack the case here in Glacier National Park.  

Here are some things we do know about Glide Avalanches:

  • Glide is a natural (and typically slow) process by which the entire snowpack slides over the ground surface.
  • Free water can lubricate the base of the snowpack and enhance glide. This can be meltwater or rain.
  • Smooth grassy surfaces or rock slabs are ideal terrains for forming glide cracks. They tend to open up on specific slopes year after year. 
  • Glide cracks are an indication that a glide avalanche is possible.
  • The timing of a glide avalanche is complicated: sometimes, they may occur immediately after a glide crack opens, and sometimes the glide crack will hang tight for the entire season. 

This recent glide avalanche is a reminder to beware of the brown frown. The best approach for glide cracks and avalanches is not to approach glide cracks and avalanches. Let us know if you spot any more glide cracks on the landscape.

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.

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