Lawinenlagebericht

Swan Range

Flathead Avalanche Center
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Lawinengefahr

Fri
Hochgebirge
3 · Erheblich
Waldgrenze
3 · Erheblich
Unterhalb
3 · Erheblich
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Kernpunkte

Dangerous avalanche conditions have developed due to abundant snowfall and blustery north and east winds, creating multiple avalanche problems. You could trigger a slab avalanche in both wind-affected and sheltered terrain. Surface slides may initiate an avalanche on a buried weak layer. Where the snow surface remains loose, you must manage your sluff on steep slopes. 

Lawinenprobleme

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Likely
    Größe
    1–2

    Wednesday's 1 to 2 feet of low-density snow has and will continue to be drifted into fresh slabs by north through east winds at all elevations. Expect to find slabs in unusual locations, including on or near the valley floor. These features may form on surfaces of low-density snow or facets atop hard slabs, resulting in avalanches that could break easily and wider than expected. 

    You've found a slab if the snow surface becomes stiffer than you've been traveling through. Cracks shooting out from your skis, board, or machine indicate instability. Look for ripples, dunes, and fresh cornices, signs the wind has been at work. 

    Natural or triggered avalanches at the surface have the potential to initiate the larger Persistent Slab avalanche described below. 

    Winds remained westerly for much of the snowfall event Wednesday. Wind slabs can be found on all aspects at upper elevations in this zone.

    (1/17/2024)

    A small drift at a ridgeline that  cracked and started to move when I stepped on it. It's a good example of wind-stiffened snow that's more cohesive - more of a slab - than the snow around it. This was one of the few places we found these conditions today.

    Photo: blase reardon

  • Storm Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Likely
    Größe
    1–2

    Copious snow fell at all elevations on Wednesday. In areas sheltered from the wind, the new snow may be cohesive enough to form a slab reactive to the weight of a rider. Where the surface snow remains loose, sluffs on steep slopes could be dangerous in areas where there are eight or more inches of new snow. 

    Slabs 1-2 feet thick are possible on open, unvegetated slopes at low elevations, especially on cutbanks and roadcuts.

    Choose your routes carefully. Stay well away from overhead hazards and evaluate your descent with caution. Cracking beneath you signifies you have found a slab and should retreat to lower-angle terrain. If you find yourself in loose snow, limit time spent above terrain traps and in long-running gullies. 

    Natural or triggered avalanches at the surface have the potential to initiate the larger Persistent Slab avalanche described below. 

     

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    2–3

    Trigging a 1-4+ foot hard slab avalanche on buried weak layers near crusts remains possible. Avalanches failing on these layers occurred last week; however, the distribution of this problem remains unclear. The only way to identify this problem is to dig into the snow.

    Natural and triggered slides in the new snow may step down to these layers and initiate a large avalanche. 

    You may or may not observe obvious warning signs like snowpack cracking or collapsing before triggering a slide. Avalanches could be triggered remotely, from gentler terrain below, above, or to the sides of steep slopes. Persistent Slab avalanches can break wider and higher on slopes than expected.

    (1/17/2024)

    One of many cracks we were able to initiate by riding along the base of low-elevation cut banks.  Failed on facets above the 12/22 crust. 

Avalanche Discussion

Cool temperatures and Pacific moisture combined to deposit bountiful low-density snow across the area Wednesday. Noisy Basin received 1.4 inches of SWE, approximately twice what most other locations picked up. Snow totals are notoriously difficult to determine with cold smoke-type snow, but most areas picked up a foot or more, with Tunnel Ridge recording 20+ inches. As the temperatures plummeted, the depth of new snow increased substantially. An example of this is Shed 11 in John F Stevens Canyon recording over 6" new snow in 2 hours at -2F. Snowfall generally ended well before midnight. 

Arctic air and north through east winds returned mid-day at most locations. Wind speeds increased overnight, with moderate to strong gusts recorded on the valley floor and at upper elevations. The cool air mass has dropped temperatures below zero this morning at all mid and upper-elevation stations. 

Due to the large amount of new snow and atypical winds, we have uncertainty with today's avalanche problems. Travel with a conservative mindset, staying well away from overhead hazards. Choosing to enjoy the deep powder on low-angle slopes is a sure-fire way to make the most of today's amazing snow.

Yesterday (Wednesday), Rob and Cam visited the Swan Range just south of Swan Lake. They found a foot-thick soft slab on top of the 12/22 crust, which cracked and collapsed on a layer of weak snow resting atop the crust at low elevations. Utilizing snowmachines, they produced shooting cracks 100 feet long on the same weak layer. Blase and Jeff visited the terrain north and west of WMR. They found the persistent slab structure on a southwest aspect at 6600 feet. It was not reactive in tests, though the weak layer was obvious. They triggered one small soft slab avalanche that involved just the new snow, though it seemed to propagate widely. BNSF snow safety noted severe wind loading with fresh wind slabs forming on the Canyon floor in John F Stevens Canyon

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