Lawinenlagebericht

Swan Range

Flathead Avalanche Center
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Lawinengefahr

Sun
Hochgebirge
3 · Erheblich
Waldgrenze
2 · Mässig
Unterhalb
1 · Gering
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Kernpunkte

Dangerous avalanche conditions developed overnight due to fresh snow and elevated wind speeds. Expect to find slabs 1-2 feet thick that are reactive to the weight of a rider or snowmachine. You can avoid this stiff, dense snow by riding in wind-sheltered locations. An accidentally triggered slide Friday is proof that buried weak layers have the potential to ruin your day. Choose terrain free of convexities and less than 35 degrees to minimize your chance of being surprised.  

Lawinenprobleme

  • Wind Slabs

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

    Southwest winds increased Friday and transported surface snow into fresh slabs. Avalanche professionals noted active wind loading and slab development in the Swan Range and John F. Stevens Canyon. Up to 8 inches of new snow overnight adds fuel to the wind slab machine. Today, expect to find slabs 1-2 feet thick reactive to a rider's weight or machine below ridgelines or saddles and in cross-loaded gullies. Look for signs of blowing snow as you gain elevation with stiff, dense snow, a clue that you have found a slab. Cracking is a sign of instability directing you to windward or more sheltered terrain.

    (2/2/2022)

    Where the snow had been wind-stiffened on the east side of the Swan, riders could easily trigger shallow wind slabs

    Photo: Guy Zoellner

  • Persistent Slabs

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

    The third rider on a slope accidentally triggered a slide on a layer of buried surface hoar Friday in the southern Whitefish Range. They had ridden similar slopes with no signs of instability earlier in the day and week. The distribution of surface hoar is spotty and appears to be confined to sheltered slopes above roughly 6000 feet and found 1-2 feet below the surface. Increasing temperatures and wind is beginning to transform Monday's low-density snow into a soft slab in isolated locations. This problem often is not giving feedback, such as cracking or collapsing (whumphing). The only way to determine if the slope you are on harbors this dangerous layer is to dig down a couple of feet and look for an apparent gray stripe. We recommend using terrain to manage this problem by riding slopes less than 35 degrees and free of convexities. 

    (2/4/2022)

    Crown of a skier triggered avalanche. The rider was the third person on the slope. They were caught, carried, and partially buried against a tree. The rider was uninjured. 

    Photo: anonymous

Avalanche Discussion

We increased the avalanche danger on leeward aspects due to blustery southwest winds and fresh snow overnight. The Middle Fork corridor, John F. Stevens Canyon, and Marias Pass received 6-8 inches of snow at mid-elevations stations in the past 24 hours. The Flattop SNOTEL reported a whopping 1.3 inches of water (SWE). The Swan Range and the Whitefish Range appear to have only picked up a few inches. 

Blase was with the Motorized Level 2 class in the Swan Range Friday and noted moderate to intense blowing snow. Fresh wind slabs were 4 inches thick and 1 Finger hard. We expect these slabs to be thicker and possibly harder today with steady wind and a little fresh snow overnight. He also noted a freezing rain crust to at least 6300', forming Friday morning. Were it not for this crust; slabs would have been thicker.

BNSF avalanche safety was also in the field Friday and observed active wind loading on easterly aspects above 5500 feet. They noted approximately 10 inches of accumulated snow from this past week, resting on 1-2 inches of near-surface facets above older snow and/or a crust interface. During ski cutting and stomping tests, new slab formation was cracking in isolated areas. Cracking was not propagating beyond the localized area where skier weight was applied. Slab thickness varied between 4" and 10".

I was in the southern Whitefish Range Friday searching for buried surface hoar and investigating the failure layer in an intentionally triggered slide from Tuesday, February 1. We only had success finding buried surface hoar on sporadic northerly slopes at mid-elevations. We found Monday's low-density snowfall has become a soft surface slab in intermittent locations and ranged from Fist to Four Finger hard. We also noted an eggshell thick freezing rain crust on all aspects at the mid-elevations. The February 1 avalanche occurred on the leeward side of a saddle and failed on a layer of buried surface hoar. Due to the windy location of this site, the surface hoar was small-sized and not apparent to the naked eye. You can find more details here https://flatheadavalanche.org/observation/wmr-avalanche-and-snow-obs.

While performing a crown profile, three local residents approached us and mentioned they had just triggered a slide. These folks provided excellent information on locating the slide and events leading up to, during and following the slide. The avalanche from February 4 failed on a layer of well-preserved surface hoar, which was easy to see and found approximately 16 - 18 inches below the surface. Please visit their observation for full details: https://flatheadavalanche.org/observation/skier-triggered-avalanche-outside-wmr-boundary. FAC staff thanks this group for sharing information from their experience with the backcountry community.

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