Lawinenlagebericht

Swan Range

Flathead Avalanche Center
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Lawinengefahr

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

Heavy snow and strong winds have created dangerous avalanche conditions. Continued snowfall and the possibility of rain-on-snow will keep the danger elevated through the day. Natural and triggered avalanches have the potential to break across terrain features and run longer than expected. Avoid traveling on, or under, slopes greater than 30 degrees.

Lawinenprobleme

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Very Likely
    Größe
    1–2.5

    Moderate to strong winds have transported snow onto leeward aspects along ridgelines and cross-loaded gullies over the past 72 hours. This snow transport will continue today. Expect to find slabs 5-15 inches or so thick in most leeward terrain. These will be reactive to the weight of a rider or snowmachine.

    For the best riding, seek out sheltered, low-angle slopes. Steer clear of areas with hollow-sounding snow or areas sculpted by recent winds.

  • Storm Slabs

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

    Adding warming temperatures and windy conditions to the mix is the perfect recipe for developing a storm slab problem. Expect these slabs to increase in thickness as you gain elevation. Slabs can also be thicker below ridgelines, in cross-loaded gullies, and mid-slope convexities. 

    On steep slopes sheltered from the wind, a person or snowmachine can trigger soft slabs 6 to 12 inches thick that break within the new snow, or at the old/new snow interface. Look for signs of instability like shooting cracks that may travel long distances.

    Avalanches that start up high will entrain a lot of snow and may run long distances in areas that have been protected from recent winds.

    (1/7/2024)

    Sot slabs like this may entrain significant amounts of snow.

     

    Photo: Kaleb Retz

  • Persistent Slabs

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

    To reduce your risk of getting overrun by dangerous avalanches that break well below the recent snow, stay off and out from under steep, shaded start zones at mid and upper elevations. In this terrain, slides can break on weak, faceted snow a few inches above the stout crust and ice deposited in early February. This layer is now buried 2 to 4 feet deep in most of the region.

    Prior to this week's storms, it was most sensitive to a person's weight on shaded, northerly aspects above 6,500 feet in the Flathead Range. Avalanches that broke on this layer resulted in several accidents and near-misses in that zone over the holiday weekend. However, the structure has been reported across the region, and it was the weak layer in a fatality north of the region, in the Canadian reaches of the North Fork of the Flathead River on Feb. 24.

    For the past three days, new and drifted snow has been actively loading the slopes where the Persistent Slab avalanche structure was most sensitive. Slabs that involve the recent snow can step down to the weak layer above the crust, resulting in slides that break much wider, involve much more debris, and can be difficult to impossible to escape. They may can also be triggered remotely or sympathetically, from below or from adjacent slopes. 

    Persistent Slab avalanches like this can break widely across terrain features and entrain most of the season's snowpack.

    Photo Credit: Rob Cherot

Avalanche Discussion

Happy Leap Day! If you get to celebrate a real birthday for the first time in 4 years, enjoy. For the rest of us, we get an additional day to play in the snow. Unfortunately, as the snow totals stack up, the terrain for safe recreation becomes more limited.

Heavy snowfall combined with 3 days of southwest winds have left a variety of avalanche  problems for backcountry users to navigate. The answer to a complex weather pattern and multiple avalanche problems is terrain management. Seek out simple terrain that is under 30 degrees and not exposed to avalanches from above.

Temperatures have risen 10 to 15* F since snow began a few days ago. New and recent snow have developed into an upside-down slab. Think: stronger and warmer,  well-bonded snow over weak, cold, dry snow. Continued warming temperatures today (Thursday) will bring the rain/ snow line to 5000 feet and possibly 5500 feet. Rain on new snow rapidly destabilizes it, by adding heat and mass to slabs and rapid strain to buried weak layers, like our old frenemy from early February (the stout/crust facet combo). Finally, drifting snow on leeward aspects has formed reactive slabs along ridgelines and steep rollovers. If/when precipitation switches to rain, expect the avalanche danger to spike across all zones. If you find areas with 8  inches or more of recent snow, expect avalanches to be easily triggered by the weight of a rider or snowmachine.

The safest bet for today is to avoid traveling under avalanche paths or near connected terrain, as you could trigger avalanches from a distance or below.

 

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