Lawinenlagebericht

Swan Range

Flathead Avalanche Center
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Lawinengefahr

Wed
Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
1 · Gering
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Veraltet — dieser Lagebericht ist 5 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

Plan an early day and exit before surface crusts break down and the avalanche hazard increases. Warm temperatures and solar radiation will thaw the snow surface making wet loose avalanches possible. Persistent weak layers buried 2 to 6 feet deep may also become reactive with rising temperatures. If the surface becomes unsupportable to your boot, stay off and out from underneath slopes 35 degrees and steeper.

Lawinenprobleme

  • Persistent Slabs

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

    Triggering a persistent slab avalanche is unlikely on slopes where a supportable crust exists. As soon as the crust breaks down, dial back your terrain choices to slopes less than 35 degrees. Avoid slopes that appear rocky with variable snow cover on upper elevation, shaded slopes where no surface crust exists. Avalanches failing on persistent weak layers 2 to 6 feet deep would be difficult to survive. 

    (3/11/2021) A recent Persistent Slab avalanche possibly triggered by a cornice fall on Mt Cameahwait in the Flathead Range. Recent warming contributed to cornice failure.

  • Loose Wet

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

    Solar radiation and daytime warming will increase the chance of wet loose avalanches that can be large enough to bury and injure a person. To reduce your risk, seek out shaded, northerly-facing slopes at mid and upper elevations, or plan your day to exit before surface crusts break down. Pinwheels and small wet loose avalanches coming from steep rocky areas are signs to move quickly to shady aspects. Early starts and early ends to a day of riding are good ways to manage this problem.

    (3/9/2021) Small Wet Loose Avalanches

  • Cornices

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

    Observers continue to report large cracks on ridge lines where cornices are beginning to un-root themselves. There is a growing concern for cornices to fail with warm temperatures. Cornice fall is an obvious and manageable hazard: plan your ascent and descent to avoid being underneath them because they can spontaneously fail and cause a significant amount of harm in and of themselves. Give ridgelines an extra-wide berth to avoid accidentally walking out onto them. The last known hard slab avalanche was triggered by a cornice fall.

    (3/14/2021) Big Cornice Sagging

Avalanche Discussion

Temperatures overnight dropped below freezing in the Whitefish Range, Flathead Range, and Glacier National Park. Noisey Basin in the Swan Range didn't get below 34 degrees F. I would expect to see a superficial crust on the surface in the Swan Range, and a thicker, more supportable crust elsewhere. The snow surface in the Swan Range will likely heat up sooner, resulting in larger wet loose avalanches. The snowpack will be temporarily locked in place this morning in the areas that saw below freezing temperatures. 

Using early starts and finishes will help reduce your chances of being involved with most of the possible problems today. Wet loose avalanches happen when the snow surface becomes wet. Persistent slab avalanches are more likely when the snowpack heats up and increased slab deformation begins to stress weak layers deeper in the snowpack. Cornice fall and glide avalanches are more likely when meltwater reaches further down in the snowpack and weakens the snow near the ground. All of these processes need one ingredient, heat.

Right now you can manage all these problems with two simple strategies. The first is to get off of the slopes before they heat up. This will especially help mitigate the wet loose and persistent slab avalanche problems. The other strategy is avoidance. Although cornice fall and glide avalanches are more likely during times of intense heat and solar radiation, they can also be rather spontaneous. It is good practice to give ridgelines an extra-wide berth and use vegetation clues for travel. Move quickly and one at a time if you have to travel beneath them. Currently, I am less concerned about getting tumbled by a cornice and more concerned about the cornice triggering a large, hard slab avalanche that would be difficult to survive. As for glide avalanches, stay out from underneath the obvious brown frowns. I would also treat slopes adjacent to glide cracks, or similar terrain, as suspects for glide avalanches.

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