Lawinenlagebericht

Swan Range

Flathead Avalanche Center
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Lawinengefahr

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

Kernpunkte

It’s unlikely that you can trigger large, hard slab avalanches. That’s because surface crusts are keeping you from collapsing buried weak layers on most slopes. If you step out into steep terrain, you can give yourself a buffer by choosing slopes with a thick, supportable crust and deep, uniform snow cover. I’d still limit my travel on steep slopes with precarious cornices overhead.

Lawinenprobleme

  • Persistent Slabs

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

    Weak layers formed in January sit below 3 to 6 feet of firm snow. An avalanche failing on one of these layers is unlikely where you feel a supportable crust under your feet. If you choose to step out into steeper terrain, you can give yourself a buffer by avoiding slopes near rocky outcrops, or with variable snow cover. Limit your travel under large cornices. Cornice fall can be the trigger that sets off a big avalanche. Expose only one person at a time and keep your partners in sight.

    (3/8/2021) Cornice fall triggered deeply buried weak layers resulting in a large avalanche

Avalanche Discussion

The last reported slab avalanches ran 5 days ago on the weekend of March 6th. These were large and failed on deeply buried weak layers. They were likely the result of a prolonged period of above freezing temperatures and intense sun.

We’re still talking about those persistent weak layers buried in late January. They rest under 3 to 6 foot thick, hard slabs. They consist of sugary, faceted snow in open areas and feathers of surface hoar on shady, sheltered slopes. Snowpack tests continue to show that they can propagate a failure. The question is whether you can trigger them. That’s unlikely where supportable surface crusts keep you from sinking into the snowpack underneath. 

All the spring sunshine has left a thick melt-freeze crust on southerly slopes at all elevations. A freezing rain event the last weekend of February left a thin ice crust on shady slopes to at least 7200 feet in the Swan and Whitefish ranges. We don’t know of any crust on upper elevation, northerly slopes in the Flathead Range and Glacier Park. 2 to 4 inches of snow sits atop these crusts.  

Lastly, cornices and glide cracks will be a concern for the rest of the season. Both are more likely to fail during periods of intense sun and warm temperatures. It’s best to avoid traveling on slopes beneath these unpredictable hazards. Cornice fall can trigger a persistent slab avalanche.

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