Lawinenlagebericht

Swan Range

Flathead Avalanche Center
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Lawinengefahr

Sat
Hochgebirge
2 · Mässig
Waldgrenze
1 · Gering
Unterhalb
1 · Gering
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Kernpunkte

Limit your travel on slopes with cornices overhead. They can fall in the heat of the day. Thick surface crusts make it unlikely that you can trigger large, hard slab avalanches on most slopes. Though cornice fall may be enough to trigger a large avalanche. If you step out into steep terrain, you can give yourself a buffer by choosing slopes with a supportable crust and deep, uniform snow cover.

Lawinenprobleme

  • Cornices

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

    Cornice fall triggered a fresh slab avalanche in the southern Flathead Range on Thursday. Cornices also fell in John Stevens Canyon. These broad, overhanging visors of snow are hazards in their own right. They can break further back from the edge than you'd expect. Give them a wide berth. Stay out from underneath of them as the sun heats ridgelines throughout the day.

    (3/4/2021) Large cornices In Glacier Park susceptible to failure during our extended warming period.

  • 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.

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

Avalanche Discussion

On Thursday, BNSF Avalanche Safety reported a cornice fall that triggered a large, deep avalanche on the southeast face of Mt. Cameahwait. Other cornices fell from ridgelines in John Stevens Canyon. Elsewhere in the Flathead Range, Cam backed off a steep, southeast face when he realized that the snowpack was shallow and had no surface crust. It was a rocky area, and he figured the risk of triggering one of the late January weak layers outweighed the reward of one run. 

In the Whitefish Range, a Level 1 avalanche course reported a new glide crack slumping on a south aspect to reveal a large swath of bare ground.

Persistent weak layers buried in late January 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. Recent avalanches and snowpack tests show that they can still 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. Where there is no crust, warming temperatures may make these layers more reactive. Temps will continue to climb each day through Sunday.

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. 2 to 4 inches of snow sits atop these crusts, and it may moisten during the day. Surface crusts are distinctly absent from upper elevation, northerly and easterly slopes in the Flathead Range and Glacier Park.  

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 obvious hazards. Cornice fall can trigger a persistent slab avalanche.

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