Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

Sun
Alta quota
2 · Moderato
Limite del bosco
1 · Debole
Sotto il limite
1 · Debole
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In sintesi

Friday's weak storm likely formed thin slabs of wind-blown snow near the highest ridgelines and summits. Reduce your chances of triggering an avalanche by steering around stiff surface snow. Today's warm conditions may produce small, wet loose slides on all aspects.

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    Friday's several inches of dense snowfall and southwest winds likely built thin slabs in locations most exposed to the wind. Expect to find these features just downwind of ridgelines and peaks at higher elevations in windy spots such as near the Aeneas Ridge or the Flathead Crest. In many places, slabs formed on a layer of surface hoar, potentially increasing instability. 

    Slabs should be small, recognizable, and easy to avoid. Dune-shaped drifts, dense snow resting on softer snow, or cracking at the surface indicate you have found cohesive snow. Cracks shooting away from you are a telltale sign of a sensitive slab, directing you to seek out lower-angle or more sheltered terrain.

    Wet Loose: Today's warm temperatures may wet the surface snow and trigger loose snow avalanches.  On southerlies, these will be shallow, as just a few inches of fresh snow rest on a crust (Observation). Northerlies hold a foot or more of unconsolidated snow that observers note has been susceptible to human triggering (Observation). Mitigate this problem by selecting slopes with clean runouts free of terrain traps and by being aware of overhead hazards.

     

    (4/3/2022)

    Thin wind slabs have formed at upper elevations from Friday's winds. Reduce your chances of triggering an avalanche by steering around stiff surface snow, especially along ridge lines. 

    Photo: burket M kniveton

Avalanche Discussion

Avalanche forecasting in December was relatively straightforward, with avalanche problems being generally confined to the new and wind-blown snow. 

January 2 Weak Layer

The snowpack likely becomes more complex as Friday’s light snowfall buried a widespread layer of surface hoar on slopes sheltered from the sun and wind at mid and upper elevations. On these slopes, the weak layer rests on 30 cm of loose, cohesionless snow. On slopes exposed to recent north winds, the surface hoar rests on windboard. On many solar aspects, the weak layer was destroyed.

The recent snow contains 0.5 to 1.0 inches of SWE (Snow Water Equivalent) and, combined with continued drifting under light to moderate southwest winds, is adding weight to the surface hoar. This is a fairly light load, and we don't expect this to affect the avalanche hazard today. However, forecast snow tonight through Sunday and into the workweek should be sufficient to test the buried weak layer in locations where a slab develops. 

Assess the snow surface for slab characteristics indicated by dense over soft snow or cracking. An observer in the southern Whitefish Range noted that the surface became more cohesive, with cracking underfoot, toward the end of the day. Additional loading and warmth may increase surface stiffness and increase the likelihood of slab formation and potential instability. When FAC staff have sufficient data, we will likely list Persistent Slab as a problem on slopes where a slab overlies surface hoar.   

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