Lawinenlagebericht

Flathead Range & Glacier NP

Flathead Avalanche Center
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Lawinengefahr

Sat
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

You can trigger large avalanches on slopes with fresh wind slabs or recently buried weak layers. You’re more likely to find trouble on steep slopes, and on slopes with convex breakovers. Look for better, safer riding on planar, wind-sheltered slopes less than about 35 degrees.

Lawinenprobleme

  • Wind Slabs

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

    Use caution when heading into steep, exposed, leeward terrain. You can trigger recent and freshly drifted slabs below ridgelines and on cross-loaded features like gullies. Watch for blowing snow above or across the slopes you’d like to ride. Feel for hollow surface snow; denser than what’s underneath. Cracking around your machine or your boards is a clear sign of the hazard. You can find better riding on sheltered slopes less than about 35 degrees.

    (2/3/2022)

    The crown of an intentionally triggered wind slab on an NW aspect at mid-elevations.

    Photo: Stefan Beattie

  • Persistent Slabs

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

    On shaded and sheltered slopes steeper than about 35 degrees, or slopes with convex breakovers,  you can trigger a large avalanche that breaks on buried weak layers. A grey stripe in snow pits down about 12 to 20 inches can help you identify these layers. Snowpit tests, however, are often unreliable when assessing their stability. Instead of betting the farm on a single column of snow, use terrain to your advantage. Avoid terrain likely to harbor weak layers and trigger points.

    (2/3/2022)

    Buried surface hoar at 6200 feet on a NE aspect. West side of Swan Range.

    Photo: Clancy Nelson

Avalanche Discussion

Strong winds will have recent, low-density snow, plus a few more inches, to drift into cohesive slabs today. Winds have blown snow onto open slopes for much of the week. On Thursday, observers reported a recent natural avalanche, shooting cracks, and small triggered wind slabs on steep convexities around Essex. Forecaster teams triggered a few soft and small wind slabs in the Swan range Thursday and Wednesday. Large natural avalanches ran on south-facing, cross-loaded slopes in John Stevens Canyon Monday and Tuesday. The good news is that riders in similar areas found soft snow and only minor sluffing in sheltered terrain. 

About that sheltered terrain…Persistent weak layers are buried about 12 to 20 inches below the surface. On sheltered, northwest- through east-facing slopes, preserved surface hoar is the problem. A rider triggered a slide that broke on that layer Tuesday, east of Whitefish Mountain Resort. Observers in the Swan Range found surface hoar on north and east slopes in Doris Creek and on northwest and northeast aspects near Broken Leg Mountain. On slopes where the surface hoar was destroyed by sun or wind, facets rest around the mid-January crust. These have not yet produced any reported avalanches or other signs of instability.

A very large natural avalanche starting on the rock slabs that make up the east face of Mt. Grant, reported Wednesday, likely released in mid-January. It's a clear example of the Deep Persistent Slab avalanche hazard above 8000 feet that we described last week. Though the crust/facet combination near the base of the snowpack appears to be dormant, I'll be thinking about similar slopes overhead when I move into upper elevation terrain for the rest of the season.

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