Lawinenlagebericht

Flathead Range & Glacier NP

Flathead Avalanche Center
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Lawinengefahr

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

Kernpunkte

New and drifted snow will keep avalanche conditions at dangerous levels. Slabs can break in the new snow on steep slopes, especially where westerly winds have grown and stiffened them below ridgelines. Recent natural and triggered hard slabs prove that you can still trigger un-survivable avalanches that fail on weak layers 3 to 6 feet deep. In sheltered terrain, you can trigger large loose sluffs. Stick to slopes less than 30 degrees that aren't exposed to the hazards overhead. 

Lawinenprobleme

  • Storm Slabs

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

    Heavy snowfall and moderate winds have grown new slabs on steep slopes. They will be largest, and most dangerous, where more snow falls and more wind has blown. As of Friday morning, Big Mountain has gotten 10 inches of new snow, Noisy Basin is reporting 4 inches, and Tunnel Ridge has received 7 inches. Westerly winds have been strongest above about 5,500’ in the northern Whitefish Range, Flathead Range, and near Marias Pass. The Swan and Flathead ranges are favored for the most new snow and wind throughout the day. Use caution near or below steep slopes. Watch for cracking in stiffer snow in exposed terrain. Watch for sluffing in steep trees. Choose slopes no steeper than 30 degrees for safer riding conditions.

    (2/25/2021) Wide-breaking soft slabs of drifted snow

  • Persistent Slabs

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

    New snow and wind continue to stress a fragile snow structure. Weak layers associated with the mid-January crust can still propagate a failure across steep slopes. They have caught riders by surprise and resulted in several near misses and accidents. Recent very large avalanches have started where the slope breaks from about 30 to more than 35 degrees. Don’t try to outsmart the snowpack. Measure the slope angle and stick to terrain that’s less than 30 degrees from top to bottom and isn’t connected to steeper slopes above. Avoid even small, steep break-overs above terrain traps.

    (2/24/2021) The crown of this triggered slide shows why slope angles and convexities are so important.

Avalanche Discussion

On Thursday, riders triggered soft slabs and dry sluffs up to a foot deep in the Flathead and Whitefish ranges. Small storm slabs failed naturally on steep road cuts at lower elevations. Avalanches grew in size and became more sensitive throughout the day. Heavy snowfall was accompanied by moderate west winds. These were stronger above about 5,500 feet, drifting the new snow onto leeward terrain.

Wednesday, avalanche mitigation operations in John Stevens Canyon resulted in very large avalanches that ran over BNSF snowsheds and dusted Highway 2 with debris. Crowns from these avalanches were 3 to 6 feet thick. Three things stick out about these slides: 1) They were triggered with small explosives, not huge payloads. 2) Several of the starting zones where they failed range from 30-35 degrees, which is not as steep as you'd expect for avalanches so large. 3) The weakness that failed was indeed the well-advertised facet layer atop the mid-January crust.

This crescendo of avalanche activity culminated an extended period of large and dangerous natural (Heaven's Peak Avalanche) and rider-triggered avalanches across the region. Slides breaking on buried weak layers have failed around the compass on slopes above 5,500'. They've caught several riders by surprise (Devil's Hump near-miss, Swan Range accident, Dorothy's near-miss). Check out the summary of recent avalanche activity here.

More snow and wind will add to the snowpack on Friday. The Swan Range, Flathead Range, and Continental Divide will see the most intense, and most continuous new loading. Natural avalanche activity will decrease throughout the day, but new slabs and buried weak layers are primed for human triggering. 

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