Lawinenlagebericht

Flathead Range & Glacier NP

Flathead Avalanche Center
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Lawinengefahr

Wed
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

Recent strong winds have abated but left dangerous avalanche conditions in their wake. Expect to find slabs of drifted snow up to 3 feet thick on leeward aspects. You are likely to trigger one of these, which may end up propagating wider and running further than anticipated. Today's sunshine will warm and weaken loose dry surface snow. Expect these sluffs to entrain substantial snow and run long distances on a crust. Stay ahead of the sun to enjoy the recent high-quality snow.

Lawinenprobleme

  • Wind Slabs

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

    Today, we should enjoy a respite from the recent blustery conditions but expect to find its aftermath on leeward slopes. Strong winds Sunday afternoon through Monday morning transported the limited available dry snow somewhere to the east - onto leeward aspects starting zones, further downslope than normal, Browning, or either sublimated into the atmosphere. Moderate westerly winds easily transported Monday's bountiful low-density snow. Expect to find fresh slabs 18 to 36 inches thick on leeward slopes below ridgelines, downwind of saddles, or behind terrain features. You will feel a distinct difference traveling through the cohesionless snow unaffected by the wind and denser wind-affected snow. Use hand pits or small, inconsequential test slopes to see how well the surface snow adheres to the underlying layer. Cracking under your skis or snowmachine signifies that you should seek out terrain sheltered from the wind.

    (3/29/2021) Moderate winds perfect for moving snow onto leeward slopes

  • Loose Dry

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

    Monday's precipitation forecast lived up to its hype, depositing dry snow amounts in most mountain locations. Temperatures during the storm were cold, with clearing overnight further cooling the snow surface. Today's forecasted partly sunny conditions should warm and weaken the new snow's bonds initiating a Dry Loose avalanche problem. Expect slides to start small but can entrain substantial surface snow and run surprisingly far on a melt-freeze crust. As the surface warms, retreat to higher elevations or try to stay in front of the sun as it moves from east to west. Avoid recreating directly above terrain traps where debris can pile up deep, or cliff bands where even a small sluff can lead to a poor outcome. 

    (3/23/2021) Loose, dry avalanches on steep north-facing slopes in the Middle Fork of the Flathead Range. These were easy to initiate with ski turns and large enough to push us around. We avoided getting tangled up in these by managing our sluffs.

  • Persistent Slabs

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

    After the wind and recent snow, persistent slab avalanches remain a low probability-high consequence problem. Recent drifts have increased to load on top of deeply buried weak layers. Skiing and riding terrain without a supportable crust under the new snow increases your risk of triggering deeply buried weak layers. That's mostly confined to upper elevation slopes on shady aspects. Avoiding slopes steeper than 35 degrees, and choosing conservative terrain, are your best bet for safe riding.

    (3/17/2021) The crown from a hard slab avalanche that's more than a few days old.

Avalanche Discussion

Monday's snowfall ended late last evening with a good dose of low-density snow for most locations. 48-hour storm totals for favored sites: Stahl Peak in the northern Whitefish Range picked up 1.3 inches of SWE and 11 inches of snow, the southern Whitefish Range at the Big Mountain with 0.5 inches of SWE and 9 inches of snow, central Glacier Park with 1.1 inches of SWE and 9 inches of snow, and Tunnel Ridge in the Flathead Range with about a foot of snow. Surprisingly, the moist Noisy Basin site in the Swan Range received a piddly 0.5 inches of SWE and 5 inches of snow.

Monday's daytime winds were moderate and easily transported this snow onto leeward aspects at mid and upper elevations. I was with the March FAC intern in the Flathead Range, where we found enjoyable cohesionless snow resting on a supportable crust on sunny aspects. The new snow did not exhibit slab characteristics until we reached ridgeline elevations where the wind was stiff, and the snow was a-blowin'. The ridgeline was scoured down to a melt-freeze crust, and we found thick drifts behind trees and terrain features. We found a thick slab of 4F hard snow below the ridgeline that rested on a 1F hard slab deposited during the strong wind event. Hand pits revealed clean failures at this interface.

Observations from yesterday were scarce, but we have decided to drop the storm slab problem due to Monday's snowfall being colder, drier, and less dense than anticipated. With forecasted sunshine today, the new snow will warm and weaken despite relatively cool air temperatures. As we learned in Avalanche 101, the snowpack does not like rapid change. With the March 30 sun high in the sky, there is potential for a widespread loose dry cycle today. These will start as dry sluffs but may entrain moist or wet debris on the way down. Expect larger and longer running slides in the Flathead Range and Glacier Park, where the terrain is bigger. Temperatures continue to warm through the week, and today's loose dry problem will transition to a wet loose problem.

Check out the weekly snowpack update from March 26 in the media gallery below or here.

 

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