Lawinenlagebericht

Flathead Range & Glacier NP

Flathead Avalanche Center
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Lawinengefahr

Thu
Hochgebirge
3 · Erheblich
Waldgrenze
2 · Mässig
Unterhalb
2 · Mässig
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Kernpunkte

The best, safest riding will be on sheltered slopes without cornices overhead. On steep sunny slopes, dangerous wet snow avalanche conditions will develop as intense sun warms the surface snow. Plan to leave these aspects before roller balls tumble from rocky areas. On leeward slopes, recent winds have built large cornices and rounded drifts 1 to 3 feet thick. A smaller avalanche or cornice fall may be the trigger for a destructive avalanche that fails on old weak layers in upper elevation, shady slopes.

Lawinenprobleme

  • Loose Wet

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

    From 5 to 11 inches of new snow sits atop older crusts on sunny aspects. In some areas that snow began to transition from dry to moist on Tuesday. We were able to trigger a loose avalanche in moist snow at 7,000 feet in Skyland, despite increasing cloud cover. Wednesday will be clear and temperatures will be much warmer. Slopes will thaw as they get intense sun and wet snow concerns will migrate from east to south to west throughout the day. Rocky and treed areas will warm first and retain the most heat. Watch for roller balls and small sluffs tumbling down very steep, sunny slopes. These are warning signs to move to shady aspects before larger slides become likely. Plan your day so that you can be out from under steep, solar aspects in the afternoon as you’re leaving the backcountry.

    (3/20/2021) Large skier triggered wet loose avalanche. East aspect. Essex area.

  • Wind Slabs

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

    Strong to moderate winds redistributed the recent snow onto leeward terrain through Tuesday. Rounded drifts may be stubborn, but triggered wind slabs can be large enough to bury or injure you. You’ll find them on steep slopes below cornices and on the sidewalls of chutes. Use small test slopes and hand pits to look for cracking in denser, textured snow before committing to an exposed line. Be wary of steep slopes with large cornices overhead. It’s easy to underestimate how much the sun is weakening the roots of cornices from shady slopes below. 

    (3/30/2021) Leeward slopes likely loaded by strong westerly winds Sunday.

  • Persistent Slabs

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

    Although it is unlikely, it is not impossible to trigger a very large hard slab avalanche failing 3 to 6 feet deep on shaded aspects above 6500 feet. Weak layers buried in late January are closer to the surface in the northern Whitefish Range. Rapid warming can make persistent slab avalanches more likely. Warming can also cause cornice fall or a loose snow avalanche which can be the trigger that sets off a destructive slide. The best way to manage uncertainty over the problem is to stick to slopes less than 35 degrees if you do not feel a supportable crust in hand pits under the recent snow.

    (3/17/2021) The crown from a hard slab avalanche that ran in mid March in the Flathead Range

Avalanche Discussion

Intense sun and warm temperatures will make recent dry snow hazards wet. It can also weaken the wind drifted snow around cornices.

On Tuesday, the snow became moist on sunny slopes. We were able to trigger a loose wet avalanche on a southeast aspect at 7,000 feet in Skyland. Increasing cloud cover kept some slopes cold and dry, and formed a new sun crust on solar aspects. Large cornices hang from leeward ridglines and were reactive to our stomping on test slopes. Observers reported blowing snow across the Flathead Range. In the southern Whitefish Range, riders found few signs of instability with minimal sluffing in the recent snow.

The recent storm left 9 to 11 inches of new snow in the Whitefish Range, Flathead Range, and Central Glacier Park. Noisy Basin and John Stevens Canyon only picked up 4 to 5 inches. Moderate to strong winds beefed up cornices and redistributed some of the new snow onto leeward terrain.

Small to large soft slab avalanches failed during the last storm on Sunday and Monday. Some of these were triggered by snow machines or skiers. Others failed naturally on steep slopes under cornices. 

Deeper in the snowpack, mid-winter weak layers sit below 3 to 6 foot thick hard slabs. These layers have been unreactive for about 2 weeks. In the northern Whitefish Range , they are closer to the surface, so they may be more easily affected by a rider or snowmachine. 

Glide cracks litter steep east to south to southwest-facing slopes above about 5,500 feet. Rapid warming can make them more likely to fail as a very large avalanche.  Recent snow may make them difficult to see. It’s best to avoid the wrinkled slopes near and below them.

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