Lawinenlagebericht

Flathead Range & Glacier NP

Flathead Avalanche Center
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Lawinengefahr

Tue
Hochgebirge
2 · Mässig
Waldgrenze
1 · Gering
Unterhalb
1 · Gering
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Veraltet — dieser Lagebericht ist 4 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

Look for and steer around areas of wind transporting snow and dune shaped drifts. These may vary from drainage to drainage. Weak layers roughly a foot below the surface may rest beneath recent slabs of wind drifted snow at upper elevations. Give these slopes a wide margin for the time being. Generally stable avalanche conditions are found on slopes below 6500 feet. 

Lawinenprobleme

  • Wind Slabs

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

    Winds are forecast to blow at 12 to 18 mph out of the west at upper elevations. If winds reach these speeds, expect to see drifting snow and thickening slabs on the downwind side of ridges and gullies. Slabs may drift up to a foot thick in places, which would be enough to knock a rider off their feet. Look for pillowy-looking surfaces and drifting snow as signs to seek out slopes sheltered from the wind. IF winds don’t reach forecast speeds and you don’t see signs of drifting, manage your sluff in steep terrain with defensive riding strategies.

    (1/16/2023)

    Noisy Basin's sagging cornices along Aeneas Ridgeline.

  • Persistent Slabs

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

    Weak layers of buried surface hoar and near-surface facets exist roughly a foot below the surface. These weak layers are proving harder to find and less troublesome in the Flathead Range, Whitefish Range, and southern Glacier National Park than in the Swan Range. They are more noticeable and weaker at upper elevations, where they are found. Hedge your bets on windward and sunny aspects and choose slopes with clean runouts free of surface obstacles. 

    (1/21/2023)

    Long, shooting cracks that occurred when a snowbiler landed a small jump. Mid elevation, southeasterly slope.

Avalanche Discussion

The most important news to share today are the reports of remotely triggered slides and loud collapses from the Swan Range. A rider remotely triggered a 2-foot slab avalanche, while another rider triggered a shooting crack that shot off and out of sight. Both of these occurred on Saturday. We are entering a particularly dangerous time in the Swan Range where slabs are growing thicker and more dangerous, but the persistent weak layer the slab rests on is still at depths where a human can collapse it, causing a large avalanche.  The culprit is a weak layer of surface hoar. 

So far, this problem is a different story in the Whitefish Range, Flathead Range, and southern Glacier National Park. A few reasons why it is different there:

  • Significantly less snow has fallen there. Snow water equivalents (SWE) range from 1.4 to 1.7 inches vs. the Swan Range is now at 4.6 inches of SWE since January 9th.
  • A storm on January 9th almost entirely favored the Swan Range with 8 to 12 inches of snow. This snow buried the surface hoar there, while leaving it exposed for a longer period of time in other ranges. The longer that surface hoar is exposed, the greater the chance it will get destroyed by wind and sun.
  • So far, buried surface hoar is proving difficult to find in the Whitefish and Flathead Ranges. We’ve found it on a northwest-facing slope at 7400 feet, and others did not find it in a nearby location on a similar aspect, but lower in elevation. The "spotty" nature of this problem makes forecasting particularly difficult.

Managing this problem is easy in the Swan Range. Our advice is to avoid steep slopes on shaded aspects, those are ones that face North and East. For the Whitefish Range, the Flathead Range, and southern Glacier National Park, all the data suggests it is unreactive. Most of this information comes from below 7000 feet. Generally, we found this layer weaker at upper elevations, leading to a higher-than-usual degree of uncertainty for terrain above that.

As Mark wrote yesterday, an “underperforming” cold front rolled through the Flathead Saturday night. Winds were generally light, with the exception of Hornett in the northern Whitefish Range and Marias Pass in the Flathead Range. Both stations reached ideal speeds for transporting snow into slabs. Wind speeds increased overnight at Link Mountain, averaging 13 mph with gusts up to 26 mph. Westerly winds are yet again forecasted to blow at ideal speeds for transporting snow.

 

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