Lawinenlagebericht

Swan Range

Flathead Avalanche Center
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Lawinengefahr

Tue
Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
1 · Gering
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Kernpunkte

A snow biker remotely triggered a slab avalanche 2 feet thick on Saturday. This is a clear sign to not trust steep slopes, particularly ones that face North and East, where this weak layer is proving more dangerous. Your safest bet is to keep slope angles low, or turn to aspects that typically get hit by the sun or wind. 

Lawinenprobleme

  • Persistent Slabs

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

    Weak layers of buried surface hoar and near surface facets exist roughly 2 to 3 feet below the surface. A snow biker in the northern Swan Range remotely triggered a slab 2 feet thick and triggered long shooting cracks on an adjacent slope. This same day a skier felt a collapse and observed a shooting crack extend out onto the slope in front of him. He was the third person on that slope. This demonstrates the untrustworthiness of these weak layers. Slabs are growing thicker and more dangerous, and are buried at a depth where a human can trigger them. Stick to slopes less than 35 degrees on shaded aspects. Slopes that face due south to west are proving to be less troublesome.

    (1/21/2023)

    This small, but relatively thick avalanche was triggered remotely by a snowbiker. The rest of the slope cracked but did not slide.

  • 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.

Avalanche Discussion

The most important news to share today is 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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