Lawinenlagebericht

Whitefish Range

Flathead Avalanche Center
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Lawinengefahr

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

A big thank you to all the riders who've reported triggered slides, near-misses, and accidents the past two weeks. Thanks to those reports, we have a good idea where trouble lingers: convex rollovers on shaded slopes above 5500 feet. In this terrain, you can trigger large avalanches that break on weak snow buried 1.5 to 4 feet below the surface. In upper-elevation terrain, it's possible to trigger soft slabs of freshly-drifted snow on steep slopes near ridges and summits. You can reduce your chances of tangling with either form of trouble by sticking to slopes less than about 35 degrees. 

Lawinenprobleme

  • Wind Slabs

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

    Snow showers the past few days have left plenty of low-density snow for winds to move, and last night, wind speeds picked up. Expect fresh, soft slabs of drifted snow in terrain that catches blowing snow: the tops of chutes, the lee sides of ridges with open, windward fetches, and the downwind sides of saddles and passes. These should be 6 to 18 inches or so thick, and reactive to the weight of a rider or snowmachine.  Triggered slabs of drifted snow may collapse weak layers deeper in the snowpack, releasing larger and more dangerous avalanches. Sticking to sheltered slopes where winds don't consistently move snow will get you great riding in low-density snow. 

  • Persistent Slabs

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

    No reports of triggered slides Thursday, which is a welcome change. Trouble still lurks on steep slopes above about 5500 feet, where slabs 15 to 30 inches thick sit on weak facets and buried surface hoar above the mid-January crust. Signs that this structure remains unstable have come from most aspects the past few days - triggered slides on shaded slopes, whumpfing collapses on sunny and windward slopes. Nearly all of the triggered slides have occurred on convex, northeast-facing slopes well-below ridgelines. That's the kind of terrain where surface hoar layers tend to be more uniform and better preserved. Cross this terrain off your run list. You can reduce your risk further by sticking to slopes less than about 35 degrees that are concave in shape - like a playground slide - and aren't above trees, gullies or other features that can turn a surprise ugly. 

    (2/17/2021) Accidently triggered avalanche in the Whitefish Range.

Avalanche Discussion

Mark has posted a video summary of the week's activity; you can click the link below or watch it here. 

We bumped the danger to CONSIDERABLE at upper elevations in the Swan and Flathead Ranges today, because of a developing Wind Slab avalanche hazard. The slab-making machine has what it needs to produce small to large slabs that can be dangerous in themeselves, or, once triggered, release thicker, wider slabs that break near the mid-Janauary crust. Given this potential, it's worth staying out from under steep start zones near ridges where you see blowing snow. That indicates active loading of leeward paths, and spontaneous cornince fall or small natural avalanches are possible. These start zones may be thousands of feet above you, but in the runouts for these start zones, you're still exposed to large natural avalanches. Freshy-formed slabs of drifted snow may also form in the Whitefish Range, though they'll be smaller and less widespread because there's less snow available for the winds to move.

Wednesday, a rider in the southern Whitefish Range triggered a large avalanche that tumbled his machine downhill into a tree. He managed to jump off and stay near the crown. The slope that released is the kind of spot where surface hoar tends to form uniform layers and remain intact until being buried: well below a ridge, sheltered from the wind, shaded from the sun, and near a small basin where cold air pools. The slope that caught a solo skier Monday - also in the southern Whitefish Range - has similar characteristics. As does the one where, last Friday. a skier grabbed a tree to avoid getting carried further downslope into more trees. The take away from these incidents? Look for - and stay off of - terrain that's most likely to harbor well-developed and well-preserved layers of surface hoar, and you'll significantly reduce your risk of triggering a large, dangerous slide. 

Reduce, but not eliminate - people have reported other signs of instability in terrain with different characteristics. Specifically, slopes more open to the wind and sun. We've had no reports of triggered slides from these aspects, however. It appears that the weak layer that formed above the mid-January crust consists mostly of near-surface facets rather than surface hoar. Reactive slabs on these aspects are more isolated. And perversely, the terrain where they remain unstable tends to be harder to distinguish. Sticking to slopes less than about 35 degrees is the most reliable way to reduce this uncertainty. Which isn't hard, given the good riding conditions out there.

The reports of triggered slides and near-misses the past week have really helped to us to pinpoint where slabs remain reactive. Thanks to the people who've contributed those reports; they help the community of backcountry riders remain safe. We're not out of the woods yet; a powerful storm is approaching from the Pacific. Keep those obs and reports coming, please.

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