Bollettino valanghe

Swan Range

Flathead Avalanche Center
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Pericolo valanghe

Wed
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Limite del bosco
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In sintesi

Strong winds have drifted recent snow into dense slabs that, because of variable wind directions, exist on a variety of aspects and elevations. Be cautious on slopes where you find more than six inches of hard, wind-drifted snow. If you experience cracking or audible collapsing, seek out lower angle terrain protected from the wind.

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–2

    Powerful winds have raked the Swan and Whitefish zones the past two days, drifting snow into hard slabs that can react unpredictably to the weight of a rider or snowmachine. Be alert for smooth, pillowy snow surfaces. These may feel or sound hollow or drum-like. Avoid steep terrain where you find these dense slabs. They can release well above a rider and break much wider than expected. The hard chunks of debris can deliver a serious pummeling to a person caught and carried.

    These slabs will be more widespread than usual, thanks to both the strong wind speeds and shifting wind directions. The location rose for this problem highlights the terrain where they'll be most widespread. You may find isolated slabs on typically windward slopes, much further below ridges than usual, or cross-loaded onto odd sides of gullies and other terrain features. Sticking to wind-sheltered terain where the snow is soft is a great strategy for minimizing your risk and maximizing your riding quality.

    (2/24/2024) Slopes like this with conditions like these are today's go-to. Less than 35 degrees steep and wind-sheltered, with soft snow at the surface.

  • Persistent Slabs

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    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1.5–2.5

    Ride to reduce your chances of triggering large avalanches that break on old, weak snow. The new and drifted snow of the past few days have thickened and stiffened the slab sitting on thin, persistent weak layers buried 1 to 3 feet above the snow surface and inches above a very hard crust from early February. Propagation in some snowpack tests before the storm indicates that these layers are not yet dormant. The additional load may have reactivated them.

    Minimize your time on and under steep slopes, particularly those that are convex in profile and that may be scoured by recent winds. Thin spots in the slabs are likely trigger points and are more common in slopes with these characteristics. You can trigger these slabs from below or from adjacent slopes, so keep an eye on your riding partners. Park well out of runouts so if a partner is surprised, you can keep eyes on them and minimize your search time. 

    These slabs will likely be thicker and more sensitive to a rider's weight in the northern Whitefish Range. 

Avalanche Discussion

New snow and powerful winds have left dense slabs of drifted snow across the forecast region the past few days. Storm totals (since Saturday morning) have varied, with the northern and eastern parts of the forecast region favored. I'm estimating 12 to 15 inches of storm snow in the northern Whitefish Range, 18 to 20 inches in the upper McDonald Valley in Glacier National Park, and a foot or more in the Flathead Range near Tunnel Ridge. The storm was stingy around Whitefish Mountain Resort and the in the Swan Range, where storm totals look to be 4 to 7 inches. 

The winds, however, were much more equitable. Most stations recorded hourly averages of 25 to 45 mph Sunday, with gusts of 40 to 80 mph - and over 100 mph at Tunnel Ridge. Wind speeds dropped to the single digits Monday when arctic air arrived. They look to veer back to the west and southwest today, and pick up again.

The storm's legacy today is dense slabs of drifted snow that are widespread in typically leeward terrain - NW to NE to SE - and isolated in usually windward terrain. The slabs are mostly composed of snow that fell during the storm, because sun crusts were present on windward slopes prior to the storm.  So the slabs will be thicker and larger in areas that got more snow, like the northern Whitefish Range, upper McDonald Valley, John F. Stevens Canyon, and the Flathead Range. You'll find them most common at upper elevations. Be on the lookout for dense slabs of drifted snow in unusual spots as well - well below ridges, on usually scoured sides of gullies, etc. The main danger with these hard slabs is that they break unpredictably, so you can get well onto a slab before it releases. 

These dense slabs of drifted snow are adding to the load above tricky weak layers now buried 1 to 3 feet below the snow surface and just a few inches above an obvious and very stout crust. These layers were the culprits in a series of accidents and near-misses in the Flathead Range over the holiday weekend. We've had no reports of triggered slides on these layers in other ranges, though they are present and sometimes propagate in snowpack tests. These layers didn't give much warning prior to releasing  - no audible collapes and no cracking. I remain very leery of steep, shaded terrain in the Flathead Range. Elsewhere, snowpack tests might be more reliable. Sticking to slopes that are less than 35 degrees and aren't below steeper terrain is the most straightforward way of managing this hazard.

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