Boletín de aludes

Bitterroot

West Central Montana Avalanche Center
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Peligro de aludes

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Alta montaña
2 · Moderado
Límite del bosque
2 · Moderado
Bajo el límite
2 · Moderado
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Desactualizado: este boletín caducó 6 months ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

New snow issues will be the most obvious today, and their size sensitivity will depend on the amount of snow you find. Watch for wind slabs in steep, open terrain exposed to the wind. Expect to be able to trigger small loose snow avalanches that may slide quickly on crusts under the new snow. Below the new snow, well-defined weak layers exist in the upper 1-2.5 feet of the snowpack. Any slides that break down to these layers will be large, wide, and destructive. Avalanches that start in the new snow may break down to these buried weak layers. 

The elevated hazard at low elevations focuses on the Lolo Pass area. Expect to find still-wet snow under a cap of drier, fresh snow and to be able to push wet loose snow avalanches here as the day warms.

Problemas de aludes

  • Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    2–3

    Prominent weak layers are buried in the upper 1-2.5 feet of the snowpack. Dig down and you'll find facets, surface hoar, and crust+facet combinations, depending on your aspect and elevation. Last week, during an intense storm, these layers produced a widespread natural avalanche cycle (you can view recent avalanches here). Many slides broke hundreds of feet wide. The largest slides spanned 1,500-2,000' feet of terrain (photo at right/above). These layers became less reactive and harder to impact as the slab above them stiffened, but yesterday's storm is keeping them on their toes.

    Avalanches that fail on these layers of weak snow will break widely and will be able to wrap around terrain features. This produces large, unpredictable avalanches with deep piles of debris that you do not want to be in. You can trigger these slides from below or near steep slopes. Completely avoiding this problem includes avoiding terrain threatened from above. 

    This recent, very wide persistent slab avalanche failed near the Lost Horse drainage on a E-SE aspect at 8,000'. It broke roughly 2,000 feet wide. The crown spans the entire width of this photo, and continues out of sight.

    Photo: Ben VandenBos

  • Wind Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1–2

    Yesterday's storm favored the crest of the Bitterroot, with much less snow falling in other portions of the range. Along the crest, up to a foot of snow accumulated in the past 36 hours, accompanied by gusty winds blowing out of the NW-W. If you are headed for steep, upper elevation terrain, you'll want to be on the lookout for these fresh slabs. Triggered slabs will run fast and far on the firm snow surfaces underneath.

    Expect to find these near ridgelines and midslope features like gullies that slow the wind down and force it to drop the snow it is carrying. Pay attention to the snow you are moving through. If you are suddenly finding much more snow, or if the snow is becoming thicker and denser, you have likely found a wind slab. Cracks shooting out from your machine or your ski tips are clear signs that the slab you found is unstable. Triggered wind slabs will be plenty large to ruin your day, and will be effective triggers for the larger persistent slab avalanches described above as they run downslope. 

    Avalanche in steep, wind-loaded terrain in the Bitterroot Mountains. 

    (2/28/2026) Photo: Ben VandenBos

Avalanche Discussion

Yesterday's storm ended up producing the lower end of forecast snowfall amounts. It appears that much more snow fell along the crest of the Bitterroot (the MT/ID border) than further east along the Bitterroot Valley. Wherever you find yourself playing today, you can identify the new snow hazard by paying close attention to how much snow fell, and how much work the wind did. If you are finding a foot of snow and wind drifts 1-2 feet thick, triggering wind slabs will be easy in steep, open terrain. If you are closer to the range front and finding 4-6 inches of snow, you'll really have to go looking to find the wind slab problem. Across the zone, the new snow is resting on a variety of crusts and firm surfaces and triggering loose snow sluffs will be common in steep terrain. If the sun is out and the new snow is becoming wet, these slides will be more reactive and able to entrain larger amounts of snow as they travel downslope. 

Don't let the new snow distract you from the less obvious, harder to identify, but uncomfortably likely persistent slab problem. We are only a week out from an intense storm and a days-long wind event that produced a widespread avalanche cycle (you can view recent avalanches here). In the days since that big storm, the snow settled into a conditional stability. The frequency of cracking and collapsing decreased significantly, and lots of folks were poking into steeper terrain and not triggering slides. Yesterday's storm didn't appear to bring enough snow to tip the balance in the other direction, but it also isn't helping. Keep the persistent slab problem and these buried weak layers at the front of your mind as you move through the mountains today. Triggering a persistent slab remains a real possibility, and any slide involving these layers will be large. You're most likely to trigger one of these in steep, open terrain where recent and past winds built stiff slabs over these weak layers.

Nata and Cam were out in this zone yesterday, you can see videos documenting what they found in the media gallery below. 

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