Bulletin d’avalanche

Bitterroot

West Central Montana Avalanche Center
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Danger d’avalanche

Sun
Alpin
4 · Fort
Limite forestière
3 · Marqué
Sous la limite
3 · Marqué
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Points saillants

The threat of natural and human-triggered avalanches is greatest in upper elevation terrain where the wind has been at work. Expect to find 1-4 foot thick slabs that are reactive to your weight on steep, wind-loaded slopes. Some of these will fail naturally today as snowfall continues and the wind adds mass to existing slabs. Enough snow has fallen that slabs breaking within the new snow in terrain sheltered from the wind are also on the menu. Natural and triggered slides can step down to weak layers buried in the snowpack, producing unsurvivable avalanches.

The elevated hazard at low elevations is for the terrain around Lolo Pass. Much of the low elevation terrain in this zone was bare and snow-free prior to this week's storm.

Problèmes avalancheux

  • Storm Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Very Likely
    Taille
    1–2.5

    With 4-6 feet of new snow in this zone since Wednesday, you will encounter a wide range of storm-related instabilities today:

    WIND SLABS:  Slabs in the new snow will be thickest, most reactive, and most dangerous where the wind has been at work. These slabs will be hard to identify visually thanks to the ongoing snowfall, but you may feel an increase in density as you move through them. STORM SLABS:  Temperatures appeared to stay warm overnight as an additional 12-18" of snow fell in this zone. You may find large, reactive slabs even in terrain completely sheltered from the wind. With both wind and storm slabs, cracks shooting out from your machine and your skis are clear signs of instability.LOOSE SNOW:  With copious amounts of new snow, expect to be able to trigger loose snow avalanches on steep slopes. Down lower where the recent snow is moist to wet, these will have a wet character. Up higher, where temperatures are cooler, these sluffs will be dry.

    Enough snow has fallen in recent days that these storm instabilities can produce avalanches that are easily large enough to bury you. The thick coat of fresh snow is also adding weight to the weak layers buried deeper in the snowpack. Slides triggered in the recent snow can step down to these weak layers, producing very large, unsurvivable avalanches. More on this in the Persistent Slab problem section below.

    (3/13/2026) Photo: West Central Montana Avalanche Center

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    2–3.5

    Weak layers buried in January and February produced natural and human-triggered avalanches across our region right up to the start of this week's storm. Initially buried 1-3 feet down, you'll now find these layers 3-5+ feet below the snow surface. Take a look at the photo at right/above of a skier-triggered slide in the backcountry terrain adjacent to Lost Trail. Use your imagination to add 4-6 inches of snow water equivalent to the slab that failed and you get a good sense of the size of slides that we are worried about.

    Any avalanche that breaks down into these weak layers will break widely and put massive amounts of snow into motion. Slides may run long distances into flat terrain and timber at the base of steeper slopes. Avalanches within the new snow that break naturally or that you trigger can step down to these deeper weak layers. Completely avoiding this problem includes avoiding avalanche terrain threatened from above. 

    (3/7/2026)

    Tracks and crown of a large skier-triggered avalanche in the backcountry terrain adjacent to Lost Trail that was triggered on March 6th. The avalanche failed around two feet down on a layer of surface hoar that was buried in mid-February.

    Photo: Ben VandenBos

Avalanche Discussion

Wow, it has been quite a week in our mountains! The storm that tapers today began on Wednesday. Water amounts for this zone are in the 4-6" range, stacking up over 4 feet of snow up high. In many places, you'll start your day down low where the recent snow fell on bare ground. Here, avalanche issues are quite limited. The exception to this is the Lolo Pass area, where large amounts of new snow have fallen. As you climb, you'll find increasingly dangerous conditions. Be on the lookout for wind and storm slabs as you move into middle elevation terrain. These slabs will become larger and more reactive as you move towards upper elevation terrain where the wind has been at work for days. With loads of snow to work with, avalanches involving just the new snow will be plenty big to bury you.

Several weak layers are buried underneath the pile of new snow. These layers were producing avalanches prior to the heavy load of snow that we just stacked on top of them. Predicting when these weak layers will reach their breaking point is challenging to say the least, but it's safe to say they are grumbling down there. The safe bet is to move through the mountains expecting that slides may break down into these weak layers. Build a wide safety margin into your travel plans. Pay attention to the terrain above you, expose one rider to hazard at a time, and don't linger in big avalanche paths (like those along the Lost Horse road). Remember that managing the storm related instabilities does not reduce your risk of tangling with one of these monsters. Large failing cornices and wind slabs can be effective triggers for these buried weak layers. 

Today's forecast comes with an extra helping of uncertainty. Weather models predicted another round of significant snowfall overnight, with 12-18+ inches along the crest of the Bitterroot (1-1.5 inches SWE), paired with strong winds. Unfortunately, the SNOTEL network went down yesterday afternoon and we have little weather data to work with. We'd love to hear about what you are finding out there; riding conditions, snow depth at the trailhead, and a little bit about the weather you experienced are all extremely useful. Consider submitting an observation here; we really appreciate your time. 

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