Avalanche forecast

Bitterroot

West Central Montana Avalanche Center
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Avalanche danger

Mon
Alpine
2 · Moderate
Treeline
1 · Low
Below
1 · Low
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Highlights

Human triggered wind slab avalanches are possible on north and east aspects at upper elevations today. Red flags include shooting cracks, collapsing, and recent slab avalanches. This problem could trigger a small dry loose avalanche that entrains more snow on the way down. If you see signs of instability, stick to low angle, wind sheltered terrain.

Although unlikely, you could still trigger a large persistent slab avalanche at upper elevations that fails on weak snow buried near the ground. Don't expect any warning signs such as cracking and collapsing before triggering this problem. Use safe travel practices, and dig down to look for poor snowpack structure (firmer snow sitting above weak, sugary snow). If you find poor structure and want to avoid the problem, stick to low angle slopes.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    Small fresh wind slabs are possible today on upper elevation leeward slopes. Around 1 foot of unconsolidated recent snow is at the surface in all three zones. Increased wind speeds that occurred later in the day yesterday may have begun wind slab formation. With an incoming weather system entering our forecast region this afternoon, another increase in wind speeds will continue to build larger slabs. While these slabs will be relatively small in size for most of the day, expect them to entrain soft snow lingering below if triggered. A smaller slide that might normally have low consequences on its own is likely to have larger consequences today. 

    Wind slabs can be found on or just below the downwind side of exposed ridgelines, in cross loaded terrain features such as gullies, behind rocky outcrops, and on rolls or convexities. They can look smooth, rounded, and pillowy.  Snow will feel more dense and consolidated underneath your skis or sled than surface snow that has not been wind affected. It may even be hard and sound hollow like a drum. Be on the lookout for rippled, uneven snow surfaces, indicating that wind has transported snow, and wind slabs may be lurking nearby.

    Red flags include cracking, collapsing, and recent slab avalanches. Cracks shooting out from your skis or machine clearly indicate that you've found an unstable slab. If you find signs of instability, stick to wind sheltered slopes less than 35 degrees.

    (2/3/2024)

    We intentionally triggered this D1.5 wind slab in the new snow on a wind loaded east aspect at 7700 ft. It was 6-10 inches deep and around 75 feet wide. 

    Photo: Arden Feldman

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    1.5–2.5

    Although unlikely, skiers and sledders could still trigger a large persistent slab avalanche on upper elevation slopes. These avalanches could fail 2 to 4 feet deep on buried persistent weak layers near the ground. 

    The problem exists in isolated areas starting at 6500 ft and becomes more widespread as you get higher in elevation. North and east aspects have the most concerning snowpack structure, with a stiff slab above dry, weak, sugary facets near the ground. These aspects will have experienced some new loading at upper elevations from wind today and may only need the additional weight of a rider for failure to occur. A poor snowpack structure also exists on south and west aspects where the facets are still wet from last week’s warm temperatures but continue propagating in snowpit tests. 

    The consequences of triggering a persistent slab avalanche remain high. You aren’t likely to see signs of instability, such as cracking and collapsing, before triggering this problem. Previous tracks on a slope are not indicators of stability - avalanches may be triggered after many riders have negotiated a slope. Shallow, rocky sections are the most likely trigger points.

    The best way to identify the problem is to dig down into the snowpack and look for poor structure with a firm slab sitting on top of weak, sugary snow. If you find this, individual risk tolerance will dictate how steep of a slope you are willing to expose yourself to. You can reduce your risk by defaulting to shorter, planar slopes with runouts free of surface obstacles and terrain traps. If you want to avoid the problem altogether, stick to slopes less than 30 degrees.

    (1/27/2024)

    A persistent slab avalanche.

Avalanche Discussion

A period of moderate wind blew through the forecast region yesterday in the afternoon. Overnight, the wind remained light with some moderate gusts. Today, clouds and wind speeds will increase in the afternoon with an incoming weather system. Scattered snow showers will develop this evening through Monday morning, resulting in some light accumulations that will favor the northern end of the forecast region.

In addition to wind slab avalanches entraining soft surface snow on their way down, you could still trigger small dry loose avalanches in steep terrain on their own. Soft and unconsolidated recent snow sits on a hard melt freeze crust in many areas that may act as a slick sliding surface. Hazards such as cliffs or rocks that still exist under little snow could cause traumatic injury if you are carried over them. These avalanches will be larger on colder and shadier slopes where the snow has not settled as much and remains softer and drier. Utilize techniques to manage sluffing from your skis or sled, like minimizing time spent in long-running gullies where debris can concentrate.

While the persistent slab problem has been becoming largely unreactive, it remains a concern as buried weak layers such as facets, depth hoar, surface hoar, and crusts persist within the snowpack. You are most likely to trigger a persistent slab avalanche in areas with shallower snowpacks, and these avalanches will be most consequential in high, north and east facing terrain. It should be noted that we found a much more well-bonded snowpack on the west side of the Seeley Lake zone this week. The facets near the ground had rounded and gained significant strength. The higher snowfall amounts in this part of the zone have contributed to the healing process.

Current surface instabilities have the potential to step down and create a large, destructive persistent slab avalanche. As you head out into the mountains, be sure to evaluate not only what is "above",but also what is "below". Managing the persistent slab problem and choosing to enter avalanche terrain depends on personal and group risk tolerance. Use safe travel practices, such as riding one at a time, regrouping in protected areas, and spreading out on a slope. Facilitate clear communication with your group members and respect any vetoes that arise.

Additional snowfall and wind are expected to arrive during the day tomorrow, favoring the northern end of the forecast region. The Bitterroot should expect to see 2-4 inches of new snow, and the Rattlesnake and Seeley Lake zones should see 3-6 inches. As conditions change and complexity increases, we advise you to pause and carefully evaluate terrain you've been playing in recently. This process started yesterday and will continue over the coming days as active, wintery weather impacts our region. We are all hungry for powder, but we encourage you to take a moment to clear your head and think about the avalanche problems that exist in the terrain you plan on visiting.

Our intern at the avalanche center is conducting graduate research on avalanche forecast usage. Data from this research will help the avalanche center learn how to better serve the backcountry community in west central Montana. Please consider taking a few minutes to fill out the survey.

Ski and sled safe!

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