Bollettino valanghe

Bitterroot

West Central Montana Avalanche Center
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Pericolo valanghe

Sun
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

Triggering an avalanche is possible anywhere the new snow has been blown into drifts six inches deep or more. Fresh snow may hide older wind slabs from the past 72 hours, which could create large avalanches if triggered. Watch for red flags like shooting cracks, collapsing, or recent avalanches. If you see these, stick to slopes less than 30 degrees.

Persistent slab avalanches are possible in the southern Bitterroot and unlikely in the rest of the zone. See the problem description below for more details. 

Problemi valanghivi

  • Wind Slabs

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

    In the past four days, Lolo Pass and Twin Lakes have received about 35 inches of snow (3.5 – 4 inches of water), and Lost Trail has received 12 inches (1.4 inches of water). Snowfall was accompanied by moderate westerly winds. This is the ideal recipe for building wind slabs on easterly slopes. Both natural and human-triggered wind slabs have been reported in the past three days. Click here for avalanche observations from Lost Horse, and here for an observation from the Lost Trail area.

    While we don’t expect new wind slabs to form today, triggering a wind slab from the past few days is still possible. Identify wind slabs by looking for pillow-shaped deposits and drifts. They are most likely to form just below ridgelines, below roll-overs, and in cross-loaded gullies. Avoid slopes over 30 degrees where you see signs of wind loading.

    (1/7/2026)

    A wind slab human triggered by cornice on an upper elevation east aspect

  • Persistent Slabs

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

    The southern Bitterroot has a crust over facets layer buried just over a foot down. This weak layer is most prevalent at high elevations between 7000 and 8000 feet. In the northern Bitterroot, where we’ve received more snow, this same crust is much deeper and does not overlie such an obvious facet layer. With this snowpack difference, we only expect this persistent weak layer to be a problem in the southern part of this range.

    At high elevations we’ve found small facets buried just over a foot deep. This layer will be less than a foot deep in the Southern Bitterroot, where there has been less recent snow. These facets formed on the snow surface in late December and early January before getting buried by this week's snow. We have had no reports of avalanches on this layer, but we’ve gotten propagation on it in snowpack tests.

    On solar aspects the sun likely destroyed any facets before they were buried. So, look for this weak layer on high-elevation shady slopes.

    Regardless of the weak layer, you are unlikely to see obvious signs of instability, like cracking or collapsing, before triggering a persistent slab avalanche. Dig down and look for weak layers before committing to a line. Strong over weak snowpack structure or propagating extended column tests indicate you should avoid slopes over 30 degrees.

    (1/9/2026)

    Snowpit on a northeast aspect at 7600 ft in the northern Bitterroot

Avalanche Discussion

Snowfall is coming to an end, and we’re moving into a period of high pressure for the weekend. As we put time between ourselves and the storm, the snowpack’s sensitivity to triggers will decrease. However, lingering wind slabs will remain possible, and a few buried weak layers still have our attention.

With limited observations, uncertainty exists regarding the near surface facets buried about a foot deep. They formed during the New Year’s dry spell, and are likely only preserved where the snow remained cold. We haven’t seen avalanches on this layer, however visibility has been limited. Up near Whitefish, a skier triggered an avalanche on what appears to be the same layer. Click here for more information.

The weak layer in the Southern Bitterroot is associated with a crust buried on December 26th, with underlying facets. While this crust exists in the rest of our forecast area, we haven’t found such obvious facets underneath it elsewhere. Snowpack tests have found propagation under this crust in the Southern Bitterroot only.

Clearing weather will give us a chance to look around for fresh avalanches. If you see any, please let us know. If you dig in the snow, please let us know what you find. Tests are helpful but not necessary, even a photo of your snow pit wall is useful. Your information helps us reduce uncertainty, which leads to a more accurate forecast.

 

On January 5th, a snowmobiler was killed in an avalanche near Lake Tahoe. Our condolences go out to the family and friends. If we look at the preliminary accident report, we find terrain and wind slab avalanche size that resembles much of the motorized terrain in our forecast zones. We mention this as a reminder and learning opportunity that even small avalanche paths can be dangerous if they could push you into trees or other terrain traps. 

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