Avalanche forecast

Bitterroot

West Central Montana Avalanche Center
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Avalanche danger

Fri
Alpine
3 · Considerable
Treeline
2 · Moderate
Below
1 · Low
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Out of date — this forecast expired 4 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Skiers and riders are likely to trigger large persistent slab avalanches on upper elevation slopes steeper than 30 degrees. Large avalanches are possible at middle elevations. Northerly aspects are the most sensitive. Signs of instability, like recent avalanches, cracking, and collapsing, are sure signs of danger. 

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2.5

    The snowpack across much of the forecast zone has a poor structure. There are widespread layers of surface hoar and depth hoar buried 1.5 to 3 feet deep in the snowpack. These weak layers are getting more stubborn to trigger, but the consequences remain high. 

    Human triggered avalanches are likely on upper elevation slopes and possible at middle elevations. Northerly aspects are more likely to harbor instabilities. In areas where you find snowpack depths greater than 4 feet, triggering avalanches is less likely. Avoid terrain steeper than 30 degrees to reduce your risk. Avalanches can be remotely triggered from below, so give steep terrain a wide berth. Shallow areas and convexities are likely trigger spots. Recent avalanches, whumpfing, and cracking are all signs of unstable snow and should be weighted more heavily than snowpit stability test results. 

    (12/14/2022) Photo: Arden Feldman

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–1.5

    Small, stubborn wind slabs are possible on all aspects along upper elevation ridgelines due to shifting winds. Yesterday’s snowfall came in with light northwesterly winds, and previously, we saw extended periods of westerly, southerly, and easterly winds. Wind slabs often appear smooth and rounded, but fresh snow may have buried signs of old wind slabs. Avoid areas below ridgelines and along the edges of gullies steeper than 35 degrees. Stay alert for signs of instability and hollow feeling snow. Wind slabs could step down to deeper weak layers and trigger large persistent slab avalanches. 

Avalanche Discussion

Yesterday, we received 1-3 inches of snow throughout the region and light to moderate northwesterly winds. Today we can expect partly cloudy skies and light northwesterly winds. Temperatures will be in the 20s. 

The snowpack across much of the forecast zone has a poor structure. There are two widespread persistent weak layers buried in the snowpack. The most active is a layer of buried surface hoar found on all aspects at middle and upper elevations in the Bitterroot and Rattlesnake. The second weak layer is depth hoar at the ground on northern aspects at middle and upper elevations. Recent observations show that these weak layers are getting more stubborn but are still reactive. Persistent weak layers are slow to heal. Don’t let stubborn snowpit test results convince you to ignore the obvious poor snowpack structure. Consider the consequences - any avalanche that does break on buried weak layers will be large and dangerous. 

This is a challenging forecast to write because there is significant spatial variability in the hazard. In the Seeley Lake zone, the southern Swans have a shallower and more dangerous snowpack than the southern Missions. In the Bitterroot zone, surface hoar layers in the southern Bitterroot and Lolo Pass areas are more active than in the central Bitterroot. Considerable hazard exists in areas with shallower snowpacks and more persistent weak layers. However, in areas with deeper snowpacks, over roughly 4 feet deep, moderate hazard exists. Stepping out onto terrain up to 35 degrees in steepness may be reasonable in these deeper areas. 

Natural avalanches are unlikely, but large human triggered avalanches remain likely. Continue to be cautious. Be alert for signs of instability like cracking and collapsing. Remote triggering avalanches from above, below, or beside is possible. Likely trigger spots include shallow areas and convexities. Slopes steeper than 30 degrees are not recommended. There is great powder riding on low angle slopes. 

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Ski and ride safe. 

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