Avalanche forecast

Bitterroot

West Central Montana Avalanche Center
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Avalanche danger

Fri
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
1 · Low
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Out of date — this forecast expired 6 months ago. You’re viewing past conditions, not the current bulletin.

Highlights

The combination of new snow, strong winds, and weak layers are creating complicated avalanche conditions. Weak layers are buried a foot or more deep, and can create large avalanches. You may not see obvious signs of instability before triggering one. The best and safest riding will be found on low-angle slopes sheltered from the wind.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1.5–3

    Multiple weak layers are buried in the upper 1 to 2 feet of the snowpack. Over the past week these layers have produced many natural and human-triggered avalanches (click here to see observations).

    Weak interfaces exist on all aspects at middle and upper elevations, but change in character around the compass. Avalanches will be easiest to trigger on shaded northerlies, where weak layers are feathery surface hoar. On sunnier aspects these same weak layers are sugary facets or a facet-crust combination. Where facets exist instead of surface hoar, avalanches may be more stubborn to trigger, but will be just as large and dangerous if triggered.

    Avalanches that fail on persistent weak layers can break wide and run far. You are unlikely to see signs of instability such as cracking and collapsing before you trigger these avalanches. Importantly, you may be able to trigger these slides remotely — at a distance away from the slope that actually avalanches. To avoid this avalanche problem completely, you’ll have to avoid slopes over 30 degrees, as well as the flats below them. Cross run-outs one at a time, with someone watching.

    Buried surface hoar under 10 inches of recent snow. Compared to when this photo was taken, weak layers are now buried under a deeper and thicker slab.

     

  • Wind Slabs

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

    Yesterday we observed both human-triggered and natural wind slabs. With moderate winds and snow still available for transport, wind slabs will continue to build today.

    Look for blowing snow, drifts, and cornices as clues that wind slabs exist. Wind slabs feel like upside-down snow in localized areas. You are most likely to find wind slabs on the downwind sides of ridges, rollovers, and gulleys. With westerly winds we expect wind slabs mostly on easterly aspects. However, wind directions shifted slightly over the past 24-hours, and mountainous terrain can funnel wind in unpredictable directions. Watch for wind slabs on all aspects today. To avoid this problem, avoid freshly wind loaded slopes over 30 degrees.

    A human-triggered wind slab avalanche with cracking in the foreground.

Avalanche Discussion

The storm is over, but it’s left behind a snowpack ripe for human-triggered avalanches.

Heavy snowfall drew to a close by yesterday morning, just in time for winds to pick up. Moderate wind speeds and ample snow for transport make the perfect recipe for overloading a weak snowpack. With persistent weak layers present, avalanche hazard will remain elevated. Even as storm snow bonds to the old interface, weak surface hoar and facet layers remain below.

Persistent slab avalanches will be large and difficult to predict. Remember that avalanche terrain includes run-outs, and you could remotely trigger an avalanche above you. With a persistent slab avalanche problem, it is especially important to pay attention to overhead hazard.

Wind slab avalanches can step down to these buried weak layers, but are dangerous even on their own. Wind slabs can break wider than expected, and hard debris can increase the chances of injury, especially where there are terrain traps, such as trees and gulleys.

Pay attention to the snow as it changes today. Wind can quickly load a slope, and slabs can become more sensitive to triggers as wind speed or direction changes. Southerly slopes will see sun, which could create small wet loose avalanches or roller balls.

If you get out in the snow, take a minute to let us know what you see. Your observations are a key component to an accurate avalanche forecast. Information about avalanches, the amount of new snow and wind, and riding conditions are all very helpful. Thanks in advance for your time, we appreciate you. Click here to access the observation submission page.

It has been a tragic week in the world of the snow. At least 13 people have been killed in avalanches in the Western US. You can find more information here. Let's do everything we can to not add to that ghastly number. 

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