Bollettino valanghe

Bitterroot

West Central Montana Avalanche Center
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Pericolo valanghe

Fri
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 8 months ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Large and destructive human triggered avalanches are possible in areas with buried weak layers. Smaller avalanches are likely in wind drifted or wet snow. 

Look out for signs of instability like recent avalanches, shooting cracks, whumfing, and rollerballs. Dig in the snow, and if you find weak sugary snow with a slab above, avoid avalanches by sticking to slopes less steep than 30 degrees.

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1.5–2.5

    It remains possible to trigger a large persistent slab avalanche that fails on weak layers buried 2-4 feet deep. These weak layers include crust facet interfaces and facets near the ground. You may not see any red flags before triggering an avalanche on these layers. 

    The crust facet layer is the most widespread and concerning. This layer is buried 2-3 feet deep and produces full propagation in snowpit tests, according to several recent observations. It can be a tricky persistent weak layer to manage and highly elevation dependent. This problem could be more sensitive today in areas where rain fell last night on a poor snowpack structure. If you hear whumfing or feel collapses, these are indicators that you are in an elevation band with avalanche potential. You can also dig in the snow and look for poor snowpack structure with a slab above a crust facet layer. Manage the problem with slope angle and stick to slopes less than 30 degrees. Persistent slabs could also be triggered remotely, from low angle slopes above, below, or to the side of steeper slopes. 

    Triggering a large and destructive persistent slab avalanche on weak snow near the ground is still possible on high-elevation northerly slopes. These are locations where early-season weak layers were buried, and were not saturated by rain earlier in December. This setup only exists in isolated locations, but the consequences will be high if you trigger one of these avalanches. Avoid slopes over 30 degrees if you find weak sugary snow with a slab above.

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–1.5

    Small wind slabs are likely today at upper elevations. Recent shifting winds built wind slabs on all aspects. 2-4 inches of snow fell at upper elevations above the rain/snow line last night, with light to moderate southwest winds. Yesterday, south and east winds deposited snow on unusual north and west aspects. Observers found small fresh wind slabs along ridgelines that were touchy and failing naturally. Additionally, strong west winds over the weekend built wind slabs on east aspects. 

    Today, you could trigger a fresh wind slab or an older, lingering wind slab. Wind slabs can be identified by their smooth or pillow like appearance. They often form just below ridgelines, in gullies, and below convex rollovers. If you see signs of instability like recent avalanches or cracking around your skis or sled, stick to low angle slopes less than 30 degrees. Small avalanches can still knock you off your feet and lead to serious consequences. Also, a smaller wind slab could step down and trigger a larger persistent slab.

  • Loose Wet

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–1.5

    Small wet loose avalanches are likely today due to warm temperatures and rain up to middle elevations. Last night, rain fell with rain/snow levels potentially reaching up to around 7000 feet. Scattered showers will continue today with rain/snow levels above 6000 feet. 

    If you find yourself in mushy, wet snow or see rollerballs or wet loose avalanches, it’s time to avoid avalanche terrain and be aware of what terrain is above you. Remember that even small avalanches can have serious consequences.

Avalanche Discussion

Merry Christmas! The WCMAC team is excited to be back to issuing full avalanche forecasts. Avalanche forecasts will be issued on Thursdays, Fridays, Saturdays, and Sundays. A general information product will be issued on Mondays to cover the Monday-Wednesday period. We hope to begin forecasting for the Seeley Lake zone next week. Thank you for your patience while we worked through our agreement with the Forest Service! 

It’s a complicated day in the backcountry with several overlapping avalanche problems to manage. The persistent slab problem is the most concerning because it could produce a large and destructive avalanche. It’s a low probability, but high consequence problem. The wind slab and wet loose avalanche problems are more likely, but smaller in size. There is also significant uncertainty in the forecast today. The uncertainty stems from a lack of recent observations, fluctuating rain/snow levels, and recent shifting winds. 

A cold front is forecast to arrive tomorrow with fresh snowfall, strong winds, and cooler temperatures. Stay alert for changing conditions. 

Let us know what you see by submitting an observation. We appreciate all observations, no matter how small, as our forecast area is large, and it helps us understand conditions across all zones. 

Ski and ride safe. 

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