Bulletin d’avalanche

Bitterroot

West Central Montana Avalanche Center
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Danger d’avalanche

Mon
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 3 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Complicated and dangerous avalanche conditions exist today due to rainfall and warm temperatures impacting an already unstable snowpack. Large and potentially destructive human triggered avalanches are likely at middle and upper elevations. Smaller wet loose avalanches could trigger larger slab avalanches on buried weak layers. 

Look for red flags like recent avalanches, cracking, collapsing, rain on snow, or rollerballs. If you encounter any of these or find a poor snowpack structure with a firm slab over weak, sugary snow, stick to low angle slopes without steep terrain above.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1.5–2.5

    Large and potentially destructive persistent slab avalanches are likely at middle and upper elevations today. Yesterday, rain fell up to at least 7000 feet throughout the region. Light rain is forecast to continue today, with rain/snow levels rising even higher. The rain and warm temperatures are decreasing the strength of the snowpack and increasing the likelihood of triggering an avalanche. Natural persistent slab avalanches are also possible today. 

    A variety of overlapping buried weak layers, including surface hoar, facets, crusts, and depth hoar exist on all aspects at middle and upper elevations. You can recognize persistent weak layers and poor snowpack structure by digging into the snowpack and looking for a dense slab on top of weak, sugary snow. Buried surface hoar can be more difficult to identify in a snowpit but often appears as a thin horizontal line across the snowpit wall face. 

    Don't try to outsmart the persistent weak layers right now. It has been a busy week with both natural and human-triggered avalanches on all these layers. Today’s rain and warm temperatures will make conditions even more sensitive. 

    Signs of instability, like cracking and collapsing, may not be present before you trigger an avalanche. If you find recent avalanches, signs of instability, poor snowpack structure, or unsupportable wet snow, stick to low angle slopes. Be aware of the terrain above and around you, as you can trigger an avalanche from a distance (remote triggering).

    (1/20/2024)

    Looking up at the crown of a large human triggered persistent slab avalanche on an upper elevation east aspect in the Rattlesnake. 

  • Loose Wet

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–2

    Wet loose avalanches are likely today due to ongoing light rainfall and unseasonably warm temperatures. The rain is falling well into upper elevations, with rain/snow levels forecast at 7500 to 8000 feet today. Last night, temperatures did not drop below freezing at most elevations. 

    If you sink into a wet snow surface or experience rain on snow, expect wet snow avalanches in steeper terrain. Rollerballs, pinwheels, and small point releases are indicators that the snow is losing strength, and wet loose avalanches are likely to follow. 

    Even a small avalanche can be consequential if it pushes you into rocks, trees, or terrain traps like streambeds and gullies. In areas where buried persistent weak layers exist, rollerballs and wet loose avalanches could step down and trigger larger persistent slab avalanches.

    (1/21/2024)

    Numerous wet loose avalanches released yesterday.

    Photo: Payton Schiff

Avalanche Discussion

In the last 24 hours, 0.1 to 0.5 inches of precipitation fell across our forecast region. This is due to a warm and wet atmospheric river just to our west in Washington and Oregon. With unseasonably warm temperatures, the precipitation fell mostly as rain, even at upper elevations. One observation from yesterday noted rain all the way up to 8300 ft in the Seeley Lake zone. The highest peaks may have picked up some dense snowfall, or it may have all fallen as rain. Last night, temperatures stayed above freezing at most elevations. Today, light rain is forecast to continue with warm temperatures and rain/snow levels rising even higher. 

There is some uncertainty in today’s forecast. We know that rain falling on a dry snowpack is a significant red flag. It decreases the strength of the snowpack and can create complicated and dangerous avalanche conditions. However, we don’t know exactly how the rain will interact with the persistent slab problem. In any case, the rain, combined with our already abnormally weak snowpack and the multiple close calls in the past week, should be more than enough reason to keep it mellow today. Dial back the terrain choices and stick to low angle slopes. Check out the observations page and the week in review video for more information about the numerous recent avalanches. 

The persistent slab problem is touchiest in the Southern Bitterroot where a layer of buried surface hoar is especially well preserved and has produced several skier triggered avalanches and close calls. 

Looking at the week ahead, the rain will dissipate by tomorrow, but unseasonably warm temperatures will remain into the middle of the week. Both persistent slab avalanches and wet snow avalanches will remain a concern, especially if the snowpack does not freeze overnight. We are forecast to return to colder weather by next weekend. 

A report on the avalanche incident outside of Phillipsburg has been released. Thanks to the Beaverhead-Deerlodge forest for facilitating the investigation.

Please continue to submit observations. We appreciate every observation we get, and it really does help us get a better understanding of what is happening across the forecast area. 

Ski and ride safe.

Régions couvertes