Bulletin d’avalanche

Bitterroot

West Central Montana Avalanche Center
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Danger d’avalanche

Mon
Alpin
3 · Marqué
Limite forestière
2 · Modéré
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

Human triggered slab avalanches are possible today at middle and upper elevations. These avalanches have the potential to be large and consequential, especially if a smaller avalanche steps down to weak layers near the ground. Signs of instability may not be obvious. The best way to avoid this problem is to dig into the snow and avoid slopes with weak, sugary snow underneath firmer snow. If you encounter this, stick to slopes under 30 degrees. If you find yourself in wet, unsupportable snow, especially if it rains, anticipate wet snow avalanches and get out of avalanche terrain.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2.5

    Large human triggered persistent slab avalanches remain possible today at middle and upper elevations. Numerous persistent weak layers are buried within the snowpack, including surface hoar, facets, crusts, and depth hoar. New snow and recent rain have added complexity to an already poor snowpack structure.

    The best way to identify this problem is to dig into the snow. Poor snowpack structure is identified as firmer or harder snow sitting above softer, sugary snow. Buried surface hoar can be difficult to identify but often appears as a thin grey line across a snowpit wall. Buried crusts often have soft, sugary snow just above or below them.

    Signs of instability like cracking, collapsing, whumpfing, and recent avalanches may not be obvious today. Expect little to no red flags before a slope avalanches. You can trigger an avalanche from a distance (remote triggering) that can break wider and higher on slopes than expected, so be aware of the terrain above and around you. A small slide in loose surface snow can step down to weak layers and initiate a large avalanche. If you find poor snowpack structure or signs of instability, stick to slopes under 30 degrees and keep a distance from steep slopes above or around you.

Avalanche Discussion

Temperatures reached above freezing yesterday, and light rain fell through upper elevations in many parts of the forecast region in the last 24 hours. Light to moderate wind blew overnight, and temperatures hovered around freezing.

Today is expected to be slightly cooler than yesterday. However, above freezing temperatures are anticipated again, particularly in the central and southern Bitterroot. A small amount of precipitation in the form of snow or freezing rain with gusts up to 24 mph from the southwest is forecasted.

We received reports of instability yesterday due to warming and poor snowpack structure. Observers noted wet loose avalanches through upper elevations, poor snowpack structure on many aspects, and widespread loud collapsing. These clear signs of instability may not be so obvious today. You might ski or sled a slope several times before triggering an avalanche.

The warming adds some uncertainty to conditions. Any type of significant warming and liquid water on snow increases sensitivity quickly, especially when surface snow is new/recent and low-density. But the slab will have become more stubborn if refrozen. While temperatures dropped to around freezing overnight, cloud coverage may have inhibited nighttime cooling within the snowpack. Ideally, gusty winds will aid in keeping the snowpack cooler throughout the day, and precipitation will fall in the form of rain only at lower elevations.

It will also be important to watch for wet snow today. If frozen slabs start to break down or never truly froze overnight, you will likely encounter wet loose avalanches. A small wet loose avalanche could be the trigger the persistent slab problem needs to release. Roller balls, pinwheels, and recent wet loose avalanches indicate that surface snow is becoming wet. You can also keep an eye out for snow falling off of trees in large clumps.

Ultimately, conditions today are complex. Use tools in your toolbox, like digging into the snowpack, being observant of the terrain and conditions around you, and using clear communication with your group. You can easily default to low angle slopes that aren’t connected to steeper terrain to increase your margin of safety. Know that the persistent slab problem will heal with time. It remains essential that you practice patience and self-control while we wait.

Please submit an observation of what you’re seeing out in the backcountry. Reports of avalanche occurrences, weather conditions, and snowpack observations are all very helpful for the forecast team.

Ski and ride safe!

Régions couvertes