Bulletin d’avalanche

Bitterroot

West Central Montana Avalanche Center
Voir sur la carte

Danger d’avalanche

Sun
Alpin
3 · Marqué
Limite forestière
2 · Modéré
Sous la limite
1 · Faible
Open full map
Périmé — ce bulletin a expiré 4 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Large human triggered avalanches are likely on upper elevation slopes and possible at middle elevations. Traveling on slopes over 30 degrees is not recommended. Avalanches can be remotely triggered from adjacent slopes. Signs of instability, like recent avalanches, cracking, and collapsing, are sure signs of danger. 

Problèmes avalancheux

  • Persistent Slabs

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

    Large persistent slab avalanches remain likely on all aspects at upper elevations and possible at middle elevations. The snowpack across the forecast zone has a very poor structure. There are widespread layers of surface hoar and facets buried 1.5 to 3 feet deep in the snowpack. Observers continue to report significant signs of instability and recent avalanches on these layers. 

    These persistent slab avalanches are large, dangerous, and demand respect. Avoid slopes steeper than 30 degrees to reduce your risk. Avalanches can be remotely triggered from a distance. Use cautious route finding and give steep terrain a wide berth. Recent avalanches, whumpfing, and cracking are all signs of unstable snow. 

    Persistent weak layers are slow to heal. They may be getting more stubborn and harder to trigger, but they are still a serious hazard. Be patient and give steeper slopes the time they need to heal. 

  • Wind Slabs

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

    Moderate to strong southerly winds are building wind slabs on upper elevation slopes facing west to north to east. These slopes will be especially sensitive. There is a lot of snow available for transport, and the wind slabs are growing rapidly. However, these wind slabs rest on even more dangerous persistent slabs. If you trigger a wind slab, it is likely to step down to a deeper weak layer and create a large, dangerous avalanche. Because of this, the travel advice remains the same as persistent slab: slopes steeper than 30 degrees are not recommended. Stay back from cornices and the edges of ridges. Avalanches may be triggered remotely and break farther back into lower angled terrain than anticipated. 

Avalanche Discussion

Yesterday, we received 2-4 inches of snow throughout the region. The exception was the Seeley Lake zone, where 9 inches of snow was reported in the southern Mission range. Winds overnight were moderate to strong out of the southeast, and temperatures were in the teens. 

Today we can expect moderate to strong southerly winds, mostly cloudy skies, and isolated light snowfall. Temperatures will be in the 20s, but wind chills will be colder. The winds will weaken and shift to easterly on Sunday, and snow showers will pick up in the afternoon. Snowfall amounts Sunday night are uncertain but look to be light to moderate. 

The snowpack across the forecast zone has a very poor structure. Over the last week, we have received continued reports of avalanches and widespread signs of instability on buried weak layers throughout the region. Yesterday WCMAC staff reported continued whumpfing and cracking in the Rattlesnake. An extended column test produced a propagating result on a well developed layer of surface hoar buried 20 inches deep (ECTP 18). WCMAC staff in the southern Bitterroot found similar results. Additionally, they reported moderate southerly winds loading up northerly slopes, making them more sensitive. These observations point to a high likelihood of avalanches. 

We also have a layer of basal facets and depth hoar buried throughout the region. This layer is primarily on northern aspects and sheltered areas where early season snow from October did not melt. This layer has not been as reactive as the surface hoar and facets above. However, there is potential for full depth avalanches on this layer. 

This weekend you may find fewer obvious signs of instability while traveling in the backcountry. Observers report that the slab is getting deeper and stiffer. The persistent weak layers are more difficult to trigger - but don’t be fooled. The weak layers are still reactive, and the resulting avalanches will be larger and more dangerous. Avalanches may also pull back onto lower angled terrain. Be aware of potential trigger locations such as shallow or rocky areas. 

The avalanche hazard at lower elevations is generally LOW across the region. However, be alert for signs of instability in areas with deeper lower elevation snowpacks, like Lolo Pass. 

Large human triggered persistent slab avalanches remain likely at upper elevations and possible at middle elevations. Remote triggering avalanches from above, below, or beside is possible. Continued snowfall and wind transport will make the problem touchier. On slopes with significant wind loading, natural avalanches are possible. Given enough time and additional snowfall, these problems will heal. However, it will take time. For now, continue to be cautious in the backcountry. Slopes steeper than 30 degrees are not recommended. There is great skiing and riding on low angle slopes. 

Thank you to everyone that has submitted an observation in the last week. They are critical to producing these forecasts. Please continue to tell us what you’re finding out there by submitting an observation. 

Ski and ride safe. 

Régions couvertes