Avalanche forecast

Seeley Lake

West Central Montana Avalanche Center
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Avalanche danger

Sun
Alpine
3 · Considerable
Treeline
2 · Moderate
Below
1 · Low
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Highlights

Large human triggered avalanches are likely on upper elevation slopes and possible at middle elevations. Traveling on slopes over 35 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. 

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2.5

    The southern Missions have a deeper snowpack with fewer persistent weak layers than the Bitterroot and Rattlesnake. As a result, better stability was found in this area yesterday. October's snow forms a faceted crust on sheltered and north slopes and was the failure plane in tests yesterday. Due to its depth, 36”,  it is stronger and gaining strength but, if triggered, could produce large destructive avalanches. Yesterday signs of instability, such as whumpfing and cracking, were isolated to shallow, windswept ridgelines around rocks and stumps. These weak spots could be the trigger point for avalanches. 

    Avoid slopes steeper than 35 degrees to reduce your risk. Recent avalanches, whumpfing, and cracking are all signs of unstable snow; dial back slope angles if you encounter these. Avalanches may be remotely triggered from a distance. 

    The southern Swans may have a shallower snowpack and more reactive facets; choose lower angle terrain in this area.

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

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    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, and large wind slabs are likely. 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 could 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 35 degrees at upper elevations 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. 

Over the last week, we have received reports of avalanches and widespread signs of instability on buried weak layers throughout the region. The observations show that the persistent slab avalanche problem is still reactive in the Bitterroot and Rattlesnake. However, the Seeley Lake zone may have less of a widespread persistent slab problem than the rest of the region. WCMAC staff in the southern Mission Range yesterday reported a deeper snowpack with fewer persistent weak layers. However, last weekend observers in the southern Swan range reported widespread whumpfing and cracking, and weak snow below a slab. The persistent slab problem in the Seeley Lake zone may be more isolated to upper elevation northerly facing slopes. Or, there may be differences in stability between the Mission range and the Swan range. Either way, both persistent slabs and wind slabs are dangerous, so be cautious. 

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 may still be reactive, and the resulting avalanches could 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 are possible at middle and upper elevations. Remote triggering avalanches from above, below, or beside is possible. Continued snowfall and wind transport will make the problem touchier. Wind slabs are likely on upper elevation slopes facing west to north to east. 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 35 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. 

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