Avalanche forecast

Bitterroot

West Central Montana Avalanche Center
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Avalanche danger

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

Human-triggered storm slab avalanches are likely in the new snow at mid- and upper elevations today, and natural storm slab avalanches are possible.

Expect avalanche size to increase as snow depths increase with elevation or in leeward terrain. Use test slopes and look for red flags like cracking and recent avalanches.  If you see these, stick to slopes under 30 degrees. Storm slab avalanches have the potential to step down and trigger a persistent slab avalanche today. 

Larger persistent slab avalanches on buried persistent weak layers are possible and remain a low probability but with high consequence. This problem exists in upper elevations where the snowpack has remained dry through the recent warm weather. Red flags like cracking or whumfing may not be obvious before a persistent slab avalanche fails.  Use slope angle to avoid this problem.

Avalanche problems

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    Since the storm began yesterday, between 6 and 12 inches of new snow have fallen at upper elevations with minimal wind effect. This snow has fallen “right side up, " with lower-density snow falling onto higher-density snow from the start of the storm. 

    Storm slab avalanche size will be determined by the amount of snow in the area where you are looking to ski or ride. If you find new snow depths of a foot or more, expect avalanche size to increase accordingly.  These avalanches have the potential to run far and fast on the firm bed surface of melt-freeze crusts that formed during the warm weather ahead of the storm.

    Yesterday, observers noted natural avalanche activity in the Rattlesnake and were able to trigger two avalanches in the new snow. This sensitivity will continue today. 

    Although snow levels have dropped to valley floors, there is not enough snow at lower elevations for a continuous bed surface for avalanches. 

    Storm slabs are fairly straightforward to identify, and instability is short-lived. Test slopes are an excellent tool for a problem like this.  Look for shooting cracks, collapsing, and fresh avalanches. Avoid convexities and areas above terrain traps like gullies, streambeds, or road cuts. Use slope angle to avoid the problem and stick to slopes under 30 degrees where avalanches are unlikely. 

     

    (2/3/2024)

    A natural slab avalanche in the new snow on a wind loaded southeast aspect at 7800 ft in the Rattlesnake. It was 6-12 inches deep and around 100 feet wide. 

    Photo: Arden Feldman

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1.5–2.5

    Large avalanches failing on persistent weak layers in the lower third of the snowpack remain a high consequence possibility. 

    You are most likely to encounter and trigger an avalanche on this problem in areas where the snowpack has remained dry and no melt-freeze crust exists at the surface. Avalanches will be much less likely on south and west aspects where the top portion of the snowpack has been wet and will freeze into a supportable crust by the end of the day today. 

    Digging in the snow and looking for weak, sugary facets below a firm slab remains the best way to identify the persistent slab problem. Red flags like collapsing or whumfing may not be obvious, and the first person to ski or ride the slope may not trigger the avalanche. 

    We have adjusted the distribution of this problem in the forecast infographic. Our elevation bands overlap slightly (see blog post). The distribution in the field remains the same. The persistent slab problem exists in isolated areas at 6500’ and becomes more widespread as you get higher in elevation. 

    With a low-probability, high-consequence problem like this, individual risk tolerance will dictate how steep of a slope you are willing to expose yourself to. However, if you want to avoid the problem, avalanches are unlikely on slopes 30 degrees or less.

    Photo: Cam Mackenzie

Avalanche Discussion

Today should be a great day for skiing and riding in the mountains of western Montana, but there are some red flags for an avalanche accident. A foot of new snow on a weekend after a warm, snowless winter will make any of us want to rush out the door for first tracks.  Take the time to carefully assess conditions, and don't let the powder panic cloud your decision-making. Storm slab instability, especially when the snow falls right side up, is short-lived, with peak instability measured in hours.

Below the new snow, the snowpack is generally stable at this point. Warm temperatures for the past week caused a significant avalanche cycle of both dry slab and wet loose avalanches. The snow that didn't slide naturally shrunk down and became wet. With the arrival of cold temperatures yesterday, the previously wet snow is in the process of freezing and locking up into stout and supportable melt-freeze layers.  There is still some water and warmth in the middle of the snowpack, especially around Lolo Pass and at mid-elevations in the Bitterroot, but these will continue to freeze throughout the day today.

Many low- and mid-elevation areas are back down to dirt ahead of last night's new snow. New snow avalanches are unlikely in areas where it falls on dirt due to the lack of a weak layer and sliding surface.

What slope angle should you choose to avoid the persistent slab problem? Statistics show that avalanches are most likely on 38-degree slopes.  Studies have shown a range of a few degrees of error when people measure slope angles. People can think they are on a 35-degree slope when they are at the bullseye of 38. Our advice is that avalanches are very unlikely on slopes under 30 degrees. If you find a slab over persistent weak layers and want to avoid the avalanche problem completely, stick to slopes 30 degrees or less.

Please let us know what you see in the mountains today by submitting an observation.

Ski and ride safe. 

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