Avalanche forecast

Seeley Lake

West Central Montana Avalanche Center
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Avalanche danger

Wed
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
1 · Low
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Highlights

The avalanche hazard has decreased daily since last week's storm and is trending to more stable. It is still possible to trigger wind slab in upper elevations, and isolated storm slab exists below treeline. Carefully assess stability before committing to steep terrain. The deep slab problem remains a remote concern in the southern Bitterroot.

Avalanche problems

  • Wind Slabs

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

    Winds over the weekend were strong out of the southwest. They affected upper elevations and exposed ridgelines at middle elevations, with significant wind transport, scouring, and deposition. These have had a couple of days to stabilize but should still be treated cautiously. It is possible to find reactive areas. Winds today are light to moderate out of the southwest. With the arrival of light snow later today, strong gusts will aid further wind transport. 

    Wind-affected areas are easy to spot right now, with sculpted wind lips and snow surfaces on the windward side, cornices, and large pillows on the leeward. 

    Avoid wind-loaded slopes greater than 35º.

     

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–3

    The deep persistent slab in the southern bitterroot is still a question mark. This well-developed depth hoar layer is found on northerly aspects above 7400’. It is now up to six feet deep. Before the storm, it showed a high likelihood for propagation in tests. Advanced facets up to 8mm in size were present. The depth and warm temperatures will help heal this layer, but it remains a low likelihood, high consequence problem. Problems like this are unpredictable, and standard snowpack tests are less effective. The slab may break much wider than predicted and pull back onto shallow terrain. Slides on this layer are unsurvivable. Until we can positively rule this out as a possibility, avoid slopes greater than 30º in this zone.

Avalanche Discussion

There's been a rather drastic change in weather and avalanche conditions in the last few days. The high pressure has given the snowpack a chance to adjust to the load it received last week. Warm temperatures today will aid cohesion. 

A strong inversion kept valley temperatures cold yesterday while areas above 5000 feet rose as high as 40º. South faces above the inversion were significantly sun-warmed and may have a crust today. Clear skies and cold nights have promoted surface hoar growth. In addition, extensive near-surface faceting was found in the central Bitterroot and Swans. These layers may cause lingering instabilities once buried.

With the arrival of light precipitation today, the high pressure ends, and the inversion weakens. The small amount of snow forecast will not affect stability. The winds, however, are possible of adding to the existing wind slab in upper elevations. 

We still don’t know the full extent of the avalanche cycle that took place during the storm. As we tour and receive observations, how much slid should become clear. So far, we’ve seen that not all drainages and areas display evidence of slides, but many do. The avalanches range from D1 to D3 and are present at all elevations, down to valley bottom, where wet slabs released. 

The rain crust varies in thickness and elevation. In the northern portion of the forecast area, it reaches about 5200 feet. In the southern, it climbs to 6000 feet and above.

The forecast area has a mostly right-side-up and nonreactive snowpack, with many areas approaching low hazard. Tests show a low propensity for propagation in most spots. However, there are exceptions, such as an 18” deep avalanche in the southern Bitterroot that ran 250 feet on Sunday. Other pockets of instability may be lurking. Mid-storm density breaks create stiff over weak areas. Pockets of surface hoar and near-surface facets might exist throughout the Bitterroot if they did not slide mid-storm. 

Assess slopes in the area you are touring by checking the snow often with hand shears and formal pits. Use small representative test slopes and steep roles to look for cracking and snow movement. Progressively work up to larger and steeper terrain in the absence of red flags.

Many thanks to all the folks who submitted observations over the weekend. As we are a small team forecasting for a large area, they are invaluable. You can submit your observations here: https://missoulaavalanche.org/submit-an-observation/

Ski and ride safe.

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