Bollettino valanghe

Bitterroot

West Central Montana Avalanche Center
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Pericolo valanghe

Sun
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

Triggering a large wind slab avalanche is possible on wind loaded upper elevation slopes. Skiers and riders could also trigger very large persistent slab avalanches on upper elevation northerly slopes. The likelihood of triggering an avalanche is decreasing, but be sure to carefully assess the snowpack before committing to upper elevation slopes steeper than 35 degrees. 

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    Recent snowfall combined with moderate to strong westerly winds formed wind slabs on easterly aspects. Observers have reported plumes of snow blowing off ridgelines and peaks. It is possible for skiers and riders to trigger large wind slabs on upper elevation easterly aspects. Wind slabs are often found just below ridges and in cross loaded gullies. Wind lips and cornices indicate wind direction and loading. Wind slabs can be identified by their smooth and rounded appearance. Signs of instability like cracking warn you that dangerous conditions exist. Carefully assess the snowpack before committing to upper elevation slopes steeper than 35 degrees. 

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    1.5–3

    Skiers and riders could still trigger very large and dangerous persistent slab avalanches on northerly aspects at middle and upper elevations. These slides could be up to 5 feet deep and break at the ground. At this point, triggering one of these monsters is unlikely, but not impossible - it’s a low likelihood, high consequence scenario. Choosing to ride steep, upper elevation northerly slopes is a matter of risk tolerance. Choosing low angle slopes under 35º, ideally under 30º, is the best way to manage these layers. If you start stepping out to steeper terrain, be sure to dig down and see what weak layers may be underneath you. If you find weak, sugary faceted snow down in the snowpack, or if you experience whumpfing or collapsing, take a step back. Avoid potential trigger spots where the snowpack is thinner, like convex rolls and around rocks and windswept ridges. 

Avalanche Discussion

Over the last 24 hours, the mountains received 3-6 inches of snow (0.3 - 0.5 inches of snow water equivalent) and moderate to strong west winds. A quieter weather pattern is beginning today. We can expect clouds with some light snow showers and lighter winds through the weekend. 

On Tuesday, a strong, warm storm brought heavy rain up to 8000 ft and significant snow. We now have a several-inch thick rain crust up to at least 7000 ft that will make it difficult to trigger any weak layer beneath it. At upper elevations where the rain crust is thinner or nonexistent, you could still impact a buried weak layer. 

Before the rain event, the snowpack throughout much of the forecast area had a poor structure. The rain and warm temperatures helped heal weak layers on lower and middle elevation slopes. However, at upper elevations and on some middle elevation slopes, the moisture did not reach far enough into the snowpack to bond these layers. The poor structure remains, especially on northerly aspects at upper elevations. The areas that were of greatest concern for persistent weak layers before the storm, the southern Bitterroot, the Northern Bitterroot, the Rattlesnake, and the Southern Swans, still contain persistent weak layers capable of producing an avalanche. 

The heavy rain and snow gave the buried weak layers a serious test. We observed several very large natural avalanches that occurred during the storm. They were a mix of wet slab and persistent slab avalanches. Overall, the avalanche cycle was not as widespread as we anticipated. The snowpack was able to handle much of the load applied by the storm. That gives us some confidence that it is now unlikely for the weight of a skier or rider to trigger persistent slabs. However, it’s not impossible. If you find the right trigger spot, you could still trigger a very large avalanche that is highly consequential. 

Low likelihood, high consequence problems like this are very difficult to manage. Be very careful trying to outsmart a problem layer. Never ignore a poor structure with weak, sugary facets in the snowpack. If you are ever in doubt, take a step back to lower angled terrain. 

Public observations are a critical tool for us to produce accurate forecasts. Please help us out by submitting an observation. If you see any significant avalanches from the recent natural cycle, we would love to hear about them! 

Ski and ride safe.

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