Bollettino valanghe

Bitterroot

West Central Montana Avalanche Center
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Pericolo valanghe

Fri
Alta quota
1 · Debole
Limite del bosco
1 · Debole
Sotto il limite
1 · Debole
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In sintesi

Large human triggered persistent slab avalanches are unlikely at upper elevations today. Signs of instability, such as cracking and collapsing, will not be obvious before triggering, so dig into the snowpack and look for weak snow 1.5-2.5 feet down that is sitting above a crust. You can use slope angle to avoid the problem entirely by sticking to slopes under 30 degrees. 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    1.5–2.5

    Large persistent slab avalanches are unlikely on upper elevation slopes.  A slab of snow 1.5 to 2.5 feet deep rests on a weak layer of small facets above a crust. This was the weak layer in a large, widely propagating skier triggered avalanche in the Seeley Lake zone last Thursday. 

    You may not experience signs of instability like collapsing or cracking before you trigger an avalanche on this layer. If you do, stick to lower angle slopes. You can identify the problem by digging in the snowpack and looking for a layer of weak snow just below recent storm snow from last week and above a stout crust. However, the weak structure may be difficult to identify because the faceted snow crystals are small and rounding in some places.

    There are also still areas with weak faceted snow buried near the ground. We haven't seen an avalanche on these deeper layers since January, but the poor snowpack structure remains. You are unlikely to trigger one of these deeper layers, but a persistent slab avalanche in the upper snowpack could step down and release a much larger avalanche. You will most likely trigger a larger, more consequential avalanche in areas with a shallower snowpack and complex terrain.

    You can use slope angle to avoid the problem completely by sticking to slopes under 30 degrees. Short of that, you can reduce your risk by avoiding large avalanche paths that will carry you a long distance or complex terrain with lots of hazards, such as cliffs, that can cause traumatic injury. 

Avalanche Discussion

Our forecast region has seen a mix of weak storm systems and sunshine this week. Scattered mountain showers will linger through the afternoon today before clearing to partly cloudy conditions tonight.

We are still honing in on the distribution and sensitivity of weak, faceted snow that formed above a melt-freeze crust in early February. We suspect this weak snow formed as near surface facets in a few new inches of snow before more significant snowfall created a new slab on top. Warm temperatures and occasional sunshine this week may have started the healing process. You are most likely to find this snowpack structure in upper elevations, but it may be possible to find it in middle elevations as well.

You are also most likely to find this poor structure in the Seeley Lake zone. The close call last Thursday was on the west side of this zone. Furthermore, this zone has a more similar snowpack to the Flathead Avalanche Center's forecast region, where multiple large, human triggered avalanches recently occurred. The avalanches failed in the same structure: weak, faceted snow above a crust. Follow the links for information about these incidents: Skier caught, carried, and injured, another skier caught and carried, sympathetic avalanche occurrence.

Because you are unlikely to see signs of instability prior to triggering an avalanche, the best way to identify this problem is to dig into the snowpack and look for poor structure. The facets are small (less than 1mm) and will feel slightly softer and weaker than the slab sitting above them. The crust is hard and difficult to penetrate in many places. The most suspect areas are where the slab is deeper and stiffer. The simplest way to manage the problem is to stick to low angle slopes.

You may encounter small wind slabs in upper elevations on east facing, leeward ridgelines. While you are unlikely to be buried, a small avalanche could carry you into consequential terrain or step down into a deeper avalanche. Wind slabs can be identified by their smooth, rounded appearance. They often feel harder than snow that has not been wind affected, and can even sound hollow like a drum. If you see signs of instability, such as cracking and collapsing, choose to enter terrain that is more wind sheltered or does not have ugly consequences.

We are looking at generally stable conditions across all three zones, but remember, low danger does not mean no danger. There is still a lot of uncertainty around the persistent slab problem, and detection is difficult. Use safe travel practices such as spreading out, going one at a time, and clear and concise communication with your group. When in doubt, go low angle or default to simple terrain that is planar and free from hazards.

Please submit an observation of what you're seeing out in the backcountry. Snowpit reports will be very helpful for us to pinpoint the distribution of the persistent slab problem. Reports of stable conditions and weather are super helpful as well.

Ski and sled safe!

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