Bollettino valanghe

Bitterroot

West Central Montana Avalanche Center
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Pericolo valanghe

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

Although unlikely, you could still trigger a large slab avalanche at upper elevations that fails on weak snow buried near the ground. You aren’t likely to see signs of instability before triggering such an avalanche. Skiers and riders could also trigger smaller avalanches in new and recent snow, especially where storm totals are highest. To avoid the problems, stick to low angle slopes. 

Problemi valanghivi

  • Persistent Slabs

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

    Although unlikely, skiers and riders could still trigger large persistent slab avalanches on upper elevation slopes. These avalanches could fail 2 to 4 feet deep on buried persistent weak layers near the ground. 

    The persistent slab problem exists in isolated areas at 6500 ft and becomes more widespread as you get higher in elevation. North aspects have the most concerning snowpack structure, with a stiff slab above weak, sugary facets near the ground. However, a poor snowpack structure also exists on south aspects where the facets are still wet from last week’s warm temperatures and continue to propagate in snowpit tests. 

    The sensitivity of this problem has decreased, but the consequences of any avalanche triggered are high. You aren't likely to experience signs of instability like collapsing or whumpfing as you travel through the mountains. Tracks on a slope are not indicators of stability - avalanches may be triggered after many riders have crossed a slope. 

    The best way to identify the problem is to dig in the snowpack and look for poor structure with a firm slab on top of weak, unconsolidated snow. If you find 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, we recommend sticking to slopes less than 30 degrees in steepness. Short of that, you can reduce your risk by avoiding large avalanche paths and those with ugly consequences if you are caught in a slide.

    (1/20/2024)

    Looking up at the crown of a large human triggered persistent slab avalanche on an upper elevation east aspect in the Rattlesnake. 

  • Storm Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–1.5

    3 to 6+ inches of new snow fell in the last 24 hours, adding to the 6 to 12” of snow that fell over the weekend. Both storms favored the Southern Bitterroot. Skiers and riders could trigger small slabs of this recent snow at middle and upper elevations. In some areas, the snow may rest on weak layers like surface hoar or crust facet layers and be more sensitive. Wind loaded areas may also be more sensitive. 

    Small 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. To avoid the problem, stick to slopes under 30 degrees in steepness.

    (2/3/2024)

    A natural slab avalanche in the new snow on a wind loaded southeast aspect at 7900 ft in the Rattlesnake. It was 6-12 inches deep and around. 50 feet wide. 

    Photo: Arden Feldman

Avalanche Discussion

After the storm over the weekend, we had some mild weather with off and on sunshine early this week. Yesterday, a weak storm system moved into the region, bringing light snow favoring the Southern Bitterroot. Light snow is forecast to continue today with light to moderate west winds. 

24 hour snow totals range from 1 to 6 inches, putting snow totals since Saturday at 6 to 18 inches. You could trigger small slabs of this recent snow, especially where you find the upper end of snow totals. This is most likely in the Bitterroot, where a storm slab avalanche problem is included in today's forecast. In the Rattlesnake and Seeley Lake zones, avalanches in the recent snow are less likely. However, stay alert for signs of instability, like shooting cracks, collapsing, and fresh avalanches. If you see these signs, take a step back to lower angle terrain. In some areas, the recent snow may rest on weak layers like surface hoar or crust facet layers and be more sensitive. Wind loaded areas could also be more sensitive. 

The more concerning issue at the moment is the lingering persistent slab problem. Weak layers near the ground have gained strength, and the likelihood of triggering a slide on them has decreased significantly. However, the possibility remains on the table, and the consequences remain high. Remember, unlikely does not mean impossible. At upper elevations, observers continue to find poor snowpack structures with propagating snowpit test results. 

Choosing to enter avalanche terrain with a low likelihood, high consequence problem like this is a matter of risk tolerance. If you find a slab over persistent weak layers and want to avoid the problem entirely, stick to slopes less than 30 degrees in steepness. 

Please submit an observation to let us know what you see in the mountains today, especially regarding new snow totals. 

Ski and ride safe.

Regioni coperte