Bollettino valanghe

Selkirk Mountains

Idaho Panhandle Avalanche Center
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Pericolo valanghe

Tue
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2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
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Non aggiornato — questo bollettino è scaduto 2 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Mid and upper elevation slopes with shallow, wind-drifted slabs over a crust or weak snow layers are the most problematic. Deeper drifts likely formed on leeward-facing slopes, in gullies, and will have a smooth, rounded appearance. Watch for signs of unstable snow like cracking in the snow surface or audible collapses and be mindful of how close you get to steeper overhead hazards.

Problemi valanghivi

  • Persistent Slabs

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

    Weak, faceted grains above a rain crust lurk one to two feet deep. The hardness and thickness of the rain crust varies with elevation and is thinner and more breakable at upper elevations. While these weak grains have been difficult to fail in snowpack tests recently, rain percolating to the crust combined with the new snow load may be pushing them toward their breaking point. At elevations above Saturday’s rain line, large Surface Hoar grains (10 to 20 mm) may have been preserved and now have a cohesive slab of snow teetering above them.

  • Wind Slabs

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

    Fresh, wind-drifted slabs 6 to 12 inches thick formed on top of a variety of old snow surfaces like sun or rain crusts, Surface Hoar in wind-sheltered areas, and firm wind-affected snow. Avalanches triggered from a drifted area of the slope can break wider than you expect and may provoke a deeper failure in the snowpack. Look for and avoid places where the snow surface transitions from soft to stiff, and watch for signs of unstable snow.

Avalanche Discussion

The latest round of snowfall is slowly winding down. Winds may keep efficient drifting ongoing for a bit longer, but we expect this to be less widespread than previous days. Now that the brunt of storm is over, the new snow will get a chance to settle and may become less reactive to human or machine triggering. The main unknown at this point is how will the wetted, now buried Surface Hoar layer and faceted crust in the upper snowpack tolerate the new load, and how long before those concerns become stubborn or unreactive? This trend will likely be influenced by how deep Saturday’s rain percolated in the snowpack and whether the old, wet snow surface has locked up.

Since the primary weak layer(s) of concern exists in the top one to two feet of the snowpack we should anticipate rider-triggering remains an elevated potential till proven otherwise. Wetting drier faceted grains above a crust will help weaken their bonds and the water may pool on top of the crusts and disrupt that interface as it migrates downhill. Throw in additional weight of an overlying slab and we have the perfect stiff-over-soft combination near the surface. Cooler temperatures will likely help consolidate and stiffen near surface snowpack layers faster than those deeper, and this trend may require finding the shallow part of the slope to trigger an avalanche in buried weak snow layers.

Prior to the last storm, snowpack tests progressively required more force to initiate failures in these collapsible snow layers and buried crust as a function of the overlying slab becoming softer during last week’s High-pressure ridge. The old snow surface consisted of a pile of F to 4F hard facets, large Surface Hoar feathers, and crusted or wind-stiffened surfaces. An avalanche you trigger on a slope with freshly drifted slabs around a foot thick have the best odds to grow larger in size and be more dangerous. Use extra caution traveling on or below wind-loaded pockets on the backside of convex rolls and near slab margins.

You can use small, low-consequence slopes to evaluate the bond of the new snow and monitor how conditions change as you move up in elevation. Give an extra buffer traveling near slopes steeper than 30 degrees if you observe active wind-drifting or see signs of unstable snow. Sticking to lower-angle, wind-sheltered terrain will help reduce your avalanche risk.

Regioni coperte