Avalanche forecast

Selkirk Mountains

Idaho Panhandle Avalanche Center
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Avalanche danger

Mon
Alpine
3 · Considerable
Treeline
2 · Moderate
Below
2 · Moderate
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Highlights

Rain and snow have added stress to shallowly buried weak snow layers and increases your chance to trigger an avalanche. Wind-drifted slopes facing northwest through north to southeast at upper elevations are the most dangerous. Choose conservative, low-angle terrain if you see cracking in the snow, see natural avalanches, or hear audible collapses.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    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

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

    Fresh, wind-drifted slabs around a foot deep will likely be reactive today from the weight of a rider or machine. These slabs have formed on a mix of 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 with a lens-shaped appearance and watch for signs of unstable snow.

Avalanche Discussion

As advertised, Saturday’s precipitation came is all shapes and flavors – freezing rain, sleet, rain, then finally snow. Field observations and remote weather stations suggest the rain made it well above 6,000 feet in the Selkirk and Cabinet Mountains, and near 7,000 feet in the Bitterroots. The far northern Selkirks and Purcell Mountains near the Canadian border may have avoided the brunt of the rain. Fortunately, most areas only received light rain amounts (0.2 to 0.6 inches), but the Bear Mountain SNOTEL in the West Cabinet Mountains received an inch of rain. Behind the cold front, all mountain areas received snowfall and totals range 3 to 7 inches across the north and 2 to 5 inches across the south.

Yesterday’s mixed bag weather show creates two concerns today regarding the hazard and potential to trigger a Persistent Slab avalanche. First, the primary weak layer of concern exists in the top one to two feet of the snowpack. Yesterday’s rain likely wetted these drier near-surface facets above the rain crust helping weaken their bonds and the water may pool on top of the crusts and migrate downhill. Throw in additional dense snow on top and we have the perfect stiff-over-soft combination that is likely vulnerable to the extra weight of a rider or machine. Despite colder temperatures, the liquid water will hang around in the snowpack for a bit before it either drains deeper or refreezes – especially since the new snow will help insulate the old snow surface.

Over the past week, 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 several days of cool, dry, and sunny weather. The old snow surface consisted of a pile of F to 4F hard facets, large Surface Hoar feathers, and crusted or wind-stiffened surfaces. The best part about the rain is it may have disrupted the Surface Hoar, but also could’ve capped these worrisome facets with a new crust. An avalanche you trigger on a slope with freshly drifted slabs around a foot thick have the best odds to be larger and more dangerous.

Leeward facing areas below ridgelines, in gullies, and cross-loaded terrain features are likely the bullseye places to trigger a failure at the new-old interface. The main question lies in whether or not the slide remains confined to the new snow, or if it breaks deeper. Tread lightly and keep terrain choices conservative. Monitor how conditions change as you move up in elevation and give an extra wide buffer if traveling near slopes steeper than 30 degrees with evidence of drifting or where the snow surface looks smooth and rounded. Sticking to low-angle and wind-sheltered slopes will help reduce your avalanche risk.

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