Avalanche forecast

Selkirk Mountains

Idaho Panhandle Avalanche Center
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Avalanche danger

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

Bountiful snowfall and southwest winds have built deep, problematic slabs at all elevations. These slabs now sit above multiple weaker snow layers or crusts and will likely be more reactive to a rider or machine on slopes around 35 degrees or steeper. Areas below ridges and in cross-loaded gullies may harbor a slightly stiffer slab and where your chances to find trouble increase. Triggering a long-running loose snow slide in steeper, more wind-sheltered terrain may provoke a soft slab failure creating a more difficult slide to escape. Lower-angled slopes offer safer travel options.

Avalanche problems

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2

    Over the last 10 days, cohesive storm slabs have formed above a variety of weak snow interfaces and each flavor whether it be a notable hardness change, subtle rain or rime crust, or faceted combo is behaving slightly different. Varying sensitivity is likely influenced by the slab stiffness or thickness, but in the end, we have copious amounts of snow teetering above softer snow layers. As you gain elevation, southwest wind during the storm efficiently drifted the new snow into thicker, stiffer slabs. You will encounter deeper drifts on leeward-facing slopes, in cross-loaded gullies, and they are typically identified by a smooth, rounded appearance. Cracking in front of your boards or machine, collapsing, and recent avalanches are telltale signs of unstable snow.

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2–3

    Faceted grains persist around a couple of older rain crusts, creating a weak and slippery sliding interface two to three feet deep across our forecast zones. Before our current storm, the slab directly above both crusts had settled and strengthened more than the faceted grains surrounding the crust. Where the crust has degraded and is more breakable, we’ve found the facets to be larger and weaker. Increased load from new snow may make these layers more reactive, and it's almost impossible to know how close to the failure edge they sit and wait.

    Be especially cautions traveling near exposed rocks or below cliffs where it may be easier to impact weaker snow below. Recent avalanche activity suggest storm slabs in motion can provoke deeper layers to fail resulting in a more dangerous avalanche. Persistent Slab avalanches can behave in surprising ways and span adjacent terrain features, so treat steeper terrain suspect till proven otherwise.

Avalanche Discussion

New snow over the weekend capped off an impressive storm cycle, that delivered copious amounts of much needed snow to all mountain regions. The Selkirk and Cabinet Mountains were favored with deeper snow totals in the beginning, then the storm shifted its sights on the Silver Valley-Bitterroot Mountains at the end of the period before things slowly petered out. 9-day storm totals ranged three to almost five feet of snow across our forecast zones, and three to six inches of Snow Water Equivalent (SWE).

This stormy period was different than prior storm cycles in December. Each pulse arrived with much colder air and lower snow densities. Despite this, the storm invoked a sudden shift in conditions due to the sheer volume of snow and SWE that accumulated quickly. Periods of high snowfall intensity built soft cohesive slabs of snow and were easily triggered by riders and snow safety teams. We observed an uptick in avalanche activity across all mountain ranges and this was in-line with the sensitivity we were observing in snowpack tests. Now that we get a short break in the weather, we expect things to continue to settle out. However, there is still a degree of uncertainty about lingering sensitivity of these storm slabs.

If you choose to step out into terrain steeper than 30 degrees, do so with extra caution. Human or machine triggering can persist once natural activity tapers off and when signs of unstable snow like cracking or audible collapse wane. If you observe signs of unstable snow or recent avalanches on slopes you planned to recreate on, dial back how steep of terrain you travel in or simply choose another aspect or go lower in elevation. Higher in elevation, areas where the wind deposited snow into deeper or stiffer slabs are where larger avalanches are more likely.

Digging deeper, the snowpack consists of a variety of weak layers or mid-storm interfaces in the upper snowpack and their depth below the surface varies ever so slightly. With more over-burden weight recently added to these persistent grains, the extra push of a rider or machine may be all that’s needed to push them over the edge. To account for the uncertainty around deeper weak layer sensitivities, choose low-consequence terrain and give bigger terrain an extra buffer if traveling below.

Lastly, the Loose Dry avalanche concern is starting to emerge as a terrain management requirement. It’s becoming easier to initiate sluffing of the lower-density surface snow and isolated sluffs have grown large enough to push you around. These will likely be confined to steeper, wind-sheltered terrain at lower elevations but can be particularly consequential if they push you over a cliff or stuff you in a terrain trap.

Regions covered