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

New snow and strong winds are stressing shallowly buried weak snow layers and increasing the chance to trigger a dangerous avalanche. Triggering an avalanche on slopes with around 8 inches of new or drifted snow has the best potential to break into deeper weak snow layers and produce a larger, more difficult to escape avalanche. Ongoing rain-snow mixture at mid and lower elevations is creating complex snowpack conditions and flavors of avalanche hazards that warrant extra caution and mindful terrain choices - especially if traveling adjacent or below steeper slopes

Avalanche problems

  • Wind Slabs

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

    Snow levels on Saturday dropped below mid and upper elevation ridgelines allowing precipitation to fall mostly as snow, but in unison with increasing wind speeds. Moderate snowfall Sunday will also be accompanied by elevated winds and stronger gusts. This combination will continue to thicken dense slabs on leeward-facing slopes and cross-loaded terrain features like steep sidewalls of gullies.

    Deeper snow totals are expected across the Northern Mountains where colder air provides better conditions for accumulations - 4 to 8 inches in the Selkirk and Cabinet Mountains and 2 to 4 inches in the Bitterroot Mountains. In general, slabs will be deeper and broader at upper elevations but don’t discount similar slab formation around 6,000 feet as the snow line progressively lowers on Sunday. If you observe cracking in the snow, hear audible collapses, or the snow surface transitions from soft to stiff, retreat to lower-angled slopes without bigger hazards directly above you.

    Stiff slabs of wind drifted snow will be building throughout the day and increasing in their sensitivity as they gain size.

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2–2.5

    Buried faceted crust and surface hoar lurk in the upper snowpack and carry the highest odds to fail from added weight of a rider or machine. Observations on Saturday found that recent rains had not percolated deep enough to wet or disturb an upright layer of surface hoar which was buried overnight on January 1. Additionally, the top 1 to 3 feet below the surface consists of a worrisome mish-mash of rain crust surrounded by fragile faceted grains. 

    Before this storm, these weaker layers had become more stubborn, requiring extra force to provoke a failure in tests. The addition of rain and/ or dense snow will help move these problematic weak layers back toward a more reactive state. As the slab above stiffens from liquid water input or denser snowfall this will help slides break wider than expected and potentially be triggered from farther distances. Be aware of where you are in the broader scope of terrain. Your chances to awaken the sleeping dangers may be higher where the overlying slab is thinner near rocky outcrops, around slab margins, and on the backsides of steep rollovers.

    Multiple faceted crusts exist within the upper 3 feet of the snowpack.  These weak layers will be put to the test today, with the additional weight of new snow, and rain on snow at middle elevations.

  • Loose Wet

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–1.5

    Rain and above freezing temperatures have persisted below 5,500 feet for more than 24 consecutive hours. Dry snow getting wetted causes the individual grains to lose cohesion and makes the wet snow more susceptible to sluff or slide downhill. Until the snow surface gets a solid lock-up from colder temperatures, wet snow hazards will linger. Fortunately evidence of wet snow hazards are fairly obvious in the presence of a rotten or punchy snow surface and naturally releasing pinwheels and roller balls.

    Most wet snow slides provoked by a rider or machine will remain small. However, wet snow slides can entrain additional snow volume moving downhill and can create a much larger pile in depressions and terrain traps than expected. Don’t linger on or below steeper slopes where the snow surface is wet and sticky and have a solid exit plan. Frequently step off your boards or machine to see how deep you sink to avoid an unexpected avalanche encounter.

    Naturally occuring pinwheels and wet punchy snow are both clear indicators that the snow surface is saturated and vulnerable to wet loose avalanche activity on steep slopes.

    Photo: Jackson George

Avalanche Discussion

After a prolonged period of calm, clear, and cold weather, this weekend brought the return of mountain snowfall, and I’ll just say it, rain to the Inland Northwest. As of this morning, much of the precipitation is falling as snow above about 4500 feet. In many places this new snow is falling on top of surface hoar or sun crust (observation) that developed during high pressure last week. However, thanks to the fluctuating freezing levels yesterday, today’s snow will also be falling on yet another rain crust within our snowpack. It’s easy to say these things, but hard to explain. Put simply, conditions are very complex right now.New snow is quickly introducing new instabilities to our snowpack. At the surface, dense slabs are building above all of the aforementioned snow surfaces, and as more snow accumulates today, we can expect these slabs to increase in their sensitivity, especially in areas above Friday night’s rain line, where weak snow was preserved at the new/old interface. Southwest winds are driving the hazard further, working to speed up the process of creating thicker and denser slabs on the lee sides of ridges and other terrain features mid-slope like the sidewalls of gullies.The other problem we are up against right now is our lingering Persistent Slab structure pertaining to several weak layers within the upper 3 feet of our snowpack. Prior to this recent storm, we have been thoroughly testing these weak layers in snowpack tests, with a mixed bag of results, indicating that the layers are stubborn and seem to be more reactive in specific locations rather than widespread across the terrain (observation). The key difference now though is not only the introduction of additional weight at the surface, but the significant warm up that occurred during the last few days.Slab properties can change dramatically during periods of rapid warming, This was evident yesterday when an IPAC field team in the Selkirks found increased sensitivity with a buried weak layer in sudden collapses and propagating ECT results despite only 3 to 4 inches of new snow (observation). Our forecast team is concerned that the likelihood of human triggered avalanches will be quickly on the rise as additional snow falls today, and the buried weak layers continue to adjust to the volatile temperature swings.  In the Silver Valley, where the rain line has been slightly higher and less snowfall is forecasted. The Persistent Slab sensitivity is going to be driven by rainfall percolating through the snowpack and destabilizing weak layers directly with liquid water. For this reason, the danger is highest at middle elevations, where the most rain in falling and a shallow, fragile snowpack exist. With how complex conditions are today, we encourage you to travel cautiously, avoid wind drifted slopes, keep close eyes on your partners, and consider the consequences of a slide as you move through the terrain. Especially if you are burying the throttle on a machine, you’ll be more easily impacting buried weak layers in the snowpack. If you wish to simplify your decision making today or better yet avoid these problems entirely, travel on slopes that are less than 30 degrees in steepness and free from overhead hazards.

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