Bollettino valanghe

Silver Valley & Bitterroot Mountains

Idaho Panhandle Avalanche Center
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Pericolo valanghe

Mon
Alta quota
2 · Moderato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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Non aggiornato — questo bollettino è scaduto 8 months ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Moderate rain amounts up to about 6,000 feet has added extra weight to slabs where buried weak layers are the weakest and have been the most reactive recently in snowpack tests. A rider or machine traveling on steeper slopes at middle elevations will likely be the extra nudge needed to trigger a large and dangerous avalanche. Lower in elevation, soaking rains will keep small wet snow hazards on the menu, and slopes above the snow line are slowly building shallow drifts above similar weak snow layers but may lack the overwhelming load to drive a similar uptick in sensitivity.

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    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.

    Pit tests show propagation potential still exists in weak facet/crust combo layers from December. This one from 6,000ft on NE aspect failed in the weak facets around a thick crust from one of the early December atmospheric river events. An avalanche on this layer would be a low-probability, high-consequence event.

  • Loose Wet

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

    Rain and above freezing temperatures have persisted below 6,000 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.

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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