Lawinenlagebericht

Silver Valley & Bitterroot Mountains

Idaho Panhandle Avalanche Center
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Lawinengefahr

Tue
Hochgebirge
4 · Gross
Waldgrenze
3 · Erheblich
Unterhalb
3 · Erheblich
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Veraltet — dieser Lagebericht ist 2 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

Continued heavy snowfall and drifting is forming very dangerous slabs of snow at higher elevations and traveling on, adjacent to, or below slopes steeper than 30 degrees is like rolling the dice. The snowpack has experienced a lot of change and stress recently, and now is the time to peel back your ambition till the snowpack has a chance to mellow out. Even if you trigger an avalanche and it remains confined to the bountiful storm snow of recent, those avalanches will likely be large enough to bury, injure, or kill you. Travel in or near avalanche terrain is not recommended.

Lawinenprobleme

  • Storm Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Very Likely
    Größe
    2

    On Saturday, skiers triggered a large avalanche that initially failed in the storm snow then broke deeper in pre-existing weak snow layers. Over the last week, 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. Cracking in front of your boards or machine, collapsing, and recent avalanches are telltale signs of unstable snow.

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    2–3

    Faceted grains persist around a couple of older rain crusts, creating a weak and slippery sliding interface around 20 to 40 inches 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. These potentially dangerous weak layers still exist, are being stressed by the impressive new snow load, and when they’re nudged against the edge is the big unknown.

    Avoid shallow areas 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 give an extra wide buffer if you travel near avalanche path runouts.

Avalanche Discussion

The focus of heavier snowfall has transitioned from the Northern Mountains to the Bitterroot Mountains over the last 24 hours. The snow that’s falling now to cap off an impressive 9-day stretch arrived with much cooler air and lower snow densities. This equates to less weight than what accumulated in previous days. Lower-density snow stacked above much heavier snow will eventually bode well for future avalanche conditions, but snow does not embrace rapid change. I will take time for things to become less sensitive. Wherever you plan to travel in the backcountry enough storm snow sits above numerous suspect interfaces and deeper lurking problems that heads up travel, conservative decision making, and good group dynamics is strongly advised.

Trying to outsmart current hazards will likely increase the risk of bringing deeper persistent weak snow layers into play and produce an avalanche that would be more difficult to survive. Newly formed slabs will remain at an elevated level of sensitivity until the snowpack gets a breather and finds some sort of equilibrium. That fortunately, but unfortunately is not in the cards soon as more of more is on the way.

We we’re quickly reminded over the weekend of our current snowpack vulnerabilities when a group of skiers triggered a large avalanche in the storm snow that stepped down to deeply buried faceted grains around a crust. Three members of the group were caught and carried several hundred feet without incident. This fortunate outcome is not withstanding. In hindsight, the avalanche behaved as anticipated and suggest dialing back our objectives to help avoid a not-so-fortunate outcome.

It's hard to know with full certainty when buried weak grains are pushed to the edge ready to come crashing down. We can perform stability tests to evaluate sensitivity, but this is really a one-off snapshot in time with many other factors weighing in. Recent avalanche activity is a solid piece of evidence cluing us in to a reactive slab, weak layer combination. But after big snow loads like the snowpack has endured, the best way to navigate various avalanche problems is simply through avoidance. Manage your terrain selection and slope angle to avoid triggering our Storm Slab avalanche problem and you will likely avoid our Persistent Slab avalanche problem that always involves a higher degree of uncertainty.

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