Bollettino valanghe

Silver Valley & Bitterroot Mountains

Idaho Panhandle Avalanche Center
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Pericolo valanghe

Mon
Alta quota
4 · Forte
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
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In sintesi

Heavy snowfall arrives midday Sunday along with increasing westerly winds and the combination will build very dangerous slabs of snow at higher elevations. The snowpack has experienced a lot of change and stress recently, and now is the time to peel back our ambition till the snowpack has a chance to mellow out. Even triggering a slide confined to the bountiful snowfall we’ve received recently, avalanches can grow large enough to bury, injure, or kill you.

Problemi valanghivi

  • Storm Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Very Likely
    Dimensione
    2

    A large skier-triggered slide Saturday initially failed in the recent storm snow then broke deeper in pre-existing weak snow layers. Over the last week, cohesive slabs have formed onto a variety of old snow surfaces and each flavor whether it be a layer hardness change, subtle rain 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 weaker snow waiting for the extra push from a rider or machine. Cracking in front of our boards or machine, collapsing, and recent avalanches are telltale signs of unstable snow.

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2–3

    Faceted grains persist around a couple of older rain crusts, creating a weak and slippery sliding interface around two feet deep across the forecast zone. In the last week, the slab directly above both crusts has settled and gained strength faster than the faceted grains surrounding the crust. Where the crust has degraded and is more breakable the facets are also larger and weaker. Buried weak snow layers have undergone a lot of additional stress from the new snow and when they’re nudged up 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 shallow slides will provoke deeper, more dangerous ones. Persistent Slab avalanches can behave in surprising ways failing from adjacent to or below a slope, so give steeper terrain and avalanche path runouts a wide berth.

Avalanche Discussion

Over the last 8 days, the Selkirk Mountains and West and East Cabinet Mountains received 30 to 50 inches of snow and 3.6 to 7 inches of Snow Water Equivalent (SWE). The Bitterroot Mountains received 20 to 30 inches of snow and 2 to 3 inches of SWE. The least amount of snowfall has fallen in the Purcell Mountains, but you know things have been bountiful when you consider 10 to 20 inches and 1 to 2 inches of SWE as bringing up the rear. None the less, 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 has chance to settle. That fortunately, but unfortunately is not in the cards soon. More of more is on the way and the focus of heavier snowfall now shifts to the Bitterroot Mountains where Saturday's human-triggered slide helps reinforce the vulnerability of our current stiff over soft snowpack setup - especially in steeper terrain and on more drifted slopes.

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 manage deeper lurking problems is simply through avoidance. Manage your terrain selection and slope angle to avoid triggering our Storm Snow avalanche problem and you will likely avoid our Persistent Slab avalanche problem that always involves a higher degree of uncertainty.

Regioni coperte