Bollettino valanghe

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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Pericolo valanghe

Wed
Alta quota
2 · Moderato
Limite del bosco
1 · Debole
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 7 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Buried weak layers 1-3 feet deep in the snowpack continue to pose a notable hazard. The likelihood of encountering this problem is highest in steep, rocky terrain where recent wind events have built stiff slabs, adding stress to these layers. You can reduce your risk by avoiding this type of terrain.

Problemi valanghivi

  • Persistent Slabs

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

    Residents of the Wood River Valley know that their region was skipped over by last week’s storms. In the absence of snow, this area was “graced” with a significant amount of wind, which dug into the existing snow and built stiff slabs. These slabs added weight to the weak snow buried underneath, increasing the likelihood of triggering a slab on these buried weak layers. Ski Patrollers at Bald Mountain were able to trigger exactly such an avalanche during avalanche mitigation work last week (photo at right).

    Triggering a slide on one of these weak layers is unlikely, but still in the realm of possibility. The chance of encountering a reactive slab is highest in exposed, wind-loaded terrain steeper than about 35 degrees, including cross-loaded gullies lower on slopes. If your sled or your skis aren’t sinking very far into the snow, it's likely that you’ve found yourself one of these stiff wind slabs.

Avalanche Discussion

If you are a regular reader of the forecast, I’d imagine that you are getting tired of hearing about persistent slab problems. Unfortunately, we’ve been dealing with them all season and it is my job to talk to you about them. But, as we know, variety is the spice of life and I’ve got a bit of extra spice for you this morning. Drumroll please...a new persistent weak layer!

This layer isn’t buried in the snowpack yet, it is currently sitting on the surface. Cold, clear nights and days create conditions that are conducive to the formation of faceted snow grains, or facets for short. We call them facets because they have sharp, angular edges on them, much like a cut gemstone. Facets are problematic because they don’t bond well to the snow around them, which makes them a “great” weak layer. Transforming new snow into facets is a process that can take place quite quickly, but rounding (the process by which facets gain strength) takes place much more slowly. And there you have it, a weak layer of snow that will persist for some time.

It's much easier to look for faceted grains in the snowpack before they get buried. If you’re out and about before the storm this weekend, spend some time looking at the snow that is sitting on the surface and see if you can see identify some differences between it and freshly fallen snow. If you look hard can you still see recognizable snowflakes? Can you make a snowball out of it? Does it feel different on sunny aspects than shady aspects? Try to map out the distribution of facets at the surface and then watch how different areas behave as they get loaded.

In our northern areas, where significantly more snow fell last week, a weak layer of these facets may have already been buried. Yesterday, I was able to spot an avalanche on Copper Mountain that had the character of a persistent slab avalanche. Read the problem discussion for the Banner Summit zone for a bit more information. This avalanche failed well below the ridgeline where wind loading would be most intense, indicating that wind loading wasn’t likely the primary factor in its release. Whether or not this is a widespread problem or not is yet to be determined.

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