Bulletin d’avalanche

Banner Summit

Sawtooth Avalanche Center
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Danger d’avalanche

Mon
Alpin
4 · Fort
Limite forestière
4 · Fort
Sous la limite
3 · Marqué
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Périmé — ce bulletin a expiré 5 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Very dangerous avalanche conditions exist. Avoid traveling on or beneath any steep slope. New snow and strong winds are pushing a weak snowpack past its tipping point. Large natural and human-triggered avalanches are likely. Avalanches may be triggered remotely—from flatter terrain below, above, and to the sides of steep slopes. 

The Backcountry Avalanche Warning does not apply to the Highway 21 Avalanche Program.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Very Likely
    Taille
    2.5–3.5

    You must have all been good girls and boys because Santa delivered a multi-day storm event that dropped 2-3' of new snow since Wednesday. I must have been a bad boy because this morning I'm having to issue an Avalanche Warning. This prolonged storm is loading our weak October snow that lurks beneath a dense 3-5' slab. This facet layer is most prevalent on mid and upper elevation shady slopes (NW-N-NE-E facing).

    The first round of this storm ended Thursday night, and on Friday morning, clearing skies revealed numerous large to very large natural avalanches. Perhaps most notable is a huge avalanche on Galena Peak (in the  Galena Summit & Eastern Mtns zone) that would have destroyed a small neighborhood. Numerous large to very large avalanches occurred in the neighboring Western Smokys (photo) and Sawtooths (photo, photo). 

    The next round began yesterday, and last night delivered the most intense period of snowfall so far. Most of this zone picked up about 8-12" overnight—with snowfall intensities of 1-2 inches/hour. Strong southerly winds blew through the night. Light snowfall will continue for the next 24 hours, and the wind may kick in again with the passage of a cold front this morning. With the near continual loading of the snowpack over the past 4 days, it's hard to know when all heck will break loose. But there's a good chance it's today.

    Today is a no-brainer. Stay out of avalanche terrain—avoid being on or under any slope steeper than 30 degrees, including the runout zones of avalanche paths at valley bottoms. Our snowpack is capable of producing very large, destructive avalanches. Some of these could run full-path to the valley floor.

    (12/24/2021) Very large natural avalanche on "Marshall Slide" in the Sawtooths, on the shoulder of Williams Peak, above the Marshall drainage. This avalanche appears to have failed in the October facets deep in the snowpack. NE-aspect at 9,000'.

  • Storm Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Very Likely
    Taille
    1–2

    Overnight, intense snowfall and strong winds will have created dangerous avalanche conditions on most slopes, even on southerly-facing terrain where the persistent weak layer (Problem #1)  is absent. Storm slab avalanches will be most likely in middle and upper elevation, wind-exposed terrain but may occur on any steep slope. They could break 1-3'+ deep, which is plenty large enough to bury or injure a skier or rider. The travel advice is simple and the same as for the persistent slab problem: Stay out of avalanche terrain.

Avalanche Discussion

I hope you enjoyed the Christmas poem yesterday because today's message is a bit more somber. Nothing cute, nothing clever—just the facts. So here we go: we have a snowpack with a well-defined weak layer, we know it can produce large, destructive avalanches, and we just whacked (and are still whacking) it with a big load of new and wind transported snow. Even with such a simple situation, the snowpack is a complex beast. It's not possible to predict with 100% accuracy what will happen today, but it sure seems like things are going to come unglued.

The October facet/crust weak layer has seen significant loading events in our three northern zones, where it's currently buried beneath a dense 3-4+' deep. In these areas, avalanches may not be as frequent, but the ones that do release are likely to be monsters capable of flattening trees or houses. But in the Soldier & Wood River Valley zone, the weak layer has received less loading since the big storm in mid-December. This storm will feel like a bigger wallop to the snowpack in these areas, and we're likely to see a significant and widespread avalanche cycle.

Adding insult to injury, this multi-day storm event has delivered enough new snow and wind to create dangerous conditions even where this weak layer is absent. These storm slab avalanches may seem less "intimidating" than the persistent slab problem discussed above, but they could easily be large enough to bury you. If you've been sticking to sunny slopes that lack the October weak layer, that risk management strategy will no longer be appropriate. 

What's the takeaway? Conditions are very dangerous, and the only reasonable approach is avalanche terrain avoidance. Stay off of and out from beneath slopes steeper than 30 degrees, and avoid traveling in the runout zones of avalanche paths.

Be sure to catch up on this week's conditions in our latest Weekend Update.

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