Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Mon
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Highlights

Triggering a large, destructive avalanche is likely today. These will fail on weak layers buried beneath a 1.5-3 foot thick slab and can be triggered from long distances away from steeper slopes. These weak layers exist on most slopes, from valley floors to mountaintops—avoiding avalanche terrain is the only way to avoid them. Freshly-forming wind drifts and slabs will be reactive and can act as triggers for deeper weak layers.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2–3

    You can trigger large, dangerous avalanches on just about any steep slope you encounter. Yes, even at lower elevations and in terrain sheltered from the wind. Widespread weak layers are now buried beneath 2.5-3.5 foot thick slabs. This combination produced a remarkable avalanche cycle in our southern zones last weekend, with avalanches likely numbering in the hundreds. 

    Yesterday, professional observers in this zone reported that they did not experience any collapsing. However, unstable snowpack test scores in their pit and a poor snowpack structure (slab over weak layer) were enough to keep them out of all avalanche terrain. The slab is harder and denser near Banner Summit than in our other zones, making it a little less sensitive to triggering by skiers and snowmobilers. This means you may not experience as many obvious signs of instability like snowpack cracking or collapsing. Don't let that lull you into a false sense of security. The consequences of triggering a slide are the same, or even greater, thanks to the amount of material in this stiff, overlying slab.

    Persistent weak layers produce avalanches that behave in surprising ways. They can break wider than you'd expect and can be triggered remotely—from gentler terrain above, below, and to the sides of steep slopes. Plan your day expecting to be able to trigger these types of avalanches. Thanks to the widespread nature of the weak layer+slab combination, avoiding slopes steep enough to slide is the only way to ensure a safe return home. Playing on or below steep terrain continues to be a risky dice roll.

    (12/12/2022) In the Banner Summit zone, the weak November snow is buried beneath a 3' thick dense slab. 

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–1.5

    Freshly forming wind slabs will be reactive to your weight today and will grow in size, distribution, and sensitivity today. These slabs are most widespread at upper elevations, though they may extend down into middle elevation terrain in places where the wind is blowing harder. Be on the lookout for blowing snow up high on peaks (known as flagging) as indications that the wind is at work. You can use these to get an idea for what the wind is doing, even before you leave your car. Surface textures like ripples and dunes can help key you into the presence of wind slabs. These slabs aren't likely to be large today, but they could be pretty touchy. Importantly, small slabs that fail (both naturally and the ones you trigger) can act as effective triggers for the deeper weak layers, producing a much larger, more destructive avalanche.  These slabs are also adding stress to the buried weak layers described above.

Avalanche Discussion

The intense storm that blanketed our area drew to a close 5 days ago. This snow fell on an exceptionally weak snowpack that developed during early-season droughts. The resulting avalanche cycle was truly impressive. Hundreds of large avalanches occurred, on slopes from the valley floors all the way to the mountaintops. Houses were hit, the Big Wood River was dammed by avalanche debris, and multiple human-triggered avalanches occurred. It's hard to drive around or ride through the mountains and not see the aftermath of an avalanche.

We've enjoyed a mostly calm period after the big storm. That is changing. Wind speeds picked up and shifted to blowing out of the western portion of the compass overnight. If you've been out skiing and riding powder lately you know that there is plenty of snow available for the wind to transport. Winds appear to be blowing at good loading speeds and will continue to do so throughout the day. Expect to see slabs grow in size and distribution, and become increasingly reactive as they do. The weak layers in our snowpack are widespread. That means that just about anywhere the wind is building slabs it is adding stress to the weak layers underneath. 

I wouldn't be an honest man if I told you that waiting for stability to improve is easy. Standing in the sun and looking at steep slopes covered in perfect powder and deciding not to ski or ride them is flat-out hard. Trust me, I know. Nothing brings me more joy than playing in steep terrain on snow-covered mountains. But that's not where you will find me this today. The chance of triggering large, destructive avalanches is far too great. Instead, I'll be searching out low-angle ridgelines for skiing and deep, powdery meadows for riding, all while paying close attention to the terrain above me. I strongly encourage you to do the same. We are all lucky that this region provides plenty of opportunities for both. 

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