Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Wed
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
2 · Moderate
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Out of date — this forecast expired 3 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Triggering a very large avalanche that involves the majority of the season's snowpack continues to be an unnerving possibility. The first sign of instability may be the unsurvivable avalanche you trigger. Smaller avalanches near the surface of the snowpack will be easy to trigger. These slides can increase in size, especially in large terrain, and can move the entire snowpack downhill.

Avalanche problems

  • Deep Persistent Slabs

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

    A disconcerting weak layer lies buried beneath very dense, 2-4 foot thick slabs on most slopes in this zone. Beneath this meaty slab, you'll find thick facet stacks with or without surface hoar on shaded aspects, and crust with associated facets on sunny slopes. Storms in January produced many avalanches involving this weak snow in this zone. With a lighter load from February storms, triggering an avalanche involving this snow has become less likely than in our other zones. 

    Triggering an avalanche on deep weak layers isn't easy, but if you do find a trigger point you can expect the entire slope you are on to shatter, sending the majority of the season's snowfall on a fast track downhill. The thick slab acts as armor for these weak layers, making triggering a slide more difficult. Likely trigger points are in steep, rocky, wind-swept terrain where the overlying slab is thinner.

    The large avalanche you trigger may be the first obvious sign of instability you see today. Don't expect feedback from this layer, but know that it is down there. You may be able to trigger avalanches from flat or low-angle terrain long distances away from steep slopes.

    (1/22/2024) The debris of this slide ran into a creek, a lower and mid-elevation terrain trap that is characteristic of the Banner Summit Zone. Terrain features like these multiply the consequences of these slides.

  • Loose Dry

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

    Recent storms brought well over a foot of snow to this zone. Expect to encounter a range of smaller issues related to this new snow:

    Loose snow: Sluffs of new snow will be easy to trigger on steep slopes without crusts at the surface. Natural loose snow slides may occur on slopes that face the sun if the clouds allow more sun through than expected. If the snow you are traveling through is becoming moist and you are seeing balls of wet snow rolling downslope, these small slides may be releasing naturally.

    Wind slabs: Winds have been generally light in this zone and recent observations point to a lack of wind slab development. If you head to upper elevations,look for reactive slabs in places where the wind has been transporting snow and building drifts. Wind slabs that formed earlier in the storm when conditions were much breezier will be hard to identify visually, thanks to the low-density powder sitting on top. 

    Slabs at new/old interface: Snowpit observations and natural avalanches point to the potential for avalanches failing at the base of the new snow, even in terrain sheltered from the wind. We have not seen slab activity in this zone. Our observations are limited, and we can't yet say with confidence if this interface will be a longer-lived weak layer or just a flash-in-the-pan. Some of the observed avalanches that failed on this layer broke many hundreds of feet wide. 

    Even small slides can knock you off your feet and send you for a ride in ugly terrain. Surficial slides will travel fast and far on the stiff surfaces underneath. Remember that all of these smaller avalanches take place on the stage set by the Persistent Slab problem above. Smaller slides can be effective triggers for much larger Persistent Slab avalanches as they rocket downslope. 

Avalanche Discussion

Our snowpack currently has two contrasting parts that are equally important to keep in mind heading into the hills: the new snow that is providing excellent skiing and riding, and the weak snow that lingers underneath. If you've ever walked or ridden along a ridgeline at sunset, you know what it means to walk the line between the sunshine and the shadows. Like in life, we like to operate in the sunshine. Shadows however do not make the sun any less bright. Seeing both turns up the contrast and allows us to see more of the world. Heading into the mountains today, it will be easy to be tricked into just ignoring the weak snow that lays beneath the high-quality, soft powder. Seeing the whole picture will allow you to make better decisions and come home safely. 

The sunshine is the new snow that fell from our recent storms. Providing high-quality riding and skiing, this is what you have had blow into your face on your way downhill lately. It lays on top of a supportable melt-freeze crust  that was buried on the first of the month. With past sustained moderate to strong winds that shifted from the SE to NW, this snow formed wind slabs in most exposed middle to upper elevation terrain and led to a small solar-initiated dry loose cycle with our first dose of sunshine a couple of days back. These issues are more prominent in our Galena and Eastern Mtns and Soldier and Wood River Valley Mtns zones where more snow fell. While these wind slabs and new snow still linger at the new/old snow interface, managing surficial problems is approachable with careful identification by evaluating for surfaces for drifting / texturing and terrain management. Without any cracking or collapsing in Eagle Creek to warn me of a deeper issue, silky slopes doused in golden light appeared inviting. Looking back over the opposite side of a ridge at sunset, I unexpectedly spotted a very large avalanche staring back at me, almost hidden in the shadows.... (see photo)

The shadowsare what lay below. Underneath the new snow is a series of weak layers that will not provide much if any warning before releasing a large avalanche. Ten days of snowfall created a thick, meaty slab on top of a series of profoundly fragile weak layers. This slab shielding the weak snow is dense and firm, and will be hard to punch through on your machine or with your ski pole. This makes warning signs like cracking and collapsing unlikely, but don't let this fool you. This weak snow near the base of the snowpack has produced many large to very large avalanches in recent days — including a slide that tore across the entire NW bowl of Galena Peak. This problem is at its worst in our southern two zones where the weak layer is not as deep. As we head further north to Banner Summit, the snowpack gets deeper the the likelihood of impacting this weak snow decreases. 

It can be hard to remind ourselves that the likely possibility of triggering a large destructive avalanche remains when there is soft snow on untracked steep faces. Today, remember that there is plenty of soft snow to be found on lower angle slopes. While the decrease in warning signs can nudge us to get caught up in dreams of steeper terrain, remember that shadows still lie below. Make a plan with your partners before heading into the hills and stick with it despite temptations in the moment. It's been a full season of this similar message, but I'll continue to say it: our goal today is to walk back in that door tonight and see another sunset tomorrow. 

Regions covered