Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Fri
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
2 · Moderate
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Out of date — this forecast expired 4 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Conditions are dangerous. You're most likely to trigger avalanches on slopes where the wind has formed drifts or stiffened the snow surface—even at lower elevations. While slides are most likely where the wind deposited snow, they could also occur in steep wind-sheltered terrain. Avalanches may be triggered remotely—from gentler terrain above, below, and to the sides of steep slopes.

Avalanche problems

  • Persistent Slabs

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

    Triggering a 2-4' thick persistent slab avalanche is possible on steep slopes. Persistent weak layers (sugary facets, surface hoar, or crusts) exist on most aspects and elevations. The widespread nature of this problem is unusual. You could trigger a large, dangerous avalanche on a variety of slopes including lower elevations and in terrain sheltered from the wind.

    Wind and snow over the past few days created fresh wind slabs (see below). The added weight is stressing the buried weak layers, especially at middle and upper elevations where thicker, heavy wind drifts are more common. The slab is harder/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. However, when you do find the "sweet spot" remember that you could trigger slides remotely—from gentler terrain above, below, and to the sides of steep slopes. These avalanches may break wider than expected or higher on the slope above you. 

    Unfortunately, avoiding slopes steep enough to slide is the only way to effectively manage this problem. I'll be sticking to lower-angled, wind-sheltered slopes to reduce my risk.

    (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
    1–2

    Snow and wind over the last few days built sensitive slabs on many slopes. These slabs will be thicker and more widespread at middle and upper elevations but yesterday's extreme wind event also built slabs at low elevations right down to the valley bottom. 

    Yesterday near Cape Horn, Chris described a snow plume launching off of Copper Mtn the likes of which he'd never seen. He encountered deep snow drifts at middle elevation terrain but a surprising lack of cracking. However, earlier in the week and before the storm started, JP and Ben found an ugly snow surface comprised of well-developed sugary facets and surface hoar.

    Look for and avoid obviously wind-loaded terrain. Yesterday's unusually strong wind built slabs in unusual places. You may find them far below ridgelines, on the sides of gullies, and near prominent terrain features in normally sheltered areas. I'd assume any drift you encounter will be touchy to your weight until proven otherwise. Smaller wind slab avalanches could "step down", triggering the large persistent slab avalanches described above.

    (12/21/2022) A set of impressive wind slab avalanches on the headwall of Eagle Ck N. of Ketchum. SW and S @ 10,000'.

Avalanche Discussion

UNUSUAL

I found myself writing the word unusual in both of our problem descriptions. In November, we developed an unusually weak and widespread weak layer that sits at the base of the snowpack. Yesterday, we experienced unusually strong wind. Upper-elevation wind speeds >30mph were commonplace and Peak 2 in the Soldiers recorded multiple gusts topping 100mph!

Safe backcountry travel depends a lot on pattern recognition and our ability to match the expected avalanche problems with terrain that's appropriate for the conditions. Sometimes, however, we observe weather or snowpack setups that are outside of our experience. It's in these times that our well-worn skin tracks and rules-of-thumb need to be re-examined. Yesterday's wind event was one of those days. I live at the base of Carbonate and have only seen the S-face avalanche twice (yesterday included) in the last 8 years. Skiers triggering avalanches just off of the valley bottom is not the norm here, but it's what we can expect again today. 

Today, you could trigger hard slab avalanches on many slopes including more isolated terrain near the valley bottom. Rather than taking your "standard route", look around you for obvious signs of recent wind affect or natural avalanches. Recent slides provide a blueprint for suspect terrain that we should be steering clear of again today.

Regions covered