Bulletin d’avalanche

Banner Summit

Sawtooth Avalanche Center
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Danger d’avalanche

Mon
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 4 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Triggering a 1.5-2 foot thick avalanche failing on a layer of weak snow is possible on both sheltered and wind-affected slopes. New snow and wind have built relatively small, fresh drifts that are likely to be sensitive to your weight today. The consequences of being caught in even small slides can be high in terrain with rocks, cliffs, trees, and terrain traps.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1.5–2

    Storms in January buried a weak layer of surface hoar on many slopes. You'll find this layer 1.5-2' below the snow surface. Nearly all of the observed avalanche activity involving this layer has occurred on slopes that face the northern half of the compass (E-N-W). However, you'll also find a weak combination of crusts with facets and some surface hoar on sunnier slopes. Yesterday, observers near Smiley Creek reported unstable snowpack test scores on this surface hoar layer and decided to ride gentler terrain where they found it.

    Surface hoar is a challenging weak layer, one that we don't deal with that frequently. It tends to have a spotty distribution—it exists on some but not all slopes. Both wind and sun can knock down and destroy surface hoar crystals. It is often at its worst on sheltered, shaded slopes. You may not find it in obvious starting zones but it tends to be well-developed further downslope (photo at right/above), which can catch us off guard. Slides failing on persistent weak layers can break wider and further above us on the slope than we might expect.

    ADDITIONAL DISCUSSION: Older weak layers (often facets found near crusts, with some surface hoar) exist deeper in the snowpack. These layers are buried beneath dense, 3-4+ thick slabs. While triggering them is unlikely, it can't be ruled out in rocky, wind-affected, alpine terrain. 

    (1/23/2023) Avalanche apparently triggered by a skier on Banner Summit. Slab depth and terrain characteristics are consistent with early January surface hoar layer. 7250', NNW. The date of this avalanche is uncertain, but likely occurred since Jan 18th. Photo taken Jan 23rd.

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–1.5

    This area picked up 6-10" of low-density snow over the past two days. Light to moderate winds accompanied this snowfall, building soft, 1-2 foot thick slabs in exposed terrain. Expect to find these slabs near ridgelines and along cross-loaded terrain features like gullies and ribs. These slabs may be reactive to your weight today, particularly where they were deposited on a weak or slick snow surface. Underneath these soft slabs you'll also find a batch of hard slabs that formed during last week's wind event. I was in this zone yesterday and I experienced a few collapses involving these stiffer drifts. 

    Be on the lookout for signs the wind has been at work—if the snow you are moving through becomes deeper or denser you've likely found a fresh wind slab. Cracks shooting out from your skis or sled clearly indicate that the slab you've found is unstable. Triggered slides may pick up speed quickly thanks to the stiff/dense snow underneath, and can be effective triggers for deeper weak layers. Even small slides can also pile up debris deeply in terrain traps like road cuts and creek beds (photo here).

Avalanche Discussion

Early season storms brought roughly equal helpings of snow across our forecast area. This helped to build a snowpack that was somewhat homogenous across the region. Over the past month, snowfall has favored our northern and western mountains and we've seen a significant divergence in the character of our snowpacks. In a broad sense, you can lump them into two categories: shallow and deep. The Banner Summit Forecast Zone falls into the deep category.

Snow depths in most of this zone exceed 6 feet. Weak layers that were buried in November and December are resting underneath very dense, heavy slabs of snow. Impacting these weak layers with our weight is unlikely, though not impossible. However, the prominent weak layer that was buried in early January is sitting under a 1.5-2 foot thick slab and impacting it with our weight remains a very real possibility. This layer has proven itself to be at its worst on shaded slopes (those on the northern half of the compass), where standing surface hoar can be found under this dense slab. However, a pattern of unstable snowpack test scores on sunnier slopes—where a crust is accompanied by facets and some surface hoar—is worth keeping in mind as you travel through the mountains. Triggering an avalanche on this weak layer is possible in both sheltered and wind-affected terrain.

With a prominent weak layer in the snowpack and fresh wind slabs at the surface, the decision to choose simpler terrain today is an easy one. The consequences of triggering avalanches can vary significantly. Triggering a small slide in terrain free of rocks and trees and terrain traps is much safer than triggering a slide in extreme, rocky, treed terrain with a terrain trap at the bottom. Spend some time thinking about snowpack complexity and the consequences of being caught in a slide as you are choosing your terrain for the day.

Today's very cold temperatures add another layer of hazard to the mountain environment. The consequences of even minor injuries or mechanical issues with your machine can be amplified by arctic temperatures. Pack an extra layer or two and some extra food today.

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