Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

Sun
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

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.

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1.5–2

    Storms in January buried a weak layer of surface hoar on many slopes. 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. The likelihood of triggering an avalanche on this layer had been gradually decreasing, but ongoing loading from this week's wind and this weekend's storm is keeping this layer on the problem list.

    Surface hoar can be 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 backcountry travelers off guard. 

    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 steep, rocky, wind-affected 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

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    1–2

    Today, you'll find a patchwork of wind slabs at and near the surface. A multi-day wind event this week constructed stiff, dense wind slabs. Yesterday's snowfall+wind built another batch of softer slabs on top of the older slabs, and similar slabs will continue to form today. Wind slabs will be variably reactive, and the most recent slabs are likely to be the most sensitive to your weight. Slabs are most likely in upper elevation terrain, though you may find them in exposed middle elevation terrain as well, thanks to the strength of the wind.

    I was in this Zone on Thursday, and I found widespread wind-affected snow at middle and upper elevations. I also observed debris from what appeared to be two recent natural wind slab avalanches. Though relatively small, these slides ran through a lot of steep and rocky terrain that you would not want to be pushed through. Small slides can also pile up debris deeply in terrain traps like road cuts or creek beds (photo here). Be on the lookout for fresh slabs—if the snow you are moving through becomes deeper or more dense you've likely found one. Cracks shooting out from your skis or sled are a clear sign that you've found an unstable slab. Triggered slides will pick up speed quickly thanks to the stiff/dense snow underneath, and can be effective triggers for deeper weak layers.

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. However, 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 much of this zone exceed 6 feet. Weak layers 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 January is sitting under a 1.5-2 foot thick slab and impacting it with our weight remains a 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—are worth keeping in mind as you travel through the mountains. 

With a prominent weak layer in the snowpack, fresh wind slabs at the surface, and an ongoing trickle of snowfall, 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.

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