Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

Thu
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 4 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

You're most likely to trigger sizeable slides in specific areas that are protected from the wind. On shady slopes facing E-N-W, 1.5-2 foot thick avalanches are possible. In more exposed locations, you may still be able to trigger hard or soft drifts or slabs that formed several days ago. Pay attention to the consequences of being caught in even small slides 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

    What is it?  In the past two weeks, skiers and riders triggered several avalanches involving a layer of surface hoar buried 1.5-2 feet deep. A skier triggered a slide on this layer Sunday in the Salmon River drainage (view report). It remains capable of producing avalanches on some slopes steeper than about 35 degrees. 

    Where is it?  Thesurface hoar is most widespread and problematic in protected, middle-elevation terrain that faces E-N-W. Surface hoar is a tricky weak layer. It can be "spotty" in its distribution—it may exist on some slopes, but not others. 

    How can you recognize it?  Assume this problem exists until proven otherwise. You may or may not experience snowpack collapsing and/or cracking while traveling. These red flags are obvious signs of instability. The layer of surface hoar is subtle and may not "jump out at you" when searching for it in a snowpit.         

    How can you manage it?  You can reduce your risk by avoiding shady slopes (facing E-N-W) that are very steep (greater than about 35 degrees)—especially those with ugly consequences if you are caught in an avalanche. While the observed avalanche activity points to the layer being more common on sheltered slopes, don't rule out its existence in terrain exposed to the wind. These avalanches may break wider than expected or higher on the slope above you. 

    ADDITIONAL DISCUSSION: You may find some old, hard wind slabs that formed last week in addition to some softer, thinner wind slabs that formed last weekend. Many of these slabs have stabilized, but triggering avalanches in very steep, wind-loaded terrain approaching 40 degrees is not out of the question. Cracks shooting out from your skis or sled are an obvious sign of instability. Hard slab avalanches can behave unpredictably, breaking above skiers and riders. The easy move is to avoid obviously wind-loaded avalanche 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.

Avalanche Discussion

Over the past month, we've seen a significant divergence in the character of the snowpack within and between forecast zones. In a broad sense, you can lump them into two categories: shallow and deep. Why does depth matter? Persistent weak layers tend to survive or thrive in shallow snowpacks. Where the snowpack is deeper, buried persistent weak layers tend to strengthen over time. 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 lie beneath very dense, heavy slabs of snow. Impacting these weak layers is unlikely but not impossible. However, the persistent weak layer (facets or surface hoar) buried in early January lies beneath a 1.5-2 foot thick slab; impacting it remains a very real possibility. This layer is ugliest on wind-protected, shaded slopes (those on the northern half of the compass) where surface hoar formed. However, a pattern of unstable snowpack test scores on sunnier slopes—where facets and surface hoar lie above a crust—is worth noting. Triggering an avalanche on this weak layer is possible in both sheltered and wind-affected terrain.

On top of these weak layers, a few generations of wind slabs formed in the past week. The most recently formed slabs are softer and may be more reactive to your weight. However, the large, recent slide in the Soldier Mtns probably involved rock-hard snow that drifted onto the slope last week; triggering beasts like that is not entirely out of the question. Triggered wind slabs can be effective triggers for deeper weak layers in the snowpack. 

The consequences of triggering avalanches can vary significantly. Triggering a small slide in terrain free of rocks, trees, and terrain traps is much less dangerous than triggering a slide in extreme, rocky, treed terrain with a terrain trap at the bottom. Think about the consequences of being caught in a slide before committing to skiing or riding in sizeable avalanche paths. 

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