Bulletin d’avalanche

Banner Summit

Sawtooth Avalanche Center
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Danger d’avalanche

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

Points saillants

Slopes where you find wind drifted snow or slabs are dangerous, and triggering large avalanches is likely. Avalanches are less likely but possible in steep terrain sheltered from the wind. You may be able to trigger avalanches remotely—from gentler terrain below, above, and to the sides of steep slopes.  

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1.5–2.5

    What is it?  1-3 foot thick slabs overlie persistent weak layers (sugary facets, surface hoar, and/or crusts) on many slopes. Recently wind-loaded slopes are the most dangerous. The photo to the right/below is a great example of a medium-sized avalanche that you might trigger in very steep terrain that is somewhat sheltered from the wind.

    Where is it?  Most slopes harbor at least one of several weak layers in the upper part of the snowpack, but you're most likely to trigger avalanches on slopes where the wind has formed drifts or slabs or stiffened the snow at the surface (example photo). Wind-exposed bowls and slopes near ridgelines, prominent gullies, and chutes are most likely to produce large avalanches. Unfortunately, you cannot assume wind-sheltered slopes are safe.

    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. Wind-loaded areas feel stiffer under your skis or sled and keep you from easily sinking into the snowpack.         

    How can you manage it?  Unfortunately, avoiding slopes steep enough to slide is the only surefire way to manage this problem. Persistent slab avalanches are tricky. These avalanches may break wider than expected or higher on the slope above you. You may be able to trigger avalanches remotely—from gentler terrain below, above, and to the sides of steep slopes.  

    ADDITIONAL DISCUSSION: While triggering slides on faceted snow deep in the snowpack is unlikely, it can't be ruled out in steep, rocky, wind-affected alpine terrain. Slopes like these have variable snow depths, and thinner snowpack areas allow our weight to more easily impact deeply buried weak layers.

    (1/10/2023)

    Snowmobilers remotely triggered this thin but wide avalanche from flat terrain a ways below the slide. Smiley Ck drainage, NW aspect near 9000'. 

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2

    What is it?  The recent 12-20" of snowfall and moderate to strong winds formed 1-3 foot thick wind slabs in exposed terrain.Despite several days of poor visibility, SAC staff and other observers spotted several recent natural wind slab avalanches and triggered a few. Last night's snowfall and shifting wind direction may hide significant wind slabs beneath softer snow. 

    Where is it?  Wind slabs are largest and most widespread at upper elevations. You may find them near ridgelines, on the sides of gullies, and near prominent terrain features. The shifting wind directions may have formed thick or thin slabs on a variety of aspects. 

    How can you recognize it?  Cracks shooting out from your skis or sled are an obvious sign of instability. Watch for drifts, dunes, and ripples on the snow surface (example photo). Wind slabs can have a smooth, chalky appearance and feel and sound hollow, like a drum. Wind-deposited snow will be stiffer and keep your skis or snowmobile closer to the surface.       

    How can you manage it?  Avoid steep slopes with thick or hard wind slabs. Any triggered wind slab could "step-down" to buried persistent weak layers (Problem 1), causing a larger destructive avalanche.  Given the increased chances of triggering a large persistent slab avalanche on wind-loaded slopes, the prudent move is to avoid obviously wind-loaded avalanche terrain. 

Avalanche Discussion

We're facing a lot of uncertainty in evaluating slope-scale stability. Some key drivers of that uncertainty:

  • Since mid-December, erratic weather has formed multiple weak layers in the upper snowpack. Identifying concrete patterns in the distribution and sensitivity of the various weak layers has proven difficult, and evaluating weak layer(s) in a given avalanche starting zone is effectively impossible without digging multiple snowpits in the starting zone in question. Given the current conditions, we DO NOT recommend digging snowpits in starting zones.
  • While obviously wind-loaded slopes are the most dangerous, yesterday's remotely triggered avalanche in the western Smoky Mtns appeared to release in subtly wind-affected terrain.  Also yesterday, Ethan and Ben were leery of stability in very steep, sheltered terrain near Copper Mtn in the Banner Summit zone. Triggering slides in sheltered terrain appears to be more likely in the western mountains. 
  • Recent snowfall and wind formed multiple generations of wind slabs. Some have likely stabilized, and some have not. Consider wind slabs "guilty until proven innocent", and evaluate them on a case-by-case basis. Wind slabs that formed earlier this week may be hidden by last night's snowfall. 
  • Today's sunshine: When slopes harboring persistent weak layers are teetering between being stable versus unstable, direct solar radiation can serve as the proverbial straw to break the camel's back. While I don't expect a widespread, sun-driven, natural avalanche cycle today, clearing skies are a small factor that adds to my apprehension to travel in significant avalanche terrain. Although this poorly-understood phenomenon is rare, today's conditions (persistent weak layers that have produced slides + lots of recent storm snow + first day of direct sunlight) are enough to give me pause. Also, we may see enough sunshine to produce small natural loose snow avalanches, potential triggers for larger slides lower on slopes. 

With 3-day storm totals in the 12-24" range (1-2" of SWE), the snowpack needs a breather. Enjoy a beautiful day outside sampling the excellent riding and sliding conditions, but recognize that there is a real chance of triggering avalanches. Don't let the sunshine and powder fever prevent you from sticking to your plan. 

Régions couvertes