Bulletin d’avalanche

Banner Summit

Sawtooth Avalanche Center
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Danger d’avalanche

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

Points saillants

Dangerous avalanche conditions exist. You can trigger avalanches involving dense, 2-3 foot thick slabs on many slopes. Winds are building another set of drifts and slabs from the 3-8" of snow that is falling today. These smaller slabs can be effective triggers for deeper, larger avalanches. Triggered avalanches may break wider than you might expect, and can be triggered from a distance away from the slope that avalanches. 

Problèmes avalancheux

  • Wind Slabs

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

    Westerly winds have been blowing at loading speeds since last night, building sensitive slabs in exposed terrain. Today's storm will bring an additional 3-8" of snow to this zone, giving the wind more ammunition.  These slabs exist now and will continue to grow in size and distribution throughout the day.  Look for these slabs near ridgelines and on the sides of mid-slope features like ribs and gullies. Textures like dunes, ripples, and pillows indicate that the wind has been at work. Cracking in these fresh wind drifts is a clear indicator of instability.

    These slabs are adding weight to the persistent slab problem described above. Wind slabs that fail can be effective triggers for larger avalanches involving the weak snow near the base of the snowpack. 

  • Persistent Slabs

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

    Weak snow from November forms the base of the snowpack on many slopes. A dense, 2-3 foot thick slab rests uncomfortably on top of this weak snow. Avalanches failing on this layer will easily be large enough to bury you. Slabs may break wider than you expect, wrapping around terrain features. Triggering slides remotely — from a distance away from the slope that avalanches  — is a real possibility. 

    This layer is at its worst on upper elevation slopes and middle elevation slopes that face away from the sun (W-NW-N-NE-E). Here, a pile of facets is resting on top of a stiff, icy, crust. Friday, I experienced 6-8 deep collapses while traveling at middle and upper elevations in this zone. An observer yesterday reported similar collapses. These are clear signs that the dense slab on top of this weak snow is not bonding well. The wind slab problem described below is adding weight to this buried weak snow, increasing the likelihood of encountering this problem. 

Avalanche Discussion

The main driver of avalanche activity this season is a layer of weak snow near the ground. This layer exists on all upper elevation slopes and most middle elevation slopes that face away from sun (northern half of the compass). Continued reports of large collapses in the snowpack are a clear indication that this weak snow is still a problem. Here's a look at how this setup varies across our region:

WETTER AREAS: the mountains in the northern and western portions of our forecast area (Banner Summit area down through the Western Smokys) have been favored by recent storms. Here, you'll find dense, 2-3+ foot thick slabs sitting on top of the very weak snow underneath. While the thickness of this slab makes it slightly more difficult to impact the weak layer underneath, continued reports of large collapses in the snowpack are a clear indication that this weak snow is still a problem.

DRIER AREAS: Recent storms have largely skipped over the mountains in the eastern and southern portions of our area. Here, the slab sitting on top of the early-season weak snow is in the 1-2 foot thick range. Initially, we needed the stiffening and loading effects of the wind to have avalanche issues, but this slab is becoming thick enough to produce avalanche concerns even in areas sheltered from the effects of the wind. 

A storm impacts the region today, adding complexity to this picture. Winds have been blowing at loading speeds for over 12 hours and will continue to blow as the snow falls today. The dry areas described above may only pick up a few inches, while the wetter areas may see as much as 12" with average totals in the 4-8" range. The wind is redistributing large amounts of snow today, building fresh slabs that will be reactive to your weight. Importantly, these slabs are adding weight to the weak snow at the base of the snowpack, increasing the likelihood of triggering a large, persistent slab avalanche. Smaller triggered wind slabs can be very effective triggers for weak layers deeper in the snowpack.

Before committing to avalanche terrain, take an honest account of your understanding of the early-season snowpack. Travel through the mountains knowing that triggering large avalanches is uncomfortably likely today. Pay attention to the terrain you are traveling through and the terrain above you. Large avalanches can be triggered long distances away from the slope that avalanches. Smaller wind slabs will be failing naturally and can act as triggers for large, more dangerous avalanches. 

Watch Brooke's Weekend Update to get a sense for what's been happening in our snowpack.

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