Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

Sat
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 remains possible on some slopes. Avalanches are more likely on shady slopes facing E-N-W and could occur in both sheltered and wind-affected terrain. Today's storm will build thin drifts that could be easy to trigger. 

Problemi valanghivi

  • Persistent Slabs

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

    A layer of surface hoar is buried 1.5-2' deep on many shady (E-N-W) slopes. In this zone on Monday, Chris observed a skier-triggered avalanche that occurred sometime in the past week (photo above/right, details). A number of similar slides have occurred in the neighboring Sawtooths & W Smokys zone. Sun Valley Heli Ski guides recently observed a natural avalanche in the W Smokys that appears to have involved this weak layer (details). Since then, observers have continued to find this layer in snowpits, but snowpack tests show signs of improvement.

    Surface hoar is a tricky weak layer. Its patchwork distribution means you may find it on one slope but not the next. Commonly it's found in small clearings and open, wind-protected terrain on shadier aspects (example). In fact, most of the avalanche activity on this layer has occurred in shady, middle-elevation terrain on slopes where signs of wind loading were not obvious. This problem can lurk well below windy ridgetops, waiting to be triggered in steep terrain further downslope than the obvious starting zone.

    You can reduce your risk by avoiding shady slopes (facing E-N-W) that are greater than about 35 degrees. Consider the consequences of going for a ride. Even small slides can have bad outcomes if they lead to trees, cliffs, or a creek bed.  

    ADDITIONAL DISCUSSION: Today's storm may create thin sensitive wind slabs. You're most likely to find them near upper elevation ridgelines. While likely to be small, they can easily knock you off your feet and take you for a ride. 

    (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

The avalanche danger is MODERATE across the forecast area. At first glance, avalanche conditions may look similar, but there are variable and divergent conditions across our region. No two MODERATE dangers are alike, and we've arrived at this danger level for different reasons. If you recreate in multiple portions of our forecast area, all of this can be hard to keep track of. To simplify matters a bit, I think it's helpful to mentally separate the deeper snowpacks of the northern and western mountains from the shallower snowpacks of the southern and eastern mountains:

  • The snowpacks in the Sawtooths & W Smokys and Banner Summit zones are more uniformly deep. The deeper snow is advantageous, as it helps to reduce the odds of triggering an avalanche on deeper weak layers buried in December and November. The weight of the overlying slab has helped these older layers to heal, and the stiff snow above them means they're harder to impact. On the flip side, snow since the start of January has buried a weak layer of surface hoar beneath a stout 1.5-2' thick slab on slopes that face E-N-W. The majority of the recent persistent slab avalanches occurred on this layer.
  • The Galena Summit & E Mtns zone has a variable and complex snowpack. The early January surface hoar can be found buried 1-1.5' deep, but it didn't get quite enough load to be a widespread issue. The surface hoar layer remains more problematic where recent or past wind loading has created a stiffer slab in the upper snowpack. Although the surface hoar is less concerning in this zone, the older, more deeply buried weak layers are more worrisome. With a shallower snowpack than further north and west, and more rocky, wind-affected alpine terrain, there are more areas where these weak layers are closer to the surface.
  • The Soldier & Wood River Valley zone can be split in two. In the mountains above Ketchum and Hailey, plenty of weak snow exists but a strong slab is less common. You can avoid most avalanche hazard by staying off of wind-affected slopes where a stiffer snow surface exists. In the western portions of this zone, including the Soldiers and the mountains around Dollarhide Summit, a deeper and much more complex snowpack exists (video). Avalanches occurred in the Soldiers on the early January surface hoar layer, and deeper weak layers may remain a concern in rockier, wind-affected alpine terrain.  

This season has been hard. Plagued with persistent weak layers, it's been an exercise in patience. When I take a step back and think of the big picture, I can't help but see the complexity and variability out there. This gives me much less confidence in choosing a safe place to play. When faced with uncertainty, the prudent move is to stick to inherently safer terrain. This includes slopes less than about 35 degrees and those with more favorable outcomes in case I get surprised.

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