Lawinenlagebericht

Banner Summit

Sawtooth Avalanche Center
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Lawinengefahr

Sat
Hochgebirge
3 · Erheblich
Waldgrenze
2 · Mässig
Unterhalb
2 · Mässig
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Veraltet — dieser Lagebericht ist 4 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

Today's storm will form sensitive drifts on slopes exposed to the wind. Your odds of triggering a large avalanche are greatest in windier areas with stiff snow near the surface. Avalanches are also possible in steep, wind-sheltered terrain. 

Lawinenprobleme

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    2–3

    Weak snow layers of surface hoar, facets, and crusts exist in the upper 1-3' of the snowpack. These layers caused collapsing yesterday in the White Clouds and on Wednesday (observation) near Galena Summit. They've also produced repeated unstable snowpack test scores both before (here, here), and after our recent storm (here, here). 

    We've found these layers across the forecast area, but their sensitivity varies from slope to slope and the patterns are complex. The development and preservation of these types of weak layers are highly dependent on wind and sun exposure. Expect to find them in some form or another on all aspects at middle and upper elevations. While they also exist on lower elevation slopes, recent rain and warm temperatures have helped them gain strength.

    You're most likely to trigger an avalanche involving this weak snow in recently wind-loaded terrain. You may not observe hard signs of instability like snowpack cracking or collapsing. But the safe bet is to assume these layers exist. Before committing to skiing or riding a slope consider the consequences of triggering a 2' thick avalanche. Is the run-out clean, or will a slide strain you through trees or pile debris into a terrain trap below?

    ADDITIONAL DISCUSSION: A widespread weak layer of snow exists near the base of the snowpack. It is deeply buried and harder to trigger. While triggering an avalanche on this layer is increasingly unlikely, the image above/right illustrates the very large and deadly slide it will produce. 

    (12/26/2022) Motorized users remotely triggered this large avalanche southeast of this zone in the White Cloud Mtns while riding in flatter terrain an estimated 1/4 mile away. Above Washington Lake, 10,500', E-NE aspect.

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Likely
    Größe
    1–2

    Today's storm may form wind slabs in exposed terrain. You're most likely to encounter a sensitive wind slab at upper elevations, but some may exist in isolated terrain at middle elevations. Look for them near ridgelines or along the sides of cross-loaded gullies. 

    Look for blowing snow, cornice growth, ripples, or dunes at the snow surface to identify where the wind has been at work. Stomping on small wind lips or cornices can give you information on the sensitivity and depth of the slabs that you'll be dealing with. 

    Cracks shooting out from your skis or sled are an obvious sign of instability. This problem could be dangerous alone, but any triggered wind slab has the distinct possibility of "stepping down" to break on weak layers deeper in the snowpack.

    (12/28/2022) Small avalanche on NE facing slope of Griffin Butte @ 7,400'.

Avalanche Discussion

We're just north of an Atmospheric River that should pummel Northern California, Southwestern Idaho, and Northern Nevada. While the main plume of moisture is to our south, we'll get a good amount of snow as a strong warm front sweeps through the forecast area today. Our greatest snow totals should favor the western slopes of the Banner Summit area, the Sawtooths, western Smokys, and the Soldiers. Wind speeds are not particularly impressive, but upper elevations should see good amounts of wind loading and slab development. Avoiding wind-loaded terrain will greatly reduce your odds of triggering a large and dangerous avalanche. Without a doubt, my avalanche antenna will perk up as I climb out of sheltered low elevations and into any exposed terrain. I'll be looking for clues, like fresh avalanches, dunes, or recent cornice growth to help identify wind-loaded terrain.

The avalanche problems listed above (although separate) should be considered together. It's where they overlap that conditions will be the most dangerous. True wind slab avalanches are likely in upper elevations, but a large one will provide a trigger if it reaches a slope with a well-connected weak layer buried deeper in the snowpack. As I pointed out in the problem text above, the exact pattern of weak snow layers in the upper 1-3' of the snowpack is impossible to know and will vary from slope to slope. Observations point toward a notable layer of weak snow on SW-S-SE facing slopes. This crust + facet combination is causing collapses and producing poor instability test scores. In shallower snowpacks (generally less than 4' thick), the weak snow near the base of the snowpack remains a threat. 

At elevations below about 7,000' heavy rain saturated the snowpack on many slopes, and the odds you find a combination of a slab and a well-preserved weak layer are lower. Above that, I'll assume I could encounter a slope capable of a large avalanche and I'll use safer terrain to help reduce my risk. Lower-angled slopes are always an option. Short slopes with clean runouts are safer than those that end in terrain traps—amplifying our mistakes.

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