Lawinenlagebericht

Banner Summit

Sawtooth Avalanche Center
Auf der Karte ansehen

Lawinengefahr

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

Kernpunkte

You could trigger a large, 12 to 16 inch thick slab avalanche in shady (NW-N-NE-E facing) upper and middle elevation terrain. Due to the presence of buried weak layers, these slides could break wider than expected. Choose lower-angled terrain, or stick to slopes where the snowpack surface is frozen hard.

Lawinenprobleme

  • Persistent Slabs

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

    There's a chance you could trigger a persistent slab avalanche on shady slopes (E-NE-N-NW) at middle and upper elevations. While the last 2 months have been dry across our entire forecast area, the Banner Summit zone has been less dry. We've been slowly building a slab atop weak layers that formed during the mid-winter drought. The wet and windy storm earlier in the week produced 6-10" of new snow on the higher slopes of this zone, which finally tipped the balance. This caused a cycle of natural avalanches breaking 12-16" thick, some of which broke surprisingly wide (see photos in Media Gallery).

    On Thursday, I went looking for the persistent slab problem near Copper and found weak layers of facets and surface hoar buried 12-14" deep on some slopes. These layers produced poor snowpack test scores (video above/right) and were likely responsible for many of the natural avalanches.

    Wind, sun, warm temps, and dry weather over the past 2 months have produced a lot of variability in the snowpack, and these weak layers don't exist on all slopes. Digging a snowpit on every steep slope you want to play on is neither practical nor safe. The best approach is to take a step back and choose more conservative slopes where this problem may exist (shady terrain above about 8000'). 

Avalanche Discussion

We all know that to have avalanches, you need both a slab and a weak layer. Weak layers usually form in the dry spells between storms; snowfall and wind during stormy periods build the slab. Our 2-month mid-winter drought has produced persistent weak layers near the surface of the snowpack—at least on slopes protected from the intense sunshine, warm temperatures, and wind that have been more common than snowfall. The intense weathering of the snowpack surface means that these weak layers are not as widespread as they could be—they're most common on shaded and wind-sheltered slopes. And—thanks to the dry weather—most of our area lacks a slab atop these weak layers. Here's how it breaks down across the forecast area:

Soldiers & Wood River Valley: The number of inches of snow that have fallen in this zone over the past 2 months can be counted on one hand, maybe two. Where snow still exists, it's often full-depth facets. TL;DR: plenty of weak snow, but little to no slab.

Galena Summit & Eastern Mtns: Weak facets can be found about 4-8" below the surface on shady/sheltered slopes. Slopes with a sufficient slab for unstable conditions have been limited to wind-loaded areas. After nearly every round of wind, folks have triggered small pockets of wind slab sitting on weaker snow.

Sawtooths & Western Smokys: While this zone has received a bit more snow than the Galena Summit & E Mtns zone, the avalanche conditions have been fairly similar. Wind-loaded slopes have been the primary place to find the necessary combination of a slab and weak layer.

Banner Summit: This zone has fared the best during the mid-winter drought. We've been slowly building a slab atop weak layers of facets and surface hoar. The wet storm earlier this week produced 6-10" of new snow at upper elevations. The scales finally tipped, and we observed a cycle of natural persistent slab avalanches breaking 12-16" thick. 

Catch up on what happened with this week's wet, wild, and windy storm in our latest Weekend Update!

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