Lawinenlagebericht

Banner Summit

Sawtooth Avalanche Center
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Lawinengefahr

Wed
Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
1 · Gering
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Veraltet — dieser Lagebericht ist 5 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

Avalanches are most likely on slopes where the wind has built slabs on top of weak layers in the snowpack. Slabs failing on these weak layers in sheltered terrain are less likely but can't be ruled out entirely. Very large avalanches failing on weak snow at the base of the snowpack have become unlikely but would be unsurvivable events.

Lawinenprobleme

  • Wind Slabs

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

    You are most likely to encounter avalanche problems in areas where the wind has been at work. There are a range of wind-related issues to be on the lookout for here. Have a look at the video at right/above and read on:

    • Wind slabs formed during last week's storm have had a bit of time to bond to the surfaces that they are resting on. If you find a wind slab today it is likely to be on a very steep slope in alpine terrain. On an off-duty ski day in the Sawtooths on Sunday, my partner and I intentionally triggered a few small wind slabs in exactly this type of terrain. This terrain is unforgiving, even if the slabs are small. You would not want to be caught on the wrong side of one of these slabs here. 
    • A crust+facet combination exists underneath last week's storm snow on slopes that directly face the sun (southern half of the compass). This layer produced a number of large, natural avalanches during the storm. Chris and I went looking for this layer yesterday (observation here). We saw a few avalanches that may have failed on this layer but we were unable to find this crust in our snowpits. This was likely because we were not on slopes that were quite steep enough, and we did not have any interest in digging in prime-time avalanche terrain. On Sunday, I saw at least 3 large avalanches in the Sawtooths (observation and photos here) that likely failed on this layer. One spanned nearly 1/4 mile of terrain. Time will tell how long this layer is problematic, but I'll be leery of slopes where this layer exists for the next bit.
    • Weak October snow is buried 5-6+' deep on most shady middle and upper elevation slopes—deep enough to be very difficult to impact with our weight. Where might it still be possible to trigger a deep slab? I'm still leery of very steep (>35 degrees), open, rocky, wind-affected slopes at upper elevations. 
    • Cornices have grown to enormous sizes in this zone. Think school bus and snowplows, not microwaves and refrigerators. Have a look at this very large cornice that collapsed in the Sawtooths at the tail end of this storm to get a better idea. These monsters make for very effective triggers, even for weak layers that are buried 6' deep. They are also quite hazardous in their own right.

    You can find excellent skiing and riding in terrain sheltered from the effects of the wind right now while simultaneously reducing your risk of encountering the hazards listed above. Sounds like a pretty good deal to me.

Avalanche Discussion

WEAK LAYER and SLAB DISTRIBUTIONAfter a wild month of nearly continual snowfall and wind we find ourselves in a period of clear skies, cold temperatures, and light winds. The calm after the storm. This is a great time to take stock of the state of the snowpack and think about the variability that we are seeing across our area. The first two rounds of storms (mid-December and near Christmas) brought a big helping of snowfall to most of our region. A bit more snow fell in our northern and western mountains (Banner Summit area, Sawtooths, Western Smokys, Soldiers) but the mountains above the Wood River Valley and those on the eastern side of the Big Wood and Salmon Rivers (Pioneers, Boulders, White Clouds) picked up their fair share. These storms buried a layer of weak snow that had stuck around after our first big snowfall of the year in late October. This October snow was preserved on shaded slopes at middle elevations and most slopes at upper elevations. The December snowfall pattern left a thicker slab of snow on top of this weak snow in the more favored areas (north and west) and a thinner slab to the south and east. Simple enough.

The most recent storm (January 3-7) heavily favored the northern mountains and left the good people of the Wood River Valley wondering, "storm, what storm"? In areas favored by this storm, we saw widespread avalanches related to the new snow (wind slabs, storm slabs, loose snow avalanches) as well as a pattern of activity that pointed to the involvement of a weak layer buried at the base of this storm snow. This layer appears to be at its worst on slopes that face the south half of the compass, where it exists as a subtle crust with facets above and below. In the areas favored by this storm, this layer is buried 2-3.5' down under a meaty slab of snow. In the areas that were missed by this storm, this weak snow is much closer to the surface. This is starting to feel a little more complex.

If we mentally zoom back out, we can see that we have patterns of weak snow and slabs that vary widely by aspect and elevation, and between forecast zones. If you want to completely avoid areas where either of these weak layers exists you will have to do quite a bit of thinking and planning. As the likelihood of triggering avalanches on these weak layers slowly decreases and the Avalanche Danger slowly comes down, we find ourselves in a tricky place. Avoiding avalanche terrain is pretty simple, but deciding when and where to engage with avalanche terrain is very complex. When you are out in the mountains today, think about these patterns and whether or not these weak layers exist in the terrain you are playing in. The danger may be MODERATE in all of our zones, but the snowpack is different everywhere you go. Whether or not you choose to enter avalanche terrain is up to you, just be sure you know what sort of dragons you are choosing to engage with.

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