Lawinenlagebericht

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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Lawinengefahr

Sat
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

To significantly reduce the odds of triggering a large avalanche, avoid steep, open slopes that are exposed to the wind. Look for evidence of recent wind loading such as cornices, pillows, and drifted snow. On these slopes, you could trigger avalanches breaking on weak layers buried 1-3 feet deep. Several natural and human-triggered avalanches occurred in wind-affected terrain in this or neighboring zones in the past four days. 

Lawinenprobleme

  • Persistent Slabs

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

    Multiple persistent weak layers exist in the top 3 feet of the snowpack. These weak layers were responsible for a number of natural and human-triggered avalanches over the past week. They appear to be most problematic on middle and upper elevation sunny slopes where they exist as a combination of crust+facets. However, there have been a few reports of facets and/or surface hoar found on shadier aspects. While these weak layers are showing signs of improvement, it's only been 4 days since our last reported avalanches—that's not that long!

    Let's say we received a report of a triggered avalanche today, and you asked me to guess the terrain it occurred in. Okay, challenge accepted. Here's my answer: a steep (greater than about 35 degrees), upper elevation slope that was recently wind loaded. When persistent weak layers are going through the healing process, these types of slopes often take the longest to stabilize and are where you are most likely to trigger a slide. You can recognize wind-loaded terrain by corniced ridgelines, drifted or pillowed snow features, and/or stiffer snow at the surface where you sink less deep into the snowpack.

    ADDITIONAL DISCUSSION: While triggering slides on faceted snow deep in the snowpack is unlikely, it can't be ruled out in steep, rocky, wind-affected alpine terrain. Slopes like these have variable snow depths, and thinner snowpack areas allow our weight to more easily impact deeply buried weak layers.

    (1/1/2023) Debris from a very large avalanche that was remotely triggered above Hyndman Creek in the Pioneer Mountains. This slide failed in a heavily wind loaded area and very likely involved a persistent weak layer. It covered the summer trail with a pile of debris that was 100-150' wide and 4' deep.

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1–1.5

    The small storm yesterday and overnight likely formed fresh, sensitive wind drifts in upper elevation terrain. Over the past 24 hours, 4-6" of new snow fell in this zone. Weather stations reported moderate S-SE winds yesterday, and NW winds increased this morning—especially in the Soldiers. Look for evidence of fresh windblown snow such as drifts, pillows, and new cornice growth. I'd expect fresh wind slabs to be on the smaller side, but given the weak snow surfaces they're forming on, they could be sensitive to your weight. 

    The new and wind-transported snow from the past 24 hours is now masking an older generation of wind slabs that formed earlier in the week. These older wind slabs have had time to stabilize, but Ben reported cracking in older wind drifts yesterday in this zone. Given the increased chances of triggering a large persistent slab avalanche on wind-loaded slopes, the prudent move is to avoid obviously wind-loaded avalanche terrain. 

Avalanche Discussion

In the past week, we recorded 12 slides in our avalanche database. That doesn't sound impressive compared to the nearly 200 avalanches recorded during the first half of December. But of those 12 slides in the past week, 7 were human-triggered—which is significant. To have human-triggered slides you need three things: an unstable (enough) snowpack, steep terrain, and—humans. We tend to have fewer triggered slides when conditions are obviously dangerous—there are less folks out, and they wisely stick to mellower terrain. (The exception is when conditions are conducive to remotely-triggering slides from gentler terrain.) And there aren't many slides triggered when conditions are stable for obvious reasons. This recent spate of triggered slides is a good example of the overlap of having unstable enough (but not too unstable) conditions and an increased number of humans in the backcountry beginning to nibble at steeper terrain.

Earlier in the winter, we had a remarkably widespread and well-developed layer of faceted snow near the base of the snowpack. This layer formed during the multi-week dry spell back in November. It existed on nearly every slope, and it was easy to predict what it would do when loaded—it would avalanche! Just when that layer was beginning to show improvement, we buried a new round of weak layers before and immediately after Christmas. On sunnier slopes (SE-S-SW-W) these layers are a mix of crusts and facets and seem to be more problematic. On the shadier side of the compass, you might find facets and/or surface hoar. These are typically buried beneath a 1.5-2.5' thick slab. These layers are not as well-developed nor as widespread as our early-season weak layer. That sounds like a good thing, right? Not necessarily. You've heard the expression "better the devil you know" right?

Conditions are still ripe for getting surprised. We're in the dreaded gray area—the persistent slab doldrums. A question that helps me in these conditions is this: sure, avalanches are still possible, but where are they most possible? Nearly all of the recent avalanche activity—natural and human-triggered—occurred on steep, upper elevation, wind-loaded slopes. You can greatly reduce your risk of triggering a slide by avoiding these types of slopes where avalanches are "most possible." 

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