Bollettino valanghe

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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In sintesi

Weak layers in the snowpack can produce large avalanches. You're most likely to trigger a slide in steep terrain that has been loaded by the wind, though slides are possible in steep, sheltered terrain as well. Thin, stiff wind slabs that formed in the past 24 hours are likely to be small but may be easy to trigger. You can reduce your risk today by avoiding steep, rocky, wind-affected terrain.

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1.5–2.5

    Several weak layers exist in the snowpack here.  You are most likely to trigger slides on weak layers that are closer to the surface, like the surface hoar layer that has been responsible for a number of recent human-triggered avalanches (details here). However, slightly deeper weak layers also exist in this zone. It's a safe bet that most slopes harbor at least one of these layers of weak snow. You're more likely to trigger avalanches on slopes where the wind has built slabs and stiffened the snow surface (example), but slides are possible in sheltered terrain as well.

    Persistent slab avalanches behave in unpredictable ways, breaking wider than you might expect and high on the slope above you. Ski and sled tracks on slopes are not indicators of stability. In both of the last two reported human-triggered slides, multiple riders had been on the slope prior to the avalanche (take a close look at this photo). Don't expect to get direct feedback from the snowpack today—you are not likely to experience collapsing or cracking of the snowpack while traveling. The first obvious sign of instability may be the large avalanche you trigger. 

    Snowpack depths in this zone are highly variable, particularly in alpine terrain that is exposed to the wind. Triggering an avalanche on deeper layers can't be ruled out, particularly in steep, rocky, wind-affected terrain. 

    (1/17/2023) Rider-triggered slab avalanche near Wells Summit. The slide broke ~1' deep on a W aspect at 7,800'. Photo: D. Bassett

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–1.5

    Moderate to strong winds have built thin, stiff slabs in exposed terrain. These slabs are likely to be small, but may be able to knock you off your feet and send you downslope if you're not paying attention. Winds have been strongest in the Soldiers, which is also where there is the most loose snow available to build slabs out of. 

    If you are heading for steep, high-consequence terrain today, be thinking about these slabs. But be sure to recognize that the persistent weak layers described above present a much more dangerous threat. Small triggered slides can step down to deeper weak layers, producing much larger, more dangerous avalanches.

Avalanche Discussion

We are seeing a significant distinction between the snowpacks in our region. Broadly speaking we can lump our snowpacks into two categories: deep and shallow. Our deeper snowpacks are found in our northern and western mountains (Sawtooth and Western Smoky Mtns and Banner Summit Area Forecast Zones). There, our primary concern is the uppermost weak layer in the snowpack, a layer of buried surface hoar. The deeper weak layers in the snowpack are buried beneath thick, dense slabs and impacting them with our weight has become unlikely (but not impossible). 

In our eastern and southern mountains, the picture is more complicated. Storms over the past month have dropped less snow here and the snowpack has remained thinner. Well-developed weak layers were buried in November, December and January. These layers are close enough to the surface that impacting them with our weight is a bit easier. In addition, there is a lot of wind-swept alpine terrain in these zones where the snowpack depths vary significantly. This creates thinner portions of the snowpack where buried weak layers are easier to impact with our weight. The thinner snowpacks also produce stronger gradients in the snowpack that make it harder for weak layers to heal quickly. 

Yesterday, I went for a long tour near the boundary between the Galena Summit and Eastern Mtns and the Soldier and Wood River Valley Mtns Forecast Zones. My partner and I longingly looked at and then walked past many large avalanche paths that held thousands of feet of beautiful snow. The presence of the weak layers in the snowpack kept us from entering this terrain. Though triggering a slide wasn't likely, it was likely enough and the consequences were high enough that we didn't even discuss skiing this type of terrain.

I'll be honest, maintaining one's self-control when dealing with long-lived persistent slab problems is flat-out hard. I'm ready to be skiing big, open avalanche terrain, but the snowpack isn't quite ready for me. I'm not avoiding all avalanche terrain, but I'm definitely avoiding large, high-consequence terrain where triggering big, destructive slides is most likely: steep, wind-affected, alpine slopes with variable snow depths. 

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