Bollettino valanghe

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Sawtooth Avalanche Center
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Pericolo valanghe

Tue
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 4 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Triggering a 1.5-2.5 foot thick avalanche is possible in both wind-affected and sheltered terrain. These slides are more likely where the wind has formed drifts or slabs. Thin, stiff wind slabs are likely to be small but may be easy to trigger. You can reduce your risk of triggering slides 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

    A weak layer is buried 1.5-2 feet beneath the snow surface. The most recent avalanche on this layer was triggered by snowmobilers just south of this zone on Friday (photo at right/above). A number of other human-triggered slides have failed on this layer over the past 10 days (details here). Avalanches have been triggered in both sheltered and wind-affected terrain.

    The primary culprit is a layer of surface hoar that was buried near the start of the month. Surface hoar can have a notoriously spotty distribution—it exists on some but not all slopes. You can read more about it in the Forecast Discussion below. Some sunnier slopes that lack surface hoar harbor a weak layer of facets and/or crusts. This setup produced unstable snowpack test scores in JP's pits in the Headwaters of the Salmon River on Friday (photo) and in Chris's pits in the Sawtooths on Saturday (observation and photos here). 

    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 the photo at right/above). 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. 

    You can significantly reduce your odds of triggering a slide by avoiding wind-affected terrain and steep slopes.

    ADDITIONAL DISCUSSION: There are also deeper weak layers in the snowpack. These layers (often facets found near crusts, with some surface hoar) are buried beneath dense, 3-4+ thick slabs. While triggering them is unlikely, it can't be ruled out in steep, rocky, wind-affected terrain. Slopes like these have variable snow depths, with thinner areas of the snowpack that make it easier for us to impact these deeper weak layers.

    (1/20/2023) This slow-moving slide was triggered in the northern Sawtooths by a snowmobiler on a NE aspect at around 8,000'. It took 6-8 tracks on the slope before it released. Based on the depth and appearance of the crown, it likely failed on a weak layer buried in early January. We'll share details as we learn more. 

  • Wind Slabs

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

    Moderate winds have transported the few inches of recent snow into small wind slabs. You're most likely to find these near ridgelines and cross-loaded terrain features like gullies and ribs. These slabs are likely to be small, but may be able to knock you off your feet and send you downslope if they catch you unaware. Pay attention to the snow you are traveling through, if it is becoming denser or deeper you are probably moving through a wind slab. Cracks shooting out from your skis or machine are clear indicators you've found an unstable slab.

    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 exist in the same terrain. Triggered slabs can be effective triggers for deeper weak layers. 

    (1/22/2023) Wind transporting snow at sunset in the mountains above Smiley Creek.

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. This Forecast Zone falls into the deep category. Here, our primary concern is the uppermost weak layer in the snowpack, a layer of buried surface hoar with some crusts on sunny aspects. The deeper weak layers in the snowpack are now buried beneath very thick, dense slabs and impacting them with our weight has become unlikely (though not impossible). 

Here are a few important things to keep in mind when dealing with surface hoar:

  • Surface hoar is easily destroyed after it forms. Wind, sun, and sluffs can all disrupt this layer before burial. This leads to a spotty distribution. It may be pronounced in one drainage and absent in another. You can find it in one pit but not the next—despite digging on the same slope. This variability can lead to unpredictable behavior. 
  • Surface hoar is commonly found in small clearings and open, wind-protected terrain on shadier aspects (example). It tends to lurk well below windy ridgetops, waiting to be triggered on steep rolls further downslope than in obvious starting zones near ridgelines. 
  • The two most recent human-triggered slides share a common characteristic—there were multiple tracks on the slope that eventually avalanched. Before they triggered these avalanches, the riders didn't observe any signs of instability on their rides despite covering a lot of terrain. Tracks on a slope are not indicators of stability. 

Widespread, surface hoar is not common in this area. If you've spent most of your backcountry days here, you probably don't have much experience dealing with this type of instability. Its spotty distribution can catch us off-guard. Some slopes lack this layer while others continue to harbor reactive surface hoar. Slides failing on surface hoar can break above you and wider than expected. On slopes that face the sun, you'll find a layer of crusts with small facets instead of surface hoar. These crusts have produced unstable snowpack test scores in our pits in the Sawtooths and the mountains above Smiley Creek. Avalanche activity has been focused on shady slopes (those on the northern 2/3 of the compass), but slides are possible where these crust+facet combos exist as well.

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