Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Mon
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
1 · Low
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Out of date — this forecast expired 4 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Triggering a 1.5-2.5 foot thick avalanche is possible in both sheltered and wind-affected terrain. These slides are most likely on shady aspects (facing NW-N-NE) and where the wind has formed large drifts or slabs. 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.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1.5–2

    A weak layer is buried 1.5-2.5 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). Several other human-triggered slides have failed on this layer in the past week (details here). The avalanche activity has been focused on NW-N-NE aspects on middle and upper elevation slopes. 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. The surface hoar layer is relatively thin, which makes it challenging to find in snowpits. 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). 

    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) 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. Alpine 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

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–1.5

    A few inches of snow and light to moderate winds have combined to build 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. Yesterday, observers on Galena Summit reported some very small, 6" thick wind slabs that had released naturally on road cuts during the day.

    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. 

Avalanche Discussion

Check the avalanche database for a rundown of the avalanche activity from the past week (view the list here). While a facet + crust combo may be to blame in a few recent slides, a buried surface hoar layer is the most likely culprit. Here's a little refresher on surface hoar along with a few things to consider when dealing with this sneaky weak layer:

  • Surface hoar usually forms on clear, calm nights. Locally, we often see surface hoar growth as humid air lingers following the passage of a storm. 
  • While surface hoar can spring up overnight, it is also easily destroyed. 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 characteristic can lead to "surprising" behavior. 
  • To better avoid this problem, it's helpful to imagine where it's most likely to survive until burial. Commonly it's found in small clearings and open, wind-protected terrain on shadier aspects (example). It lurks well below windy ridgetops, waiting to be triggered on steep rolls further downslope than the "obvious" starting zone. 
  • On Tuesday, riders near Wells Summit north of Fairfield triggered a 1' thick slab avalanche. They didn't observe any signs of instability on their ride despite covering a lot of terrain. As they returned to their truck, they triggered a slide on a slope they'd already crossed that morning. The variability of surface hoar across a slope may allow several skiers or riders to cross a slope before someone finds the "sweet spot". 

It's important to realize that we're all in a bit of unfamiliar territory. 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 "pockety" distribution can catch us off-guard. Some slopes have stabilized, while others will continue to remain sensitive to our weight. Slides failing on surface hoar can break above you and wider than expected. While recent slides have not been "huge", around half were large enough to bury or injure people, and all could carry someone into a gully, through trees, or over cliffs. Picture where even a small avalanche could carry you before committing to playing in large or consequential avalanche paths.

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