Boletín de aludes

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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Peligro de aludes

Sat
Alta montaña
2 · Moderado
Límite del bosque
2 · Moderado
Bajo el límite
1 · Débil
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Desactualizado: este boletín caducó 4 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

You could trigger 1-2 foot thick avalanches in wind-affected and sheltered terrain. Avalanches are most likely on shady aspects (facing NW-N-NE) and where the wind has formed large drifts or slabs.

Problemas de aludes

  • Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1.5–2

    In the past week, skiers and riders triggered several 1-2 foot thick avalanches. Many of the slides appeared to fail on a thin layer of surface hoar. The most recent slide was triggered Tuesday, as a rider north of Fairfield crossed a slope for the second time (report). 

    Recent avalanche activity is focused on NW-N-NE aspects between 8000-10,000' in elevation (view graphic). The avalanches released in sheltered and wind-affected terrain. The surface hoar layer responsible for many of these slides is thin, and some of the crystals are relatively small, making them difficult to identify in snowpits (photo). Sunnier slopes that lack surface hoar may harbor a weak layer of facets and/or crusts. JP found this setup yesterday in the Headwaters of the Salmon River (photo). 

    Persistent slab problems are hard to deal with. Avalanches may break wider than expected or higher on the slope above you. Tracks on a slope don't mean it's safe. You may be able to trigger a slide after several riders have safely climbed or descended a given slope. You may or may not experience snowpack collapsing and/or cracking while traveling. These red flags are obvious signs of instability.  

    You can significantly reduce your odds of triggering a slide by steering clear of large wind drifts and by avoiding very steep middle and upper elevation slopes on the northern half of the compass.

    ADDITIONAL DISCUSSION: In addition to the persistent weak layers in the top few feet of the snowpack, there are more deeply buried layers to keep on your radar. These facets (often found near crusts) are buried beneath dense, 2-4+ thick slabs—making them difficult to trigger but capable of producing very dangerous avalanches. 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 allow us to impact deeper weak layers.

    Avalanche in the Fishhook Drainage of the Sawtooths. N aspect near 8,600'.

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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