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
2 · Moderato
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In sintesi

Small slabs of wind-drifted snow may be reactive to your weight today. While not likely, you can trigger avalanches involving very dense, 2-4+ foot thick slabs on a variety of slopes today. The first sign of instability may be the large avalanche you trigger. Steep slopes receiving direct sunshine will produce loose snow avalanches. Small avalanches can trigger larger avalanches as they travel downslope. Choose simple terrain free of terrain traps to reduce your risk. 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    2–3

    What is it? A very dense, 2-4+ foot thick slab stands over buried weak layers of facets or crust + facet combinations. Triggering an avalanche on these layers is not likely, but the outcome of doing so would be catastrophic. Watch the video to the right/above to understand this problem better.

    Where is it? Most slopes harbor these layers. You'll find crust + facet combinations on sunny slopes and small facets on shaded slopes. We are still tallying up the historical amount of slides produced by these weak layers (map, list). 

    How can you recognize it? The first sign of instability may be the large, destructive avalanche you trigger. Tracks on a slope don't indicate it is good to go. These slides may break much wider and further above you on the slope than expected. Unless you dig down to confirm the weak layer exists, you might assume the snowpack feels good under your feet or sled. And if you take the time to dig down, don't trust the results of your stability tests. They serve little help when dealing with such a dense slab overlying this weak layer. 

    How can you manage it? The only way to eliminate your risk of encountering one of these devastating slides is to avoid avalanche terrain and terrain exposed to hazard from above. I know that's not satisfying to hear. You can reduce your risk by avoiding slopes steeper than about 35 degrees—especially large avalanche paths and those with ugly consequences if caught in a slide. Remember that even small slopes can have high consequences when dealing with very dense slabs. These slides can be triggered remotely—from gentler terrain above, below, and to the sides of steep slopes.

    ADDITIONAL DISCUSSION: Today's ample sunshine will trigger loose snow avalanches as the sun warms up the freshly fallen snow for the first time. Small slabs of wind-drifted snow may also be reactive to your weight today. These smaller avalanches can be effective triggers for the large persistent slabs described above. Slides may run fast and far over the crusts found below the new snow on sunny slopes. These slides ca knock you off your feet and carry you through cliffs, trees, gullies, or other terrain traps. 

Avalanche Discussion

The sun will be making a strong appearance today. Since we haven't seen much of this old friend, it will likely cause a widespread cycle of small avalanches on slopes that face the sun. But don't let this glaring problem be at the front of your mind when leaving the house. We still have the other unwelcomed guest lurking in the snowpack. These sun-induced avalanches will likely take the show, but the persistent slab shouldn't be left in your rearview mirror. The wind and persistent slabs are more likely to bury you in a large to unsurvivable avalanche. 

It's hard to believe that we are at the end of March. We've had far fewer days of sunshine than is typical this time of year. All of the recent snow isn’t going to react well to the strong effects of the sun. These loose snow avalanches will likely start failing as the sun rotates from the SE to S to SW. As these slides pick up speed and entrain more snow, they can release other larger avalanches, like wind slabs below cliff bands or persistent slabs that need an extra nudge of weight to release.

Accidents are more likely to occur on sunny days following snow storms. I know this storm isn’t the same size as a couple of weeks ago, but the western mountains have picked up a good amount of snow in recent days (see Storm Totals). As we continue to manage expectations of a low likelihood/high consequence avalanche problem, we face another influencer of instability: the sun. Stay off of steep, rocky slopes in direct sunlight to manage the loose snow avalanches. Avoidance is the only surefire tool to not mess with the persistent and wind slabs problems. Stable spring conditions will eventually make up our snowpack, but we are far from that structure right now.

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