Bulletin d’avalanche

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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Danger d’avalanche

Tue
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 6 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Triggering an avalanche is most likely where the recent wind drifts are stressing weak layers in the upper 1-3 feet of the snowpack. Triggering a slide involving this weak snow is possible even in areas that are sheltered from the wind. You may be able to trigger slides remotely: from flatter terrain above, below, or to the side of a steep slope. With warming temperatures and rain today, wet snow avalanches will be possible, primarily at lower and middle elevations.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    2–3

    Layers of weak snow are buried 1-3 feet deep. In neighboring zones, human-triggered avalanches were failing on this weak interface even before the big load of new snow fell earlier this week (photo, photo). In the days since the storm, skiers and riders have triggered multiple slides that failed on this weak interface. On Thursday, Chris investigated an avalanche near Galena Summit that was remotely triggered by skiers from flatter terrain above (video at right/above). In the past two days, I look for these layers in both the eastern and western portions of this zone. Even though the two snowpacks that I encountered differed significantly, weak snow buried about 1.5 feet deep produced unstable snowpack test scores in both places.

    The culprit weak layers in all of these slides have been sugary facets that developed in March during clear and cold weather between storms. The distribution and sensitivity of these layers are somewhat irregular.  You are most likely to find them on slopes that face E-NE-N-NW. On these aspects, it's best to assume these layers exist and choose lower angled terrain not threatened by steeper slopes above. These slabs are more likely to be reactive in areas where the wind has stiffened the snow surface and added more weight, but they are possible in sheltered terrain as well. On Thursday above the Soldier Mountain Ski Area, a snowmobiler triggered a large wind slab avalanche but fortunately was able to ride out of it (photo). The hard wind slab broke 2-3 feet thick and pulled into very gentle terrain at the ridge crest. It's possible this slide may have involved the persistent weak layers described above.

    Dealing with persistent weak layers is tricky business, particularly as they become more deeply buried. Snowpack test results become more difficult to interpret and obvious signs of instability are typically absent. The first clear sign of an unstable snowpack may occur when you trigger a large avalanche. These types of slides can be triggered remotely—from a distance above, below, or to the side of a steeper slope.

  • Loose Wet

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1

    Above-freezing temperatures at lower elevations and rain falling below 7,000' are wetting the snowpack. Keep an eye on the condition of the snow surface. If it is wet or if you are seeing wet balls of snow forming and rolling downhill it is time to think about looking for a slope where the snow surface is colder and drier. This problem is most likely on lower and middle elevation slopes, regardless of the direction they face.

    ADDITIONAL DISCUSSIONThe snowpack in this zone is very weak, particularly on slopes that face the northern half of the compass. In these areas, water may be percolating down through the snowpack and further weakening the faceted snow described in the persistent slab problem above. This process can result in wet slab avalanches that wipe out much of the season's snowpack. These types of slides are both destructive and unpredictable.

Avalanche Discussion

Our snowpack is getting trickier to deal with by the day. Persistent weak layers that formed in March are buried in the upper few feet of the snowpack. These layers produced human-triggered avalanches six out of the past nine days. Many of the avalanches that failed during the widespread natural avalanche cycle (and earthquake) last week stepped down to deeper weak layers. For days we've been warning you about a snowpack that is behaving much more like a mid-winter snowpack than one that you typically find in April (read yesterday's Forecast Discussion for more details on this).  Today and in the coming days that picture will become even more complicated. 

Snowpacks do not respond well to rapid change. Last week we saw how strong winds and heavy snowfall can result in widespread natural avalanche activity (and that earthquake didn't hurt). Today and in the coming days, we will "get" to watch as our unique snowpack begins its annual springtime transition. Temperatures are expected for be 5-10 degrees F above seasonal averages, likely hitting the warmest sustained temperatures we've seen all season. Today, we will couple those warm temperatures with a dose of rain. Sound complex and a bit gross? Good.

What impact will this rapid warming and warm precipitation have on the snowpack? It depends on what elevation band you are traveling in. At lower elevations, where rain is falling on an unfrozen snowpack, we will see the increasing possibility of loose, wet snow avalanches. At middle and upper elevations the warm temperatures and heavy wet snowfall are quickly helping the snow that fell last week consolidate into a hefty slab. Slabs like this do a better job of "communicating" a fracture, which may help to keep the persistent slab problem on the menu for some time. At the transition, where a dry snowpack is becoming wet for the first time we may begin to see a wet slab problem developing as liquid water percolates down to and destabilizes weak layers in the snowpack. Yuck.

As backcountry travelers, some of the best tools we have in our toolkit are our powers of observation and pattern recognition. We can look at the snow surface and tell if the wind has been at work. We compare these observations to those of other wind slabs we have dealt with in the past and make a prediction about how a slab will behave based on that. However, if we are looking at a pattern that we don't recognize or don't have much experience with, this tool becomes useless. If you go into the backcountry today there is a pretty good chance that you will be dealing with a snowpack that you don't recognize. Bear this in mind as you travel through the mountains and build in an appropriate safety margin to help you manage that.  

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