Bulletin d’avalanche

Banner Summit

Sawtooth Avalanche Center
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Danger d’avalanche

Wed
Alpin
2 · Modéré
Limite forestière
1 · Faible
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 7 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Wind slabs created during last week’s storms and shifting wind events are the primary hazard today. Be on the lookout for these slabs in steep upper elevation terrain, where they will be largest and easiest to trigger. Watch for thin, freshly forming slabs to be touchy as wind speeds increase this afternoon. A recent avalanche in this zone indicates the potential of encountering avalanches failing in the upper snowpack even on slopes that aren't wind loaded. Triggering an avalanche on deeply buried weak layers is unlikely, but not out of the realm of possibility. Keep this problem in mind, particularly when traveling in steep, rocky, wind affected terrain.

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2

    Moderate to strong winds blew out of a variety of directions during last week's storms. These winds readily transported the foot of snow that fell. Thanks to the shifting wind directions, the slabs that formed are distributed in an irregular pattern in upper elevation terrain. During the final day of snowfall (Saturday, 2/8), wind speeds dropped dramatically, leaving a nice blanket of low density snow on top of these slabs. On Sunday, as I traveled through upper elevation terrain in the Sawtooths, I found that this blanket made it challenging to predict the distribution of older slabs. 

    Sunday night, an increase in wind speeds transported some of that soft snow that fell at the end of the storm. This afternoon, wind speeds are again expected to increase. Increasing wind speeds allow the wind to build slabs in exposed locations. These slabs are likely to be touchier than the older generations of slabs, but they will likely be relatively small. Older, stiffer slabs underneath will be less sensitive to your weight, but the resulting slides will be larger. Expect to find these slabs in upper elevation terrain, near features that slow the wind down and force it to drop the load that it is carrying.

     

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2

    Yesterday, I observed an avalanche that failed on the east side of Copper Mountain (photo at right). Based on the appearance of this slide and the location it failed in, it may have released on a thin layer of facets that formed near the surface and were buried by subsequent snowfall. We don't have much information into how widespread this problem may be yet, but it is worth keeping on your radar if you are traveling in this area. You'll be most likely to encounter this problem where the winds have added additional load, but even in the absence of wind these avalanches may be possible.

    ADDITIONAL DISCUSSIONTriggering a 4-6 foot thick persistent slab avalanche on weak snow near the ground has become very unlikely. The weak layers we've struggled with all winter have not gone away, but they're buried at a depth where they are very difficult to impact with the weight of a skier or rider. Hard to trigger is not the same as impossible—you'll do well to keep them in mind and continue to factor them into your decision-making. It would take a dose of very bad luck, but it may be possible to trigger an unsurvivable avalanche in very steep (over 35 degrees), alpine terrain that is more likely to have thin spots where your weight may reach the weak layers.

Avalanche Discussion

If you are a regular reader of the forecast, I’d imagine that you are getting tired of hearing about persistent slab problems. Unfortunately, we’ve been dealing with them all season and it is my job to talk to you about them. But, as we know, variety is the spice of life and I’ve got a bit of extra spice for you this morning. Drumroll please...a new persistent weak layer!

This layer isn’t buried in the snowpack yet, it is currently sitting on the surface. Cold, clear nights and days create conditions that are conducive to the formation of faceted snow grains, or facets for short. We call them facets because they have sharp, angular edges on them, much like a cut gemstone. Facets are problematic because they don’t bond well to the snow around them, which makes them a “great” weak layer. Transforming new snow into facets is a process that can take place quite quickly, but rounding (the process by which facets gain strength) takes place much more slowly. And there you have it, a weak layer of snow that will persist for some time.

It's much easier to look for faceted grains in the snowpack before they get buried. If you’re out and about before the storm this weekend, spend some time looking at the snow that is sitting on the surface and see if you can see identify some differences between it and freshly fallen snow. If you look hard can you still see recognizable snowflakes? Can you make a snowball out of it? Does it feel different on sunny aspects than shady aspects? Try to map out the distribution of facets at the surface and then watch how different areas behave as they get loaded.

In our northern areas, where significantly more snow fell last week, a weak layer of these facets may have already been buried. Yesterday, I was able to spot an avalanche on Copper Mountain that had the character of a persistent slab avalanche. Read the problem discussion for the Banner Summit zone for a bit more information. This avalanche failed well below the ridgeline where wind loading would be most intense, indicating that wind loading wasn’t likely the primary factor in its release. Whether or not this is a widespread problem or not is yet to be determined.

Régions couvertes