Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Thu
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
2 · Moderate
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Highlights

A snowmobiler remotely triggered a 1-2 foot thick slide on a steep, wind-loaded slope yesterday near Cape Horn Summit. Steep slopes with wind drifting remain suspect today. In very steep, sheltered terrain, loose snow avalanches are also possible. While unlikely, we can't rule out triggering an unsurvivable, 4-5 foot thick avalanche—especially on very steep, rocky, wind-affected slopes. 

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    Yesterday, a rider triggered a slide from the bottom of a steep, wind-loaded slope in this zone (photo to right/below, Observation). With at least a couple feet of fresh snow in recent days, it doesn't take a lot of wind to form slabs. The 1-2 foot thick wind slabs appear to be fairly isolated to slopes immediately below exposed ridgelines, but they are there. Fortunately, they are easy to spot and avoid. Watch for and steer clear of slopes with cornices on the ridges. 

    On very steep slopes, triggering loose snow avalanches (sluffs) is possible. If the sun pokes through the clouds today, expect to see some natural loose snow avalanches. This problem is fairly simple to manage—if it's sunny, be aware of the terrain above you. Remember that even small slides can have high consequences if they carry you through rocky, treed terrain or push you down into a terrain trap like a gully or a creek bed. 

    ADDITIONAL DISCUSSION:There are two weak layers we are still tracking:

    1. A layer of surface hoar and small facets buried 2-3 feet deep: This layer has not produced avalanches in about a week, but Chris saw unstable test scores on it yesterday in this zone; he and his partner avoided avalanche terrain where it existed. 
    2. Depth hoar (large-grained weak snow) buried in December: A hard, 4-5 foot plus slab lies above this weak layer. You would have to be unlucky to trigger a slide this deep, but this layer has kept us out of the terrain where it is most likely to exist. What type of terrain? Slopes that are very steep, rocky, and wind-affected. The most recent slide involving this weak layer failed on exactly such a slope (photo).

    Chris found both of these layers in the Cape Horn area on Saturday—he describes them in this video.

    (2/16/2021) A snowmobiler triggered this avalanche from the bottom of the slope on Tuesday afternoon near Cape Horn Summit.

Avalanche Discussion

Naps, Hibernation, and Dormancy

The persistent weak layers that produced extensive natural and human-triggered avalanche activity in December and January are finally showing signs of going to sleep. Unfortunately, they aren’t “out cold” yet; they appear to be stuck in limbo.

Think back to a time when you were listening to a boring teacher or professor’s lecture on an early morning when you didn’t get enough sleep the night before. The drone of the speaker’s voice continues, you are not interested, and your head begins to bob and then snap back up as you fight the urge to fall asleep. This is how our persistent weak layers are currently behaving.

When you were falling asleep in class, someone gently touching your shoulder may or may not have been enough to get your attention; this reaction (or lack thereof) is akin to the past several days of low-density snowfall falling on many slopes. However, what if your instructor poked you hard in the ribs with a pencil? You’d instantly snap out of your partial-sleep mode and be more than a little peeved, much like our weak layers on slopes where the wind is rapidly forming stiff drifts and wind slabs, or where small loose snow avalanches are running downhill.

Until it stops snowing and blowing for an extended time, I expect the persistent weak layers on some slopes to be susceptible to a pencil poke in the ribs. The difficulty lies in:

  • Which slopes are nodding off to sleep, and which slopes are “out cold”?
  • How hard is it to poke the weak layers on the slopes where they are still nodding off to sleep? How deep is the weak layer buried?
  • If I wake one of the sleepy students/weak layers up, will it be a meek 80-pound student who tells me off or a 280-pound behemoth who gives me a royal beatdown?

Our snowpack has become quite complex in the past 3 weeks, and it's nearly impossible to answer those questions on a slope-scale unless you’re an experienced professional studying and sampling the same slide paths day in and day out. As most of us don’t have either the luxury or experience to do so, we are forced to travel a little more conservatively than we’d like to give ourselves a healthy safety margin. As always, I encourage you to think about the consequences of triggering a sizable avalanche before committing to riding or skiing large avalanche paths.  

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