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

Triggering a 1-2 foot thick avalanche remains possible. Steep slopes where winds have formed drifts or hard layers at the surface are most suspect. While unlikely, we can't rule out triggering a huge, 4-5 foot thick avalanche—especially on very steep, rocky, wind-affected slopes.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    We have two separate persistent weak layers to consider in this zone:

    1. Surface hoar and/or facets buried about 1.5-2 feet below the surface: Around Banner Summit, most of the recent observed avalanche activity failed on a layer of surface hoar (photo at right/above) and small faceted grains. We've seen this layer present itself as surface hoar, small facets and surface hoar, small facets, or facets above a crust (observation). Stability test results and a lack of avalanche activity the past couple days indicate this layer is strengthening, but we can't forget about it yet; human-triggered slides remain possible. You are most likely to trigger one in "prime time" avalanche terrain—slopes over about 35*—that has been loaded by the wind.
    2. December facets buried 4-5' down: This layer is deeply buried but has been slow to gain strength. Because the slab over this weak snow is so thick and dense, it is difficult forskiers and sledders to impact the weak snow underneath. Unfortunately, we're stuck with the bad structure (strong snow over weak snow) for now.  It would take a dose of bad luck to trigger a deep slab, but it could happen in very steep, rocky, wind-affected terrain where more "trigger points" allow us to better impact the weak layer.

    We are just removed from a multi-day wind event that loaded many middle and upper elevation slopes. Avoiding steep, wind loaded slopes will greatly reduce your chances of triggering a persistent slab avalanche.

    (2/4/2021) This avalanche failed 2' deep on buried surface hoar. This weak layer has presented itself as surface hoar in some areas and small facets in others. NE, 8400', Copper Mtn.

  • Wind Slabs

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

    Several days of unusually strong W-NW winds formed slabs, drifts, and hard "wind board" on exposed slopes. Our observations indicate the slabs seemed to be bonding well with the snow underneath. 

    From an avalanche danger perspective, the larger concern is the multiple days of loading slopes with the persistent weak layers described in Problem 1. Small wind slab avalanches (see video to right/above) can be great triggers for more deeply buried weak layers. Any avalanche that steps down to these persistent weak layers will be large and destructive. Slopes with soft powder on the surface are less likely to produce avalanches.  

    ADDITIONAL DISCUSSION: Cornices grew significantly in recent days. On Sunday, Ben found large, fresh cornices that were easy to trigger, often toppling off the ridge with a single kick (from a safe distance, of course). These dense blocks can penetrate deeply into the snow as the cartwheel downslope, providing a great trigger for both shallow and deeply buried weak layers. If you're traveling on corniced ridges, stay on or near the scoured rocks and trees and away from overhanging cornice edges. 

Avalanche Discussion

In the three southern forecast zones (all except Banner Summit), we have been struggling mightily with when to drop the danger to MODERATE. Why has this decision been so agonizing?

  • We were dealt the mother/grandaddy of all weak layers (the December facets) this year, one that has continued to impress, amaze, and scare us with avalanches during and after each loading event. It still looks awful in most areas—we haven't even started putting lipstick on this pig. 
  • The snowpack wants and needs a week of rest and relaxation at the beach (no storms or wind-loading) to heal, but the weather has not cooperated and doesn't look like it's going to in the coming days. 
  • Obvious signs of instability (avalanches, snowpack cracking and collapsing) are steadily decreasing, and snowpack test scores are showing gradual signs of improvement.

If we base the danger rating strictly on the chances of triggering avalanches and avalanche size, MODERATE is a good fit. A very scary MODERATE, but MODERATE nonetheless. If we base the danger rating on how we (SAC staff) and local professionals are traveling, CONSIDERABLE is a better fit; we continue to stay out of all sizable "prime time" avalanche terrain, slopes in the 35-45* range, and areas below "prime time" terrain. 

So what does this mean? The danger rating is an invaluable tool for us to communicate risk and hazard, but you should not base "go/no-go" decisions strictly on the danger rating, especially when faced with a scary MODERATE vs CONSIDERABLE persistent slab problem. I encourage you to think long and hard before committing to skiing or riding large or consequential avalanche paths; make sure you're happy with the risk vs reward facing you. 

 

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