Avalanche forecast

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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Avalanche danger

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

While obvious signs of instability (avalanche activity, snowpack cracking and collapsing) are decreasing, avalanche conditions remain tricky. If you trigger a slide, it may be a large destructive event powerful enough to break trees. Steep slopes where several days of strong winds formed hard layers at the surface are most suspect. Slopes with soft powder on the surface are less likely to produce avalanches.  

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2–3

    Our snowpack is plagued by two prominent persistent weak layers:

    1. Facets and isolated surface hoar buried 1-2' down: The late January and early February storms and recent multi-day wind event produced many natural and human-triggered avalanches on this layer. It was the likely culprit in a remotely-triggered slide in Beaver Creek (photo). 
    2. December facets, now buried 3-4' down: The good news—this ugly layer has become harder to impact as the burial depth has increased. SAC forecasters and others are seeing fewer obvious signs of instability (like collapsing of the snowpack). The bad news—the first sign of instability you see may be a large, unsurvivable avalanche like the recent slide near McDonald Peak (photo to right/below). This layer is trending towards a deep persistent slab problem: difficult to trigger but terrible consequences if you do.

    We are just removed from a multi-day wind event that loaded many middle and upper elevation slopes. Avoiding  "prime time" avalanche terrain—slopes over about 35*—with drifts or hard snow at the surface will greatly reduce your chances of triggering a persistent slab avalanche. Keep in mind that these slides can be triggered remotely—from below, above, and to the sides of steeper terrain. Hard slabs that support the weight of skiers and sledders are notorious for producing avalanches that break wider or higher than expected. 

     

    (2/9/2021) Crown and debris from a large natural avalanche on McDonald Peak in the southern Sawtooths. This slide occurred sometime overnight on Sunday, during the ongoing wind event that loaded many slopes with stiff, dense slabs, and scoured many others. Visible are textures that indicate scouring (in the upper left) and loading (in the bottom right).

  • Wind Slabs

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

    Several days of strong W-NW winds formed slabs, drifts, and hard "wind board" on exposed slopes. Some examples of recent, wind-driven avalanche activity:

    • Monday, a large avalanche was reported in upper elevation terrain in the southern Sawtooths. It likely released naturally Sunday night. 
    • Sunday, a group of riders in the Soldier Mountains triggered a 1-2' thick wind slab on a south-facing slope at 7,500'.
    • In the Sawtooths, Ben found several recent wind slabs in middle elevation terrain (photos here and here).
    • In the Smoky Mtns this weekend, Scott and Chris watched the wind rapidly loading exposed slopes. Chris remotely triggered a small(ish) wind slab avalanche from about 50 feet away (video to right/above) in the Prairie Creek drainage.

    Stiff, hard slabs formed in unusual locations. Wind slabs are more widespread at upper elevations but may exist in all but the most sheltered terrain. From an avalanche danger perspective, the larger concern is still the additional loading on the persistent weak layers described in Problem 1. Small wind slab avalanches (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). Monday, he spotted several large, improbable cornices threatening slopes in the Sawtooths. 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 exposed 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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