Lawinenlagebericht

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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Lawinengefahr

Fri
Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
2 · Mässig
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Kernpunkte

Human-triggered avalanches are possible—particularly where wind forms drifts or hard layers at the snow surface. While unlikely, we can't rule out triggering a very large, 2-4 foot thick avalanche—especially on very steep, rocky, wind-affected slopes. The danger will increase tonight as the first of a series of storms impacts the area.

Lawinenprobleme

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Likely
    Größe
    1–2

    Beginning this afternoon, a series of storms are forecast to bring moderate wind and snow to the Sawtooths and the western Smokys. You may find small, touchy wind slabs as early as this evening, and likely by tomorrow morning. This problem will be most widespread at upper elevations with greater snow amounts that are exposed to stronger wind. 

    Be alert to this problem if you're out later in the day as fresh slabs can form in a surprisingly short amount of time. Look for active wind-loading and cracking in the snow surface. 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.

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    2–3

    Two prominent persistent weak layers are worth tracking:

    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.

    You are most likely to trigger a persistent slab in "prime time" avalanche terrain—slopes over about 35*—that has been loaded by the wind.  Keep in mind that these slides can be triggered remotely—from below, above, and to the sides of steeper terrain.

    (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).

Avalanche Discussion

A series of storms are stacking up in the Pacific and snowfall will arrive in the majority of the forecast area by noon today. The first storm will bring snow this afternoon and into Friday morning. The second storm will bring an additional pulse of moisture during the day on Saturday. For both systems, expect a heavy bias toward greater snow amounts near Banner Summit, the Sawtooths, and the western Smoky Mountains. 

In the Sawtooths and Western Smokys, over a foot of snow is forecast by Sunday morning. During the storm, wind will generally blow moderate from the west. Expect the danger to rise tonight and tomorrow as accumulations stack up and wind redistributes this new snow into a fresh layer of sensitive wind slab. These slabs will be thicker and more widespread at upper elevations. 

This year, it's been difficult to disassociate new snow avalanche problems from the effect they have on buried weak layers. Today's fresh wind slabs and storm snow will add stress to the layers of surface hoar and facets described above. However, a lack of recent avalanche activity, improving snowpack test scores, and fewer reports of snowpack collapsing all suggest improvement. That said, I'd look at the next few days as a test. Where and if we see activity on existing persistent weak layers will give us great information moving forward. 

As the storm progresses, wind slabs will quickly become the primary avalanche problem and remain that way throughout the storm. In the next few days, finding safer places to play will primarily involve avoiding areas with recent wind-loading and obvious stiffer layers near the snow surface. 

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