Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Fri
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
2 · Moderate
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Highlights

Triggering a 1-2 foot thick avalanche is possible—particularly where wind forms drifts or hard layers at the snow surface. While unlikely, we can't rule out triggering a huge, 4-5 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. 

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    Around Banner Summit, most of the recent observed avalanche activity failed on a layer of surface hoar 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 few 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.

    ADDITIONAL DISCUSSION:A weak layer of facets sits near the base of the snowpack buried beneath a stout 4-5' thick slab of snow. It is difficult for skiers and sledders to impact this layer because the slab over it is so thick and dense. It would take a dose of bad luck to trigger one of these deep slab avalanches, but it could happen in very steep, rocky, wind-affected terrain. 

    (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
    Possible
    Size
    1–2

    Beginning this afternoon, a series of storms are forecast to bring moderate wind and snow to the Banner Summit Zone. 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.

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 Banner Summit Zone, up to 18" 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 layer 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 likely 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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