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

You are most likely to trigger an avalanche in steep, exposed, upper and middle elevation terrain where the wind has built slabs or formed drifts. You can dramatically reduce the chances of getting caught in an avalanche by sticking to sheltered slopes with soft snow on the surface.

Avalanche problems

  • Wind Slabs

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

    In the past two days, professionals near Copper Mtn reported that small wind features were reactive to ski cuts, and they avoided very steep avalanche paths and all wind-loaded starting zones.

    Continued light snowfall may mask some of the obvious signs that the wind has been at work, but it's a safe bet to assume many upper elevation slopes and some middle elevation terrain have received at least some wind loading in the past two days. 

    Expect to find wind slabs near ridgelines or lower on slopes in exposed terrain. Some thin slabs may have been deposited above temperature or rain crusts that formed Monday; this setup could make it easy to trigger small avalanches. 

    ADDITIONAL DISCUSSION:A number of weak layers exist in the top 3' or so of the snowpack. The Idaho Transportation Department HWY21 avalanche program reported several natural, large slides that released below 7500' on W-NW aspects while the road was closed Monday night. Last week, a snowmobiler near Cape Horn Summit triggered two avalanches remotely (one is pictured at right/above), and a skier triggered a slab in very steep terrain.

    In addition to the weak layers in the upper portion of the snowpack, there is weak snow near the ground (depth hoar) in this zone. You'd have to be truly unlucky to trigger a slide this deep, but this layer has kept us out of the terrain where it is most likely to exist—namely very steep, rocky, and wind-affected upper elevation terrain.

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

Avalanche Discussion

TODAY'S SNOW SURFACE, TOMORROW'S WEAK LAYER

A storm will arrive today bringing snowfall to much of the western and northern reaches of the forecast area including Banner Summit, the Sawtooths, the Smokys, and the Soldiers. As the current snow surface is buried, it's a good idea to take stock of the interface that may prove to be our next problematic weak layer. 

We've had a grab-bag of weather this week including rain Monday afternoon, snow and strong wind Tuesday, sun, light snow, and graupel yesterday, and a clear, cold night last night. This mix of weather created a smorgasbord of snow surfaces that will sit beneath tomorrow's storm.

If I had to guess, the touchiest interfaces we'll see are those that involve facets above or below a crust (or two). Crusts exist on many slopes near or just below the snow surface. Warm ambient air temperatures Monday climbed into the low 40s F. Snow surfaces at low and middle elevations (both sunny and shaded) melted during the day and refroze overnight. The warm air also turned some snow showers to rain up to 9,000'. 

Crusts also formed on most steep slopes that face the sun, even at upper elevations. Check out this cool photo Scott took on Tuesday where you can see the sun reflecting off of a crust high on McDonald Peak in the southern Sawtooths.

Middle and upper elevation, shaded aspects are the areas most likely to be crust-free, but they don't necessarily get to duck out of the potential weak layer category. As skies cleared last night dry, very cold air likely promoted faceting in many areas. 

Throwing in recent wind effect and snowfall differences across the area I'd expect the patterns of weak layer distribution to be highly variable. Stability may change over short distances as you climb in elevation or wrap to different aspects. This storm will also add stress to known weak layers in the middle and at the base of the snowpack. As a complex picture becomes even murkier, plan to build a larger margin for error into your travel plans. 

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