Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Sat
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
2 · Moderate
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Highlights

Ample low-density snow and steady moderate wind will create dangerous wind drifts in steep, exposed, upper and middle elevation terrain. This added snow load will also stress buried weak layers, making large avalanches more likely today. You can reduce the chances of getting caught in an avalanche by sticking to slopes more sheltered from the wind.

Avalanche problems

  • Wind Slabs

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

    Banner Summit will pick up 8-10" of new snow by 5 PM, adding to last night's 2-6". This storm is producing low-density snowfall of about 4-5%.  Wind will blow moderate to strong from the west, and it won't take much wind to get this snow moving. Expect whiteout conditions at upper elevations and any middle elevation terrain exposed to the wind. 

    Yesterday professionals on Copper Mtn found that thin fresh wind deposits and cornices were sensitive to their weight. Expect these drifts to grow in size and extent today. Wind slabs are most commonly found near exposed ridgelines, on the sides of cross-loaded gullies, and lower on slopes near prominent terrain features. Some thin slabs may sit atop temperature or rain crusts that formed Monday; making them particularly touchy.

    Recognize that active wind-loading also adds stress to the weak layers buried deeper in the snowpack (described in Problem 2 below). Any wind slab you trigger could step down to these deeper layers, producing a much larger, more dangerous avalanche.  You can greatly reduce your risk of triggering an avalanche of any variety by sticking to slopes that are sheltered from the wind.

  • Persistent Slabs

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

    Today's storm will add stress to weak layers buried near the surface, in the middle of the snowpack, and near the ground

    • Periods of sun, warm temperatures, and rain created a variable mix of crusts and facets near the snow surface. These crust + facet layers along with small facets beneath the current storm snow may produce smaller but touchy slab avalanches as snow accumulates.  
    • Multiple weak layers exist in the middle of the snowpack. Wednesday, Chris observed two large avalanches in the Sawtooths near McGown Peak (observation). 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 small slab in very steep terrain.
    • Weak snow also exists near the ground (depth hoar). 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.

    Given the uncertainty surrounding these weak layers, skiing or riding in "prime-time" avalanche terrain remains risky. You are more likely to trigger an avalanche on wind-affected slopes steeper than about 35 degrees, but these weak layers exist in sheltered terrain as well.

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

Avalanche Discussion

It's probably going to be pretty nasty out there today. With low-density snowfall, it won't take much wind to create whiteout conditions. Today's a good day not to bite off more than you can chew. Conditions will worsen throughout the day as steady wind and snow build fresh slabs on most slopes exposed to the wind. 

Wind slabs are likely to be touchy. We've had a grab-bag of weather this week including rain Monday afternoon, snow and strong wind Tuesday, sun, light snow, and graupel Wednesday, and clear, cold nights. This mix of weather created a smorgasbord of snow surfaces that will sit beneath today's snow.

I'd expect the majority of the small avalanches we see today to 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.

Larger avalanches, failing on weak layers deeper in the snowpack, are also more likely today. I think it's important to remember that many of our northern and western mountains have seen nearly continuous snowfall in the last two weeks. Sure, there have been a few lulls, but, as far as our persistent weak layers are concerned, I see this storm more as a culmination of a much longer loading event. As a complex picture becomes even murkier, I'd urge you to take a big step back in your terrain choices and avoid all steep, wind-loaded terrain. 

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