Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

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

Conditions are dangerous, and natural avalanches may occur. Triggering avalanches large enough to break trees and bury people is possible on slopes where the wind has formed drifts or stiffened the snow surface. While the chance of triggering a large avalanche is highest in exposed, windy areas, avalanches are also possible in steep, wind-sheltered terrain. Avalanches may be triggered remotely—from gentler terrain above, below, and to the sides of steep slopes. At lower elevations, avalanches are less likely due to yesterday's rainfall. 

Avalanche problems

  • Deep Persistent Slabs

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

    What is it?  Look at the photo to the right/below; we added about a foot of dense, heavy snowfall since snowmobilers triggered that avalanche from a quarter mile away.Dense, 3-6 foot thick slabs overlie persistent weak layers (sugary facets, surface hoar, and/or crusts) on most aspects and elevations. Wind-loaded slopes are the most dangerous, but the ongoing storm makes slopes sheltered from the wind suspect, too. Yesterday, professionals traveling in the northern Sawtooth Mtns reported some snowpack cracking and questionable stability at the storm snow interface. The snowpack needs a break to adjust to yesterday's big storm, but it's not getting it today. Instead, expected snowfall and wind will continue stressing buried weak layers.

    Where is it?  This problem is widespread on most aspects at middle and upper elevations. While the vast majority of December's avalanche activity occurred on W-N-E aspects where the weak layers are ugliest, sunnier SW-S-SE facing slopes are not guaranteed to be safe. Wind-loaded slopes are far more dangerous than sheltered terrain. At lower elevations, yesterday's rain event pushed water into the persistent weak layers on many slopes, reducing the chances of triggering avalanches on these layers going forward.

    How can you recognize it?  This persistent slab problem is widespread; assume it exists on all slopes until proven otherwise. You may or may not experience snowpack collapsing and/or cracking while traveling. These red flags are obvious signs of instability. Wind-loaded areas feel stiffer under your skis or sled and keep you from easily sinking into the snowpack.         

    How can you manage it?  Avoiding slopes steep enough to slide is the only reliable way to manage this problem.Deep persistent slab avalanches are tricky. If you're playing near steep slopes, you could trigger slides remotely (from gentler terrain above, below, and to the sides of steep slopes—explanatory video here). These avalanches frequently break wider, deeper, and higher on the slope than expected. 

    (12/26/2022) Motorized users remotely triggered this large avalanche southeast of this zone in the White Cloud Mtns while riding in flatter terrain an estimated 1/4 mile away. Above Washington Lake, 10,500', E-NE aspect.

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1.5–2

    What is it?  Yesterday's rowdy storm formed soft and hard wind slabs in exposed terrain. In the afternoon, Ethan observed fresh wind slabs up to 2 feet thick near Titus Ridge.Today's snowfall and wind will continue building slabs, and natural avalanches are possible. The wind slabs lie above a variable surface (facets, crusts, "regular" snow, wind-polished surfaces, etc); some slabs will be easy to trigger while others may be "stubborn" or hard to trigger. Wind slab avalanches could "step down", triggering the large deep persistent slab avalanches described in Problem 1. 

    Where is it?  Strong and extreme winds can form slabs in unusual and unpredictable locations as well as typical places near ridgelines. You may find them near or far below ridgelines, on the sides of gullies, and near prominent terrain features in normally sheltered areas.

    How can you recognize it?  Cracks shooting out from your skis or sled are an obvious sign of instability. Watch for drifts, dunes, and ripples on the snow surface. Wind slabs can have a smooth, chalky appearance and feel and sound hollow, like a drum. The lower-density snow that fell overnight and is continuing today may hide or disguise large wind slabs that formed yesterday. Wind-deposited snow will be stiffer and keep your skis or snowmobile closer to the surface.       

    How can you manage it?  Given the likelihood of triggering wind slabs today, the simple answer is to avoid wind-loaded avalanche terrain. Hard slabs are notoriously unpredictable; these avalanches may break wider than expected or higher on the slope above you. 

Avalanche Discussion

The simple part of today's Forecast Discussion: DO NOT play in or below large, wind-loaded avalanche paths. As you climb in elevation, you'll encounter thicker wind deposits and more wind-loaded terrain. Traveling at upper elevations is not recommended today; you would be walking or riding through a minefield.  

Here are some details on the different elevation bands:

  • Below 7000': Significant rain fell yesterday. The rain saturated the snowpack on many slopes. Avalanches are possible today, but this elevation band is the least dangerous. Unfortunately, the skiing and riding conditions will not be pretty. 
  • 7000-9000': We have the perfect overlap of buried persistent weak layers and a big rapid-loading event (yesterday's storm). In most forecast zones, we've seen large, destructive natural and human-triggered avalanche activity on the buried weak layers with every decent storm this season. Although triggering large avalanches has become more difficult in the Banner Summit zone in recent weeks, I am not going to bet my life that today is the day we break that pattern. Travel with caution, and give large avalanche paths a lot of respect. In the last big storm, several slides pulled into nearly flat terrain (less than about 20 degrees) at the top of the slide paths. 
  • Above 9000': Given the huge new load and continuing snowfall and wind, traveling on or below steep slopes above 9000' would scare the heck out of me. No thanks. 

There's a ton of fun to be had enjoying the amazing early-season riding and sliding conditions. But, today is not the day to push into significant avalanche terrain, especially at middle and upper elevations.

Regions covered