Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Sat
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
1 · Low
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Out of date — this forecast expired 6 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

You're most likely to trigger large avalanches where the wind has formed stiff drifts.  Large avalanches are possible but less likely terrain more sheltered from the wind. Avalanches can be triggered from gentle terrain below, above, and to the sides of steep slopes. Stormy weather will increase the danger this weekend. 

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    Heavy wet snow and rain caused a widespread natural avalanche cycle Sunday evening into Monday. Persistent weak layers are slowly adjusting to the added load but triggering a large persistent slab avalanche remains possible. The majority of our recent large avalanches failed on wind-loaded slopes on the east half of the compass.

    Persistent slabs can release in surprising ways and can be triggered from gentler terrain below, above, and to the sides of steep slopes. These slides can break wider than expected, spanning multiple terrain features that often confine storm slabs and wind slabs. 

    Steer clear of steep slopes if you see cracks shooting out from your skis or sled, or if you feel the snowpack collapse or hear whumpfing noises. Pay attention to the terrain that you are in as well as the terrain that you are connected to. 

    Additional Discussion:Moderate and strong N-NW wind formed a fresh batch of wind slabs earlier this week. While these slabs should be relatively small and isolated, they could knock you off your feet or sled. In addition, these slabs are adding stress to the weak snow described above. An avalanche that starts as a small wind slab has the potential to step down to deeper weak layers buried in the snowpack, resulting in a large and dangerous avalanche.

    (12/22/2020) Numerous crowns are visible in this photo of the N aspect of Copper Mountain in the Banner Summit Zone. The NE aspect (on the left side of the photo) also appeared to have avalanched.

Avalanche Discussion

Our role as forecasters (and backcountry travelers) can be fairly straightforward on days when the avalanche danger is HIGH or on days that it's LOW. On either end of the danger scale, we often have a greater degree of certainty and our travel advice can be simple. In the middle ground, when the danger is MODERATE or CONSIDERABLE, conditions are often much trickier, variable, and nuanced. This is especially the case now, as we sort out the complexities dished out by a "once in a decade" weak layer, diverging snow depths from zone-to-zone, and a rain event on the Solstice. 

The Winter Solstice storm brought heavy snow and rain favoring the southern Salmon River Mountains, those around Banner Summit, and the western portions of the Sawtooths and Smokys. This event prompted our first AVALANCHE WARNING of the season. Since then we've observed the aftermath of what turned out to be a widespread avalanche cycle. Much of the observed activity occurred on slopes that face N-NE-E-SE. On these slopes, the combination of weak underlying facets and intense wind-loading proved too much to handle. At the tail end of this storm, some of the precipitation fell as rain or a wet snow and rain mix. Clearing skies and frigid cold air from the north froze this moisture into a rain crust that we've found on most slopes up to at least 9,200'.

Following the storm, light, dry snow continued to fall in the northern mountains, accompanied by NW wind. These small wind slabs sit atop the rain crust and add additional weight to the weak facets buried deeper in the snowpack. 

It can be easy to get lost in the weeds here, tracking weak layers and crusts, but thankfully (or not?) there is one constant–weak facets lower in the snowpack. Whether you're in the Wood River Valley, the Smokys, or at Banner Summit, this weak layer exists. What varies from zone-to-zone (and even across a single zone) is the thickness and composition of the overlying slab which is generally thicker in the western and northern mountains that were favored with recent snow. Generally speaking, you're more likely to get yourself into trouble in areas where this overlying slab has been stiffened. This may be in areas exposed to the wind, or in areas sheltered from the wind which saw more recent warm, heavy snow and a bit of rain.

Looking ahead, another round of snow and wind will bring stormy weather through Sunday. The added load is likely to again tip the scales toward instability, particularly in wind-loaded terrain. Southwest flow may bring heavier snow to areas that have built less of a slab atop our weak facets but are primed for avalanche activity as the slab thickens (the Soldiers and southern Smokys come to mind). With proven weak layers and a weak foundation in our snowpack, I'll be expecting instability with each storm until proven otherwise. 

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