Bulletin d’avalanche

Banner Summit

Sawtooth Avalanche Center
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Danger d’avalanche

Fri
Alpin
3 · Marqué
Limite forestière
2 · Modéré
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 6 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

You could trigger wide avalanches up to 2 feet thick. You're most likely to trigger large slides where the wind has formed drifts in recent days.  Avalanches can be triggered from gentle terrain below, above, and to the sides of steep slopes. Loose snow avalanches are possible on very steep slopes that face the sun. Small slides could trigger avalanches on deeper weak layers as they travel downslope.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    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 likely. The majority of our recent large avalanches failed on wind-loaded slopes on the east half of the compass.

    Continue to build wide safety margins into your backcountry travel. 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. 

    (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.

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2

    Moderate and strong N-NW wind formed a fresh batch of wind slabs earlier this week. In many locations, these slabs rest on a crust formed at the tail end of the winter solstice storm. Yesterday, I found one of these sensitive drifts in the Smoky Mountains. 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.

Avalanche Discussion

And Now the Hard Part

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'.

In the last few days, light, dry snow continued to fall in the northern mountains, accompanied by NW wind. These fresh 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 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.

We're distancing ourselves from the heavy loading events of Sunday and Monday, but heavy snow and the added weight of wind slabs this week will keep the snowpack on edge. Mild, calm weather today and tomorrow will give the snowpack a rest, but persistent weak layers are slow to adjust to load. Signs of instability may become less frequent, but the ingredients for a large avalanche remain. 

Régions couvertes