Bulletin d’avalanche

Banner Summit

Sawtooth Avalanche Center
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Danger d’avalanche

Tue
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
3 · Marqué
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Périmé — ce bulletin a expiré 4 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

You're most likely to trigger large avalanches on slopes where the wind has formed drifts or stiffened the snow surface. Slides are also possible in steep, wind-sheltered terrain and at lower elevations. Above-freezing temperatures, periods of direct sun, and wet snow/valley rain increase the possibility of wet snow avalanches. Pay close attention to the snow as conditions change through the day.

Avalanche Danger will increase as the storm arrives overnight and Avalanche Warnings may be issued for tomorrow.

Problèmes avalancheux

  • Loose Wet

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–1.5

    Above-freezing temperatures, periods of direct sun, and the potential for light valley rain this evening create the possibility of wet snow avalanches. These are most likely on steep slopes as the day heats up, particularly during periods of direct sun or rain. This zone has a lot of small avalanche terrain at lower elevations that can be hazardous (think road cuts, creek bottoms, snow-covered cliff bands, etc.). Any moving wet snow will be very dense and heavy and even small avalanches here can be hard to escape and can pile up debris deeply. 

  • Deep Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    2–3

    Triggering a 2-4' thick slab avalanche is possible on steep slopes. Persistent weak layers (sugary facets, surface hoar, or crusts) exist on most aspects and elevations. Weak snow is common in our region, but the widespread nature of this weak layer is unusual. Extreme wind over the Solstice reshaped the upper snowpack, thinning (scouring) some areas and building thick hard slabs in others. You're most likely to trigger one of these slides in recently wind-affected terrain. 

    The stiff slab of snow overlying the weak snow near the ground means you are not likely to experience obvious signs of instability like snowpack cracking or collapsing. However, if you do find the "sweet spot" remember that you could trigger slides remotely—from gentler terrain above, below, and to the sides of steep slopes. These avalanches may break wider than expected or higher on the slope above you. 

    Unfortunately, avoiding slopes steep enough to slide is the only way to effectively manage this problem. While your odds of triggering a large avalanche are decreasing, the consequences of triggering one are not. Sticking to lower-angled, wind-sheltered slopes is a tactic to effectively reduce your risk.

    Significant complexity and uncertainty exists in our snowpack. Pease read the Forecast Discussion below for more details.

    (12/12/2022) In the Banner Summit zone, the weak November snow is buried beneath a 3' thick dense slab. 

Avalanche Discussion

UNCERTAINTY: Today's forecast comes with significant uncertainty. We have a lot going on in our snowpack right now, and the weather is not helping. Temperatures climbed above freezing in the mountains yesterday. In many locations, they stayed there overnight. Today will bring even warmer temperatures and the chance for some light snow and rain this evening. Intense precipitation starts overnight and there is no doubt in my mind that tomorrow will be a very dangerous day in the mountains. What happens leading up to then is significantly more uncertain. 

UNPREDICTABILITY: Wet snow behaves in weird ways. We are used to dealing with wet snow issues in March, not December. The upper snowpack is changing and the results are unpredictable. I would not be surprised to see a relatively benign day and I would not be surprised to see things start to come unglued. Any wet snow issues are taking place on a stage filled with weak layers. 

COMPLEXITY: Let's take a broad look at our snowpack, from top to bottom. There is a lot going on here:

  • UPPER: strong to extreme winds and new snow built stiff, hard drifts in many locations and produced a widespread avalanche cycle last week. Additional snowfall and lighter winds built another round of softer slabs on top of these drifts and masked some of the older drifts. Above-freezing temperatures and periods of direct sun today will bring on the chance for wet snow avalanches in steep terrain. The upper portion of the snowpack became wet yesterday and users across our region experienced soupy/gloppy/wet-mashed-potato conditions. Some weather stations have not dipped below freezing in 24+ hours and these conditions will exist again today. Sound ugly? Read on, there's more.
  • MIDDLE: several layers of surface hoar, facets, sun crusts, and even a rain crust sit 6-24" below the snow surface. In recent days, I've experienced collapsing involving these layers and repeated unstable snowpack test scores (observations here and here, photo here). Some of the wind slabs that were deposited earlier this week broke wider than you might expect, indicating the involvement of one of these weak layers.
  • LOWER: everything I've described above is taking place on top of a widespread layer of weak snow buried at the base of the snowpack. This layer is impressive, both in the number of avalanches it has produced already (hundreds and hundreds) and in its widespread nature (it exists just about everywhere). Snowpack collapsing is decreasing in frequency, but we have rarely gone a day without a report of large, booming, rumbling collapses. On steeper slopes these collapses would result in avalanches. 

YOU: This is the only part of this messy picture that we have any control over. The weather and the snowpack are off on their own program, but we can control where we go and what terrain we expose ourselves to. Your best bet is to remain conservative in your terrain selection today, and to pay close attention to changing conditions.

Régions couvertes