Bulletin d’avalanche

Banner Summit

Sawtooth Avalanche Center
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Danger d’avalanche

Sun
Alpin
4 · Fort
Limite forestière
3 · Marqué
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 7 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Very dangerous avalanche conditions exist. Thursday's storm loaded a weak, early-season snowpack, and human-triggered avalanches occurred yesterday. Avoid traveling on slopes steep enough to avalanche. Recognize that you could trigger avalanches from above, to the side, or from below steeper slopes.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Very Likely
    Taille
    2

    Thursday's snowfall and wind added a good thump to an early-season snowpack with several prominent weak layers (read Forecast Discussion below). While persistent weak layers exist on all aspects at mid and upper elevations, they seem most dangerous on shadier slopes. Chris and Ben were near Banner Summit yesterday where they experienced widespread collapsing and cracking of the snowpack. Walking a ridge above steeper terrain produced collapses that remotely triggered several avalanches - these broke 16 inches deep and were more than large enough to bury or injure a person. Even though the avalanche warning has expired, avoiding being on or beneath slopes steep enough to avalanche remains the best approach.

Avalanche Discussion

If you're a Cliff Notes type, here's the short story: our forecast area has a complex, early-season snowpack with several prominent weak layers. Thursday's storm added a decent wallop to these weak layers in the form of new snow and wind, especially in our northern and western zones. While there is some variability across our forecast area, we're just talking about different shades of dangerous. The best course of action ranges from a very conservative, cautious approach in our southern mountains to outright avalanche terrain avoidance in our northern and western zones. 

If you're ready for the longer story, let's take a closer look at some of the weak layers we're dealing with:

September and October Depth Hoar:The snow that fell in the early fall turned to well-developed facets and depth hoar during our dry November. This layer is most prominent on upper elevation, northerly-facing slopes and in alpine terrain - places where this snow didn't melt during the November drought. After getting loaded by the Thanksgiving storm, this layer started acting up and producing natural avalanches and obvious signs of instability. This layer is pretty obvious—it's the large-grained, sugary snow found near the base of the snowpack. You can feel it with your ski pole, or by giving your machine a bit too much throttle. 

Thanksgiving Storm Snow:Thanksgiving brought some much-needed snowfall, but cold, clear weather following the storm quickly turned this to weak facets. This layer is most prominent in our southerly zones that received the brunt of the Thanksgiving storm, and is less common in our northern mountains that were left out of the Thanksgiving fun. This layer produced some remotely-triggered avalanches in the Eagle Creek drainage following the December 7th storm.

December 7th Surface Hoar:The December 7th storm also buried a layer of surface hoar, and Thursday's storm added additional load to this layer—especially in the Western Smokys, the Sawtooths, and on Banner Summit. Yesterday, this layer was very reactive on Banner Summit. We saw every sign of instability in the book: poor snowpack test scores, collapsing and cracking of the snowpack, and natural and remotely-triggered avalanches.

Régions couvertes