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 2 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Triggering an avalanche large enough to bury a person is likely, especially in areas exposed to the wind. Natural avalanches are possible. You may be able to trigger avalanches remotely—from flat terrain below, above, and to the sides of steep slopes.

A fatality occurred yesterday near Cascade Lake, west of the Banner Summit zone (see Forecast Discussion). We send our sincere condolences to the victim's family and friends. 

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1.5–2.5

    What is it: Soft slabs (1-3 feet thick) formed over the weekend. The slabs formed above a variety of weak layers (facets, surface hoar, crusts, and crust + facet combinations). Today's storm will increase both the size and chance of natural and triggered avalanches.  

    Where is it: Conditions are most dangerous where wind formed drifts or stiffened the snow surface. Avalanches are less likely but possible on steep slopes that are more sheltered from the wind.  The widespread nature of the weak layers means avalanches may be easy to trigger on many aspects and elevations. 

    How can you recognize it: Watch for stiff snow; snow pillows, dunes, and ripples; and cracks shooting out from your skis or boards. These are signs the wind has been at work transporting snow. Assume the weak layers exist on most slopes. Snowpack collapsing or whumphing is a clear sign of unstable conditions.   

    How can you manage it: Persistent Slabs can release in surprising ways, break wider than expected, and release above or hundreds of feet away from where they are triggered. Remotely triggering an avalanche from flat terrain in the valley bottom is a real possibility. Keep a close eye on your partner and the terrain that's above you.

    Additional Discussion: While less likely, avalanches may fail on weak snow near the ground. The resulting avalanche would be large enough to destroy a house. This scenario is most likely on shaded NW-N-NE facing slopes above about 8,000'. 

    (12/15/2024) This large, natural avalanche released on Saturday or Sunday in wind-loaded terrain in the Beaver Ck drainage in the western Smoky Mtns, in the forecast zone to the south of the Banner Summit zone. E aspect near 10000'. 

    Photo: Ben VandenBos

Avalanche Discussion

Avalanche fatality: On Sunday, a snowbiker was buried in a terrain trap and killed in an avalanche in the West Mountain area near Cascade Lake. This area is located between the Payette and Sawtooth Avalanche Center forecast areas, about 30 miles west of the Banner Summit zone. You can view a preliminary report here. 

Some recent, pertinent observations:

  • Yesterday, the Sun Valley Ski Patrol trigggered several avalanches during mitigation work, before opening to the public. The slides were 12-18" deep and occurred in obviously wind-loaded terrain. While viewing the alpine terrain in the Boulder and Pioneer Mtns, they observed avalanches in wind-loaded areas and large plumes of wind-transported snow.
  • Near Banner Summit, a professional reported many plow triggered slides at lower elevations and was able to remotely trigger several additional avalanches. We observed similar unstable conditions (snowpack collapsing and 1 foot slabs fracturing near creek banks) in the valley bottom of the Spring Creek drainage near Galena Lodge.
  • In the western Smoky Mtns, multiple observers reported a large natural avalanche on an E aspect near 10,000'. 

The weekend storm snow (1-2" of water and 10-20" of snowfall) fell above several weak layers. While the weak layers are widespread, they change significantly (and appear or dissappear) with respect to aspect, elevation, and drainage; it's nearly impossible to definitively predict when any individual slope will avalanche as a result of the recent and ongoing snowfall and wind loading.

As forecasters, we get spooked when we struggle to anticipate exactly which weak layer or snowpack setup is most problematic. The current level of uncertainty has many avalanche professionals playing it safe and avoiding most avalanche terrain. 

If your choose to play in avalanche terrain today, two things can help you manage your risk: 

  1. Avoid steep wind-loaded slopes and the avalanche runout zones beneath them.
  2. Steer clear of terrain traps and large, consequential avalanche paths.

Régions couvertes