Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

Wed
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
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In sintesi

Triggering an avalanche large enough to bury a person is likely, especially in areas where wind has stiffened the snow surface or built thick drifts. You may be able to trigger these avalanches remotely—from flat terrain below, above, and to the sides of steep slopes. Avalanches could release on their own with today's snow and wind, so pay attention to the terrain above you.

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

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1.5–2.5

    What is it: Soft and stiff slabs (1-4 feet thick) formed over the weekend. The slabs formed above a variety of weak layers (facets, surface hoar, crusts, and crust + facet combinations). 

    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, board or sled. 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. 

A productive storm cycle over the weekend is nearing its end, with 1-2.5" of water and 10-30" of snowfall deposited since Friday. Freshly formed slabs rest on a mix of weak snow surfaces, including facets, surface hoar, and crust + facet combinations. These weak layer are widespread and vary with respect to aspect, elevation and drainage.  

Recent observations can be helpful tools to understand the character of these weak layers.  They can also highlight the levels of uncertainty associated with an ongoing storm and persistent weak layers.  Some examples: 

  • On Sunday, the Sun Valley Ski Patrol triggered 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.
  • On Sunday, 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'.
  • Forecasters in the Beaver Creek drainage did not see obvious signs of instability but expressed concern with poor stability test results and the well-developed, widespread nature of these weak layers. 
  • Yesterday we found weak snow at many elevations and aspects near Cape Horn Summit in the Banner Summit Zone.

Conditions are most dangerous where wind has formed thick, stiff slabs at the surface. Choosing slopes sheltered from the wind is a good way to decrease your exposure to potentially large avalanches. Weak snow can still be found in sheltered terrain, and we can't rule out the chance of triggering avalanches where the snow is very soft.

Today's travel can be simplified by choosing low angle terrain with no overhead hazard or potential terrain traps like cliffs, gullies or thick trees.

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