Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

Fri
Alta quota
3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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In sintesi

Triggering avalanches is most likely in upper elevation terrain. Fresh wind drifts may be sensitive to your weight and are adding additional load to old, weak snow buried near the ground. Very large avalanches could fail on this old snow, particularly on shadier, NW-N-NE-E facing slopes.

Problemi valanghivi

  • Deep Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2.5–3.5

    In terrain where old, weak October snow exists at the bottom of the snowpack—middle elevation slopes that face away from the sun (NW-N-NE-E) and most aspects at upper elevations—we're facing a classic low-probability/high-consequence scenario:

    • Triggering a huge, 4-6 foot thick slab avalanche is becoming more difficult
    • If you do trigger one of these beasts, the consequences will be severe 

    Ben made a video Monday to help explain where you will find the weak October snow and why that is crucial to understand (watch it here).

    Riding in locations where the overlying slab is thinner, like steep rocky alpine terrain, will increase your chances of triggering one of these monsters. Large cornices falling onto slopes and smaller wind slab avalanches running downslope can also trigger deep persistent slab avalanches.

    (12/24/2021) Very large natural avalanche on "Marshall Slide" in the Sawtooths, on the shoulder of Williams Peak, above the Marshall drainage. This avalanche appears to have failed in the October facets deep in the snowpack. NE-aspect at 9,000'.

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–2

    Today's snow and wind will build wind slabs that are likely to be sensitive to your weight. Expect to find slabs and cornices in exposed, middle and upper elevation terrain. Cracks shooting from your boards or sled are a clear sign you could trigger a wind slab avalanche.

    There's a lot of soft snow out there today. If the snow stiffens or you notice that you're staying closer to the surface instead of sinking deep into the snowpack, you've found a wind-affected slope and should avoid steep, consequential terrain. These wind slabs could be hazardous on their own but are also stressing the deeply buried weak snow described in Problem 1 above. Steep slopes where these problems overlap are a dangerous place to be. Smaller wind slab avalanches or large cornice failures could provide the rapid force needed to "step-down" to larger avalanches failing on weak snow near the ground.

    Additional Discussion:Weak layers of facets, crusts, and surface hoar may exist in the upper half of the snowpack. These avalanches observed by Sun Valley Heli Ski on Tuesday, likely failed on one of these layers. We may see similar avalanches today, breaking wider and deeper than an average wind slab. This potential problem adds yet another reason to steer clear of steep, wind-affected slopes.

Avalanche Discussion

Similar Weak Layers, Differing Depths

A weak layer of facets and crusts near the ground (Problem 1 above) is an unfortunate constant across shadier, middle and upper elevation terrain.  This layer is responsible for the vast majority of the avalanches we've seen this season. An intense 5-day storm ended on Monday leaving 2-3'+ of snow and 2.5-3.5" of SWE. In some areas, this storm doubled the snowpack depth.

Quickly adding weight is a good way to push a fragile weak layer to its breaking point. The avalanches we observed during this prolonged storm are testament to that (report, report, report). However, as we move away from rapid loading events the odds of triggering an avalanche on this layer will begin to slowly diminish. Obvious signs of instability such as remotely triggered avalanches, snowpack collapses, and cracking in this layer become less frequent, and snowpack test scores can be misleading.

As Scott pointed out yesterday, initiating a fracture on a weak layer is easiest when the weak layer is buried less than about 3' deep. Much deeper and the force from you/your sled has a harder time impacting the weak layer. In the Banner Summit zone, this weak snow is buried well over 4' deep in most locations. Evaluating stability is difficult when there's a lot of soft, low-density powder at the surface. Performing snowpack tests by banging on a shovel at the snow surface isn't likely to produce a result, and you're not likely to feel a collapse. That said, the weak layer is still there, and it's still ugly. If you do initiate a fracture, it's likely to propagate.

Be patient. Don't get lulled into complacency by a lack of obvious signs of snowpack instability. A major storm event ended just 3 days ago and we've got a nasty-looking weak layer at the base of the snowpack that's shown it can produce massive, unsurvivable avalanches. Today's storm will add stress to this layer, and wind slab avalanches or large cornice failures could provide the rapid force needed to "step-down" to weak snow at the ground.

Regioni coperte