Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

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

Dangerous and complex avalanche conditions exist. Triggering 1-3 foot thick avalanches is likely today. Large natural avalanches are possible. Avalanches will be easier to trigger in terrain affected by the wind, but are possible in sheltered terrain as well. Avalanches can be triggered from long distances above, below, and to the sides of steeper slopes. 

Problemi valanghivi

  • Persistent Slabs

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

    Layers of weak snow in the upper snowpack can produce large, wide, 2-3' thick slab avalanches. You're most likely to trigger a slide involving these layers in areas where the wind has built a slab on top (read Problem #2 below). However, enough snow has fallen here that avalanches failing in terrain sheltered from the wind are a very real possibility. Avalanches that fail on these weak layers will be large and destructive, breaking significantly wider and deeper than a simple wind slab. You can trigger large avalanches on these layers from gentle terrain above, below, or to the sides of steep slopes, a process known as remote triggering.

    Last week I spotted a large avalanche on Copper Mountain. It broke about 2 feet deep on a layer of weak facets. The slide appeared to have been remotely triggered by a skier on a low-angle ridge above a steeper slope (photo of the triggering track and crown). While the avalanche was initiated in wind-loaded terrain, crowns opened up in apparently sheltered terrain as well (photo at right/above). Friday on Copper, my snowpack tests returned repeated unstable results. A dense, 2+ foot thick slab failed cleanly and slid off into my lap with each test. The combination of a recent large natural avalanche, an ugly snowpack structure, and unstable snowpack test scores, was plenty of evidence to keep me out of big, steep, complex avalanche terrain. Additional snow and wind loading overnight have only made this problem worse. 

    The nature of these weak layers changes from slope to slope, but it's safe to assume that they exist on most slopes.  Expect to find weak, sugary facets on shaded slopes and crust+facet sandwiches on slopes that face the sun. The best management strategy for this persistent slab problem is terrain management. Today I recommend you choose simple, conservative terrain to play in. Pay close attention to the terrain around you and above you, and spend some time contemplating the potential outcomes of being caught in an avalanche in the terrain you are in. Cliffs, trees, and terrain traps magnify the consequence of even small slides.

    (2/28/2023) NE face of Copper Mountain at 8,400'. This avalanche appeared to be remotely triggered by a skier from above. This crown in sheltered terrain points to the potential for encountering avalanche problems outside of areas that are obviously affected by the wind.

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1.5–2

    4-8" of snow (0.3-0.4" of SWE) fell in this zone in the last 24 hours. Wind speeds increased overnight, blowing at moderate speeds out of the SW-S-SE. As I write this, soft, 1-2 foot thick wind slabs are building (and potentially failing naturally) in terrain exposed to the wind. These slabs will be largest, thickest, and most common in upper elevation terrain. Expect them to be easy to trigger where you find them.

    Wind slabs commonly form near ridgelines and on the sides of mid-slope features like gullies and ribs. If the snow you are moving through becomes deeper or denser you are likely traveling on a wind slab. Evidence of wind-loading may be subtle or may be masked by additional snow that fell after slabs formed. Cracks shooting out from your skis or sled and collapsing of the snowpack are both clear indications you've found an unstable slab. Wind slabs that fail can be effective triggers for the deeper weak layers described above.

    (2/28/2023) This large avalanche was observed on a N aspect of Copper Mountain near Banner Summit. The crown extends into terrain that is mostly sheltered from the wind.

Avalanche Discussion

Avalanche conditions are both dangerous and complex. This is a bad combination. Periods of drought and snowfall in January and early February built a variety of weak layers. A series of storms beginning in mid-February have gradually buried these weak layers underneath an increasingly dense slab of snow. The ongoing slew of avalanche activity is clear evidence that this combination of slab+weak layer has the potential to generate very large, destructive avalanches.

Everything you just read was true yesterday, before last night's storm. This quick-hitting, intense feature is exiting our area this morning, but don't let the fact that this storm took place while you were asleep lull you into complacency. It just dumped snow for 6-8 hours, accompanied by honking winds out of the SW-S-SE. Yesterday was dangerous and things only got worse. I expect that large natural avalanches took place overnight.

This morning's sunshine will quickly affect the new snow, generating a number of small, loose snow avalanches. Though small, these slides can be effective triggers for wind slabs or deeper weak layers. The sun will also help the new snow consolidate into a more cohesive slab. While rare, this process can increase the likelihood of encountering slides.

When conditions are dangerous and complex the best solution is a simple one. Today is a good day to dial back your ambitions and choose some conservative terrain to play in. With widespread persistent weak layers in the snowpack and a meaty slab sitting on top of them, this means paying close attention to the terrain above you as well. Large avalanches can be triggered from long distances away from steeper slopes, and the potential for large natural avalanches lingers today. 

Skiing and riding conditions are excellent in low-angle terrain right now. Enjoy the gift of new snow while respecting the hazards that the mountains are presenting. Make returning home safely to family and friends your priority today.

You can catch up on what happened this past week in Ethan's latest Weekend Update.

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