Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

Wed
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

You can trigger a 10-18 inch thick slab avalanche today, particularly where recent wind loading is adding weight to buried weak layers. Limit your exposure to this problem by sticking to gentler terrain not threatened by avalanche hazard from above. Generally safe conditions exist at lower elevations where the snowpack was soaked by last week's rain and is now frozen solid. 

Problemi valanghivi

  • Persistent Slabs

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

    The extended drought in January and February was interspersed with periodic snowfall in this zone. This pattern created a series of weak layers that are now buried 10-18" beneath the snow surface. These weak layers reached their breaking point during last week's storm and a natural avalanche cycle occurred here (observation and photos here). The type of weak snow responsible for these slides can take a long time to heal and triggering an avalanche involving this weak snow remains a concerning possibility today. Additional snow from today's storm will add stress to these layers of weak snow.

    These layers are best developed in sheltered terrain on the northern portion of the compass (NW-N-NE-E) at middle and upper elevations. I spent 4 days last week looking for these layers in this Zone and got mixed results in my snowpits. You can see all of my observations here. In some places these layers returned unstable snowpack scores, in others they didn't. In some areas they produced widespread avalanche activity and in others they produced significantly less activity. Digging a snowpit on every steep slope you want to play on is neither practical nor safe. The best approach is to take a step back and choose more conservative slopes where this problem may exist. 

    Want a little more detail on this? Read the Forecast Discussion below. More of a visual learner? Here is a video from Chris describing the problem in this Zone, and a video from me describing what it looks like where the snow is a bit thinner. 

    ADDITIONAL DISCUSSION Recent snowfall and winds have built thin slabs here. Today's storm will provide the wind with more ammunition. Be on the lookout for small, recently formed slabs if you are traveling in upper elevations in this Zone. Though small, these slabs may be reactive. More importantly, they are adding weight to the weak layers described above.

    (3/2/2022) This series of persistent slabs failed near the tail end of the warm, wet storm last week. These slightly discontinuous crowns span over 1,000' of terrain in this photo, with another 500' of crowns out of frame to the left. The crowns pictured here are on a NE/E-facing slope around 8,400-8,700'. 

Avalanche Discussion

UPPER SNOWPACK COMPLEXITY

We've got a very scientific phrase to describe the upper portion of our snowpack, and I've found myself using it numerous times in the past few days: "Our snowpack is a mess!"  Let's take a look at it and see if we can make a little more sense out of it. 

  • January Drought Layer: Our last major storm drew to a close on January 7th. Oof. Several weeks of cold weather helped to create some prominent facets in the upper portion of the snowpack during this period. This layer is now buried beneath a 6-18" slab across most of our Forecast Area. In the Banner Summit area, this layer produced a number of avalanches large enough to bury you during the storm last week (observation and photos here). 
  • Last Week's Surface: The layer of snow that was buried by last week's warm and wet storm also formed during a period of very cold weather. Unsurprisingly, it also created a number of avalanches in the Banner Summit area.
  • Today's Surface: The snow that is currently at the surface of the snowpack is about to be buried by today's storm. Last week's rainy storm and the very warm days that followed created crusts that extend from the valley bottoms up to 9,500' in places. These crusts tend to concentrate temperature gradients, which promotes faceting in the adjacent snow. Since this layer doesn't have a slab on it yet, it hasn't produced any avalanches. With that said, there is a good chance that you'll be reading about this crust and the associated facets as the snow piles up on top of it in the coming day.s

Avalanches require both slabs and weak layers. As described above, we've got the weak layer component across our Forecast area. The big difference from Zone to Zone is in how thick the overlying slab is. Near Banner Summit, where more snow has fallen in the past 6 weeks, this slab is approaching 1.5' thick. Numerous slab avalanches occurred here during the storm last week and the possibility of human-triggered avalanches is on the table today. As we travel south, the slab of snow overlying these weak layers is thinner and the likelihood of encountering this problem is lower. Today's storm will add a bit more weight to this slab and a bit more stress on the weak layer's underneath. It's difficult to predict exactly when this tipping point will be reached. If it were to snow two feet today my job would be a lot easier, but there is a lot more uncertainty when it only snows a few inches at a time. To deal with this uncertainty, I've already taken a step back from complex avalanche terrain where these weak layers exist. With more snow in the forecast I suspect I'll be continuing to step back as the slab gets thicker and stiffer. 

Today's Forecast is dedicated to Kip Rand, who passed away in the Wallowa Mountains of eastern Oregon on this date in 2016. Kip was a son, a brother, a best friend, a lover, a teacher, and an avalanche forecaster. Take a minute to give a friend a hug today, he'd appreciate it. And don't forget to pack a bag full of cookies for your backcountry travels, there is no better fuel for body and spirit in the mountains. We miss you brother. 

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