Bollettino valanghe

West Slopes South

Northwest Avalanche Center
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3 · Marcato
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3 · Marcato
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2 · Moderato
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In sintesi

Heavy snowfall, rising temperatures, wind, and a known weak snowpack will create a dangerous day in the mountain on Sunday. We may be teetering with high avalanche danger on the volcanoes if we exceed our predicted water numbers. Keep your terrain selection simple by avoiding slopes over 35 degrees where triggering avalanches in new or old snow is more likely. 

Problemi valanghivi

  • Storm Slabs

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

    As new snowfall stacks up, the potential for avalanches will increase on slopes over 35 degrees. Whether that's your classic storm slab due to a warming trend making the snow upside down, a poor bond with the underlying surface, or maybe a wind slab, it doesn't really matter. You will want to steer around those steeper slopes and convexities where triggering a slide is more likely. You can use small test slopes or quick handpits as you travel to help understand how the snow is coming in and reacting to your weight as a traveler. With various surfaces currently in place, you may find that the storm snow has bonded differently from slope to slope. Slabs will be larger and potentially more sensitive on the volcanoes or in wind-loaded terrain. You may even see a few natural avalanches occur during or shortly after periods of heavy precipitation or wind. Obviously, shooting cracks, whumpfing collapses, or recent avalanches are all red-flag signs that the snowpack is unhappy and you're flirting with danger. 

    (12/26/2024)

    Even relatively shallow storm slabs such as this one from earlier this season could be consequential on Sunday. They could break in surprising ways or step down to our PWLs.

    Photo: Crystal Mountain Pro Patrol

  • Persistent Slabs

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    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2

    Here's the avalanche problem that should worry you the most. Uncertainty abounds regarding persistent slabs, and this one is no different.  Our persistent weak layers have been dormant for over a week, but snowpack tests and observations indicate they haven't healed much. After getting buried earlier this month, there was a lot of talk that these layers lacked the slab over them to produce avalanches. Well, all that low-density snow has settled and compressed into a slab, and now this storm should provide an excellent test for the January Drought Layer (JDL) and 1/10 complex. We're not confident that this will be enough load to tip the scales, but this storm certainly warrants a bump in likelihood for this problem. If you've written these layers off, it's time to bring them back to the forefront. It may not even be the loading from the storm but another avalanche in new snow that steps down and creates a larger and potentially more dangerous slide. Stick with the travel advice from the storm slab problem by avoiding slopes over 35 degrees, and you should also be able to manage this problem throughout the storm cycle. 

Avalanche Discussion

On Saturday, NWAC forecasters were out in the West South gathering data on the JDL and 1/10 complex ahead of our incoming storm. On Mount Adams, they received a whumpfing collapse while traveling across a hard wind slab in low-angled terrain. Digging a test pit in the immediate vicinity, they got propagating test results on the JDL. They then shifted a few hundred yards to a slope that was not wind-loaded and dug again. This time, the block did not propagate a fracture. Further north in the zone, near Crystal Mountain, NWAC forecasters said the JDL and 1/10 looked essentially the same as they have for much of February. Propagation saw tests continued to produce failures with cut lengths around 35cm.

What does all this nitty-gritty snow talk mean? Well, for one, the snowpack structure is still weak. It reacted poorly once a large enough load was over it (the wind slab) compared to a short distance away where no slab existed. So, as we load up these layers with multiple feet of snow over the next few days, we'll see if avalanche activity spikes. That's the primary concern.

Typically, with a storm like this, we deal with new snow problems, such as storm and wind slabs. That all still holds for Sunday, but the lingering weak layers throw a real wrench in things. It will be almost impossible to pinpoint the exact slopes where the persistent problem may exist or what it will take to trigger a slide. We can only examine the weak structure and make assumptions based on our test results.

The upper snowpack has already undergone some changes due to significant wind events this week. Variable surfaces abound with a mix of wind-affected snow, well-settled powder, and a firm crust. A general warming trend means the new snow may come in upside-down. As can often be the case in our zone, the volcanoes are forecast to get two to three times as much water as areas closer to the crest. Any triggered avalanche, whether in new snow or old, will only increase the potential for stepping down and activating the deeper and more unpredictable JDL and 1/10 complex. 

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