Bollettino valanghe

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In sintesi

You may be able to trigger avalanches up to 2 feet thick, especially where the wind has drifted snow. Loose snow avalanches may release on very steep slopes receiving prolonged direct sun. 

Problemi valanghivi

  • Persistent Slabs

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

    You could trigger a persistent slab avalanche on shady slopes (NW-N-NE-E) at middle and upper elevations. Tuesday's storm added weight to the weak layers responsible for last week's avalanche cycle (observation and photos here). It's hard to predict whether the recent and windblown snow will be enough of a load to "wake up" these weak layers. However, we can say that this problem is likely to be at its worst on shadier aspects where the wind has stiffened the snow surface or formed drifts.

    Persistent slab avalanches can behave in ways that surprise you, wrapping around small terrain features and releasing well below obviously wind-loaded terrain. Small changes in aspect or elevation can have a big impact on stability. Slow it down and choose more conservative terrain where this problem is expected to exist. 

    ADDITIONAL DISCUSSIONKeep a lookout for fresh wind slabs in the alpine. Yesterday, observers reported lots of blowing snow at upper elevations across the forecast area. Wednesday in the Smoky Mountains, I found small wind slabs up to 1' thick (photo). Similar drifts were reported in the Smiley Ck drainage (photo). Wind slabs often form near or just below ridgelines and on the sides of cross-loaded gullies. Many of them rest on slick crusts. This means they may be easier to trigger and will pick up speed quickly as they travel downslope.

    Loose snow avalanches may initiate on very steep, sunny slopes that are sheltered from the wind. These avalanches may not be big enough to bury people, but they could be powerful enough to carry you downslope or trigger the persistent slabs described above.

    (3/2/2022) This series of persistent slabs failed near the tail end of the warm, wet storm last week. These crowns span over 1,000' of NE/E-facing slopes around 8,400-8,700'. 

Avalanche Discussion

In our northern zones, we've buried a complex mix of weak snow layers including multiple crusts, layers of facets, and occasionally surface hoar. The worst of these layers exist on shadier (NW-N-NE-E) at middle and upper elevations. The composition of these layers is highly variable and depend on aspect, elevation, slope angle, and the amount of recent snowfall. 

As a quick example, you can see the contrast between this S-facing pit and this N-facing pit I dug on Wednesday in the Smokys. These pits were less than a 1/4 mile from each other and at similar elevations. The point here is that it's complex. Not only do the weak layers change, but the thickness and stiffness of the slab vary as well. In some areas, you'll be scratching around in "dust on crust". Around the corner, you could run into a stiffer, sensitive drift.

New snow and continued wind-drifting over the last few days formed a fresh batch of wind slabs in alpine terrain. These slabs may be sensitive on their own, even without a persistent weak layer below them. More sinister, however, is where they rest atop the weak layers described above. The complex distribution and sensitivity of these layers make it hard to know if the slab you encounter will be your average small wind slab that's easy to manage, or one that breaks above you and wider than expected. With increased complexity comes increased uncertainty. The best way to deal with the uncertainty is to dial back your terrain choices. Personally, I'm not willing to trust a steep, shaded slope with an obviously stiffer slab near the surface. 

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