Bollettino valanghe

East Slopes South

Northwest Avalanche Center
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Pericolo valanghe

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

On stormy days, weak layers in the snowpack need time to adjust to a new load of snow and wind. You can look for signs of instability such as cracking, collapsing, and recent slab avalanches but dig down into the snowpack even if you don't see any to investigate the layers within. The best way to manage avalanche hazard will be to simply avoid steep, open slopes.

Problemi valanghivi

  • Wind Slabs

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

    Avalanche danger will quickly increase as fresh slabs build on down wind slopes throughout the day. Although small, these slabs may have trouble bonding to slick crusts, surface hoar, and faceted snow below. With increasing wind speeds up to 30 mph, anticipate wind-affected snow lower down on slopes than you might expect. Fresh slab formation can overload slopes harboring the persistent slab problem and increase its sensitivity.

    Blowing snow will tell you where slopes are being freshly loaded. Cornices and hard, uneven snow surfaces will point you toward where wind-affected snow can be found as well. These areas are commonly just below ridgelines and in cross-loaded gullies. Shooting cracks, collapsing, and recent slab avalanches are clear signs that you should avoid steep, exposed terrain.

    (1/7/2025)

    Small rider-triggered wind slabs in the Superbowl. 

    (6800, ENE)

    Photo: Payton Schiff

  • Persistent Slabs

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

    A weak layer of surface hoar buried 1-2 ft deep has been showing fewer signs of reactivity, but new weather inputs can reinvigorate sensitivity. This problem is most concerning in areas where the slab overlaying the weak layer is more cohesive. You're most likely to find this set-up above 6000 ft in areas that received more new snow (from last week) like Ahtanum Meadows or adjacent to ridgelines where the wind has drifted snow into stiffer slabs.

    Persistent slab avalanches can break in surprising ways across terrain features. You up the chances for a perilous surprise if you choose to step out blindly into steeper, more consequential terrain. You may or may not see any signs of instability on the surface before triggering an avalanche so testing for the buried weak layers can reduce your risk. Surface hoar can be identifiable as a thin gray line in the upper snowpack and with snowpit tests. Ultimately, your best way to manage the problem can be to just avoid open slopes greater than 30 degrees.

    (1/7/2025)

    No propagation on buried surface hoar in this pit but pulling the block away from the pit wall reveals the layer quite well.

    Photo: Payton Schiff

Avalanche Discussion

You will encounter more stormy conditions closer to the Cascade Crest where we expect to see 1-4 inches of new snowfall. While these aren't very significant precipitation numbers, the snow will be falling on a variable and oftentimes weak snowpack. So be suspicious of steep slopes where new snow has fallen and is being blown around, including at middle elevations. The larger and more cohesive the slab, the more likely it will be to trigger a persistent slab avalanche.

An NWAC forecaster traveled to Ahtanum Meadows on Tuesday to track the persistent slab problem. They found the concerning weak layer of buried surface hoar, but it produced inconsistent test results. This was mainly due to the inconsistency of the overlying slab. Check out the full observation here. Your best strategy is to make like a forecaster and dig around on many aspects to look for weak layers and unstable test results. Be sure there is no buried surface hoar before high-marking or sledding on steep slopes with convexities and terrain traps.

When considering the wind slab problem, be aware of terrain below you. A small avalanche that knocks you off your machine could carry both of you into hazards such as cliffs and trees. 

Help NWAC Forecasters reduce uncertainty by submitting an observation!

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