Bollettino valanghe

East Slopes South

Northwest Avalanche Center
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Pericolo valanghe

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

A storm favoring the East South will bring snow, wind, and dangerous avalanche conditions. As snow piles up on weak surfaces, it will be easy to trigger avalanches in steep terrain, particularly where the wind drifts snow into stiff slabs. Choose low-angle slopes sheltered from the wind to avoid getting caught in a large avalanche.

Problemi valanghivi

  • Wind Slabs

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

    Fresh wind slabs will likely be easy to trigger on Thursday. To avoid triggering one of these wind slab avalanches, stick to slopes less than 30 degrees or travel on scoured ridgelines or slopes at mid and upper elevations. Snow that feels hollow or the snowpack collapses beneath you means the snowpack is unstable, and you should steer clear of steep terrain. Moderate winds will be strong enough to drift the fresh, low-density snow into stiff wind slabs in exposed terrain. You can expect to find these slabs below corniced ridges, convex rolls, steep gullies, and at the base of cliffs. Easterly winds could form wind slabs on atypical aspects. Watch for actively blowing snow, smaller fresh cornices, and drifts around rocks and trees to point you to likely wind slab locations. 

    Example human-triggered wind slab.

    Photo: Simon Trautman

  • Storm Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–1.5

    You could also trigger smaller, but still consequential, storm slab avalanches in steep terrain sheltered from the wind. With cold temperatures producing high snow-to-water ratios, we could see over 1ft of new snow by the end of the day on Thursday. Enough snow is in the forecast to keep your travel decisions simple and stick to lower-angle terrain as the best option. Steer around likely trigger points such as steep, convex rolls or gullies even when they exist within lower-angle slopes. While wind slabs will be larger and likely the more sensitive and dangerous avalanche problem, we can't rule out reactive storm slabs in sheltered terrain. New snow will fall on widely variable surfaces, including weak faceted snow, surface hoar, and slick crusts. Adjacent slopes may react quite differently from one another. You can use small test slopes or hand pits to observe the bond with the old snow surface, but these could also give you inconsistent results. It's best just to choose lower-angle slopes until the snowpack has time to adjust to the new snow load.

    (1/7/2024)

    A rider triggering a small storm slab on a test slope in Ahtanum Meadows.  

    Photo: Joseph Labernik

Avalanche Discussion

The brunt of the incoming storm will likely occur overnight, with decreasing snow showers and wind throughout the day on Thursday. Precipitation amounts in liquid water are less than 1" of snow water equivalent, but the cold temperatures will ring a lot of snow out of those modest numbers. We'll likely end up with 12-14 inches of snow. 

Before this storm, observers found widespread surface hoar at Ahtanum and isolated buried faceted layers at nearby Mission Ridge. These persistent weak layers could cause wind or storm slabs to fracture wider than expected. Previously, a small storm on Sunday night and Monday brought more wind than snow through the area. Other old snow surfaces may consist of thin crusts, settled powder, and firm wind-textured snow. 

Earlier in the month, a prolonged period of warm temperatures and rain formed what we are calling the Early February Crust. How deep is the EFC? That depends on where you are in the zone. Closer to the crest, it may be 6-8 inches below the surface. Further east or at lower elevations, it may be the surface. Expect very thin (or even non-existent) snowpacks at lower elevations and on some steep southerly slopes. Unfortunately, we're still talking about travel hazards. Slick crusts, exposed obstacles, open creeks, and bare ground continue to make travel more adventurous. While this storm will be a welcome refresh, it's probably still best to use established roads and groomed trails for better access to the high country. 

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