Avalanche forecast

West Slopes South

Northwest Avalanche Center
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Avalanche danger

Fri
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
2 · Moderate
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Highlights

A storm just ended and an extreme warming period is about to start, without much of a break from avalanche activity. You can trigger avalanches in wind-drifted snow, and then the sun will start cooking southerly aspects. Avoid steep wind-drifted slopes, and if you notice wet loose activity on sunny slopes, avoid steeper nearby terrain and areas beneath the steep slopes as well. 

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    You may find brand new or couple-day-old wind slabs during your outing on Thursday. Some slabs could be shallow, only a couple inches deep, whereas others may be a foot or two deep. If you see active signs of wind loading, which may be present along ridgelines at upper elevations, identify where snow is landing and avoid those areas. 

    Snow usually deposits below ridgelines, in gullies or around outcropping and features. Feel for stiffer slabs and search for textured, or rippled snow surfaces. The difference in heaviness may be subtle. You may see long shooting cracks. Steer around slopes greater than 35° where you identify wind loading, as you could trigger an avalanche resulting in a dangerous ride. 

    (3/13/2024)

    Visual  clues help highlight where wind slabs may occur.

    Photo: Dallas Glass

  • Loose Wet

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    Wet loose avalanche activity will pick up midday and you can expect slides to continue until the sun leaves a slope. Freezing levels transition from 3500ft in the morning to 8500ft at the end of the day and it'll be sunny. The sun will first moisten east-facing slopes a little in the morning, before shifting south and then on to west-facing terrain in the afternoon. As temperatures increase avalanche size will continue growing into the afternoon. 

    You may be able to start the day on sunny aspects, but plan on leaving them as the sun intensifies and wet loose avalanche activity begins. Small rollerballs will be your first indicators before small wet loose avalanches fan out. Slides may become large as the day progresses, digging deeper into all the low-density snow that just fell. When you notice any of this activity, transition to shady aspects or away from any slopes greater than 30° and the runout beneath those slopes as well. 

    Wet loose avalanches may trigger slab avalanches as they pick up mass while traveling down a slope. Make sure you plan your day so you won't have to exit through terrain that earlier did not have avalanche activity but by the end of the day, could be very scary and active.

    (2/22/2024)

    Wet loose avalanche activity will start very small, like these rollerballs, before growing, possibly large enough to be large, destructive wet loose avalanches. If it doesn't happen on Thursday it will happen sometime in the subsequent days. 

    Photo: Dallas Glass

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2–3

    Forecasters and observers have yet to report seeing large or very large avalanches failing on the deep persistent layer from the storm activity on Tuesday. Previously, the last time we had a significant loading event, the West South zone produced multiple very large (D3) avalanches. Strength may slowly be improving, but we're still concerned about this layer. 

    Weaknesses are around 120-200cm (4-6ft) deep throughout the zone, and resulting avalanches will be that deep and likely unsurvivable. Similar layers have been wreaking havoc in neighboring zones for over a week. You won't see any warning signs or get feedback from the snowpack before triggering one of these large, unsurvivable avalanches. Even if other people are out on a slope and don't trigger an avalanche, it doesn't mean you won't too. Avoid large open slopes where avalanches like this can start, run, or stop.

    (3/10/2024)

    Propagation saw tests and other snowpack tests on the Early February Crust, where our deep persistent concerns lie, failed repeatedly. Notice that the slab produced other failures as it slid off the EFC.

    Photo: Andy Harrington

Avalanche Discussion

Forecasters were in the Crystal backcountry on Wednesday. They found a generally right-side-up storm snow layer, with few new instabilities. Tuesday's storm resulted in about 25cm at lower elevations and 45-50cm at upper elevations. Occasional wind slab pockets were present, and dropping cornices produced only small results. A little sun slightly moistened southerly slopes, and only small rollerballs were noted- a precursor to the large amounts of wet loose avalanche activity we expect in the coming days. Activity on the deeper weak layers was surprisingly absent where they traveled. 

The Cascades are heading into a stretch of very warm sunny weather. It will be the first big, spring-like warming event of the year, with high temperatures in the 50’s and freezing levels over 10,000’. Very warm conditions through the weekend could produce significant slab and loose snow avalanches.  As with any warming event, there is a high level of uncertainty as to where, when, or even if this scale of avalanche activity will occur. On the other hand, big natural avalanches aren’t really the kinds of things that should be taken lightly. It’s worth noting that smaller wet loose or wind slab avalanches, cornice falls, or snow shedding from cliffs can provide the trigger needed to pull out a very large avalanche. When planning for the weekend, consider that this is one of the times when an ‘improbable’ avalanche might actually occur.

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