Bollettino valanghe

West Slopes South

Northwest Avalanche Center
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Pericolo valanghe

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

A massive warmup is on our doorstep, and Friday is the first full-on day, with freezing levels reaching 11000ft with intense sun. Wet avalanches are our largest concern, but at least they're fairly predictable: once you notice the snow getting wet or sliding downhill, it's time to shift to shadier or low-angle slopes away from steep start zones.  Wind-drifted snow may react to a human trigger, so steer around steep locations with thicker, wind-thickened deposits. 

Problemi valanghivi

  • Loose Wet

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

    Friday is the first official day of us really baking. Freezing levels will transition from an unseasonable 10,000ft to 11,000ft during the day. Wet loose avalanche activity will start quickly as sun touches slopes, and you can expect slides to continue until the sun leaves a slope. The sun will first wet east-facing slopes in the morning, before shifting south and then on to west-facing terrain in the afternoon. Avalanche size will likely grow all day. A crust exists on most of these slopes from Thursday, but it probably won't take long for the sun to cook through that crust. 

    You may be able to travel really early on sunny aspects, but beware, as conditions can change very quickly. Small amounts of avalanche activity, or rollerballs, will be your first indicators before wet loose avalanches increase in size, probably becoming large and destructive. Slides will dig 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.

    (1/29/2024)

    Example photo from earlier this year of wet avalanche debris in the White Pass backcountry. You can expect to see similar, or larger avalanches on Friday. 

    Photo: White Pass Pro Patrol

  • Wind Slabs

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

    You may find brand new or couple-day-old wind slabs during your outing on Friday. 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 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. Your weight may cause long shooting cracks in slabs. Steer around slopes greater than 35° where you identify wind loading, as you could trigger an avalanche resulting in a dangerous ride. 

    (3/10/2024)

    A skier-triggered cornice and shallow wind slab on the slope below. 

    Photo: Andy Harrington

  • Deep Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2–3

    We haven't received reports of activity on this layer in a recent days. The large loading and storm event which ended March 2nd 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.

Avalanche Discussion

Forecasters in the Crystal backcountry on Wednesday 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, but dropping cornices produced only small results. A little sun slightly moistened southerly slopes, and only small rollerballs were noted. More activity was observed on Thursday. 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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