Bulletin d’avalanche

West Slopes South

Northwest Avalanche Center
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Danger d’avalanche

Tue
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 4 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

There are a lot of different ways Monday’s storm could playout for the West South zone making it tricky to forecast. However, you’re most likely to encounter a serious avalanche issue in wind drifted snow at mid and upper elevations. Steer around smooth drifts and steep slopes below sharp cornices to avoid the problem.

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–2

    For most of the West South zone, the wind seems to be the biggest factor driving avalanche concerns Monday. You could find new and reactive wind slabs on the lee sides of ridges and gullies. What might make some of these potential avalanches a bit different than normal is they could fail on old layers within the upper snowpack (see discussion for details). If that occurs, they may trigger easily, from the sides, or even below. Look for clues like sharp cornices and blowing snow to indicate which slopes and features could hold possible wind slabs. Travel around steep slopes where you find smooth pillow-like drifts. 

    There’s a chance that places near Mt Rainier and Mt St Helens could receive significant snowfall by Monday afternoon. If you find more than 8” of new snow, conditions may be passing a critical threshold. You could trigger storm slabs on any steep open slope. Dial it back, and stay off of terrain greater than 35 degrees.

    (3/13/2022) Cornices, drifts, and scouring all show where winds slabs might exist. Photo: Dallas Glass

  • Loose Wet

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1

    Near and below the rain-snow line you may trigger small loose wet avalanches. If you notice the surface becoming wet and sticky or see rollerballs, then loose wet slides are becoming possible. Fortunately, these avalanches alone shouldn’t pose much of a risk. However, make sure to take them into account before you travel across a steep slope above an open creek or cliff. Terrain traps like these can significantly increase the consequences of even small avalanches.

    (3/11/2022) Wet sticky snow like this lets you know the surface may be becoming unstable. Photo: Dallas Glass

Avalanche Discussion

The weather forecast for the West South zone is quite tricky over the next 24hrs (Sunday night-> Monday). An approaching storm will bring rain and snow to the Pacific Northwest, but exactly how much of that precipitation makes its way into this region is hard to say. It could stall further north and leave the West South fairly dry, or it could shift south and bring more precipitation than forecasted. So, what do you do when the weather forecast is uncertain? Choice terrain with lots of options that allow you to easily adjust to conditions and avoid avalanche terrain if necessary. 

As if the uncertain weather forecast isn’t enough, potential weak layers exist in the upper snowpack that could make avalanches easy to trigger or act in surprising ways. Because all of the major access points could see wildly different amounts of snow, each local area will reach its tipping point at different times. Fortunately, the snowpack structure seems to be similar from one area to the next. Most of the potential weak layers exist on NW-NE aspects at mid and upper elevations. There were occasionally noted on southerly aspects near Mt Rainer (and possibly Mt St Helens) where more snow sits over the Early March Crust. Here’s a list of prominent layers on these more shaded aspects. 

  • 3/12 Crust: This layer is just below the most recent snow and is often a thin (1-2cm) breakable crust. 
  • 3/8 Facets: 1-2mm facets have been observed near Crystal Mt, White Pass, and Mt Rainier above the very stout and prominent Early March Crust. Up until now, these weak grains have not had much of a slab resting above them. 
  • Early March Crust: This very thick and easy to identify crust is a great snowpack marker. We expect any avalanche issue to remain above this layer. 

Sunday’s brief shot of snow was a bit of a dud for the West South zone. Locations near the crest received 2-3” of snow with sites near the volcanoes picking up 4-6”. While some wind drifting occurred during the storm, no wind slabs were reported. Instead, unexpected sunshine and warm temperatures softened southerly slopes and even resulted in a few rollerballs.

Régions couvertes