Bulletin d’avalanche

West Slopes North

Northwest Avalanche Center
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Danger d’avalanche

Thu
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 5 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Expect loose wet avalanche activity to begin as the sun melts new snow. Use caution on, or under steep sunny slopes -- especially below rock-bands, or steeper areas where trees are shedding their snow load. Take a look at the West Central Forecast if you are in the southern portion of the zone, or traveling along the Highway 20 corridor.

Problèmes avalancheux

  • Loose Wet

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

    The sun may make an appearance around mid-morning on Wednesday with mostly clear skies by afternoon. Expect the March sunshine to rapidly destabilize solar aspects -- especially around exposed rocks, or where trees are shedding their snow load above steep slopes. 

    These avalanches probably won’t be very big below treeline (many areas got rain on Sunday, or that have already gone through a melt-freeze cycle). That said, sunny slopes near and above treeline have plenty of new snow for entertainment and may produce sluffs that are big enough to catch, carry, and potentially even bury a person.

    Example of the type of natural avalanche we expect on Wednesday. Photo: Zach McGill

  • Wind Slabs

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

    The recent snow is bonded well, but it's possible to trigger wind slabs above treeline in steep, heavily wind loaded start zones. These dense slabs may be anywhere from 6 inches to several feet thick and may, or may not be reactive to your weight. Use caution around near-ridgeline start zones that look heavily drifted, or ‘pillowed’.

    Maybe more importantly, cornices are overbuilt right now and we will start to see more of them fail as the spring sun intensifies. Cornices are incredibly dangerous and unpredictable -- give them a wide berth along the ridgelines and avoid climbing slopes where they represent an overhead hazard. 

    (3/2/2021) Natural wind slabs at 9000 ft in wind-loaded start zones.

Avalanche Discussion

March is here. It brings stronger sun, longer days, and is generally the harbinger of a transition from the winter snowpack to a more springlike snowpack. These transitions create short-term uncertainty (in forecasting) because they can drive even larger variations between aspects, elevations, and adjoining zones. 

Last week there was a widespread natural cycle that resulted in many very large avalanches (D3+) some of which hit Highway 20 just outside Newhalem. We believe most of these were big wind slabs, but there are several in the more southern part of the zone and one notable avalanche on Skyline Ridge (see gallery below) that may have failed on the deeper weak layer from January.  This layer is 200cm - 300cm deep in most portions of the zone and it should be hard for a human to trigger it. That said, a big cornice falling in the right spot might just do the trick. 

The take-home is that snow and avalanche conditions are variable throughout the West North Zone and through the cascades in general. In some areas, recent storm snow is stabilizing quickly, but a cluster of recent very large natural avalanches along the border of the Stevens and East Central zones continue to keep these old and deep weak layers on our minds in this zone as well. Read the West Central  or Stevens Forecasts to learn more about it. 

During the ongoing Covid-19 pandemic, continue to follow the latest guidance from local, state, and federal health officials. Avoid unnecessary risks that could impact first responders and stress medical resources.  Wear a mask when appropriate, practice social distancing, and consider recreating closer to home.

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