Bollettino valanghe

East Slopes North

Northwest Avalanche Center
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Pericolo valanghe

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

While the danger looks the same, a more potent storm will add additional load to an overall weak snowpack. You're most likely to trigger smaller avalanches in the new snow. But you need to consider the consequences of a much larger avalanche breaking widely on deeply buried weak layers. Avoid traveling on or directly under steep slopes in open terrain at mid and upper elevations until the snowpack adjusts to this new load.

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2–2.5

    We have been incrementally loading persistent weak layers in the upper snowpack for the past few days. The storm impacting the mountains Monday into Tuesday will bring a more significant dose of snow and winds, making it easier to trigger these dangerous avalanches. Put a larger buffer of terrain between your group and large start zones or complex avalanche paths above 6,500ft. Use extra caution in areas where you find less than 5 feet of snow on the ground above 6,000ft. Choose smaller, supported slopes and stay well out from under larger features. Signs of instability can be inconsistent, creating a tricky problem that is essentially 'unmanageable.' You might not receive signs of instability before the whole slope fails. 

    Facets buried throughout February exist over a crust in the top 2-4 feet of the snowpack. There's copious evidence of these layers being the culprits of numerous very large natural avalanches and close calls in the zone. 

    (3/5/2024)

    A very large avalanche was remotely triggered by skiers on 3/2 on Kangaroo Ridge. The crown was ~70-100cm deep, and it ran on a layer of facets (1.0mm) buried in mid-February, just above the 1/31 crust.

    Kangaroo Ridge, Pk 8183, WNW, 7,600ft

    Photo: John Stimberis

  • Storm Slabs

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

    The fresh snow will form shallow new storm slabs, and winds may drift the snow into larger wind slabs. Recent snow has fallen on weak faceted snow and surface hoar, potentially forming a weak bond with the old snow surface and making avalanches easy to trigger. Steer around steep slopes, particularly in wind loaded terrain, and give the snowpack time to adjust to the new load. You may be able to avoid small wind-loaded features or steep convex rolls in sheltered terrain. But consider that a smaller avalanche could end up triggering the much more dangerous persistent slab avalanche.

    (3/2/2024)

    Example of a small storm slab. 

    Photo: Willie Webster

Avalanche Discussion

NWAC will be closed on Tuesday, March 12th, as we gather and grieve following the death of one of our forecasters, Matt Primomo. We will not issue regular avalanche or mountain weather forecasts on Tuesday. We will return to normal operations on Wednesday, March 13th. We appreciate the community's understanding and support during this time.

Discussion:Most snowfall will occur overnight, tapering in the morning hours. But light to moderate winds will sustain further into the day on Tuesday. While observations of the persistent slab problem and snowpack test results have been more favorable, this more potent storm may tip the balance and reinvigorate the potential for these very large and dangerous avalanches. Give the snowpack time to adjust before stepping out into bigger open terrain. 

Outlook (Day 2): With multiple persistent weak layers in the upper snowpack, you can expect danger to be slow to improve, and triggering avalanches will remain likely on Wednesday. Sunshine and warmer temperatures could also bring a round of wet loose avalanches on southern slopes. 

On the 5th, forecasters visited the site of a remotely triggered (D3) avalanche on Kangaroo Ridge. They found that the avalanche was ~2.5-3 feet deep and ran on a layer of facets buried in mid-February, just above the Jan 31st crust. Tests still produced sudden or propagating results indicating that backcountry travelers could trigger similar avalanches, especially in areas of relatively shallow snowpack. There have been multiple reports of close calls, natural avalanches, and snowpack observations in the Highway 20 corridor that point to the deep, persistent slab problem. Widespread, very large natural avalanches occurred between February 28th and March 3rd. In the past week, natural avalanche activity has stopped, and snowpack tests have become more variable, with some showing relatively stable results. In the East Central and Stevens Pass zones, there have been multiple triggered avalanches and involvements this week on a similar snowpack structure.

Snowpack information is limited outside of the Highway 20 corridor and Harts Pass. Let us know what you find if you get out into the backcountry!

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