Bulletin d’avalanche

Stevens Pass

Dec 19 – 20, 2018Northwest Avalanche Center
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Archivé · expiré 8 years ago

Danger d’avalanche

Fri
Alpin
3 · Marqué
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Points saillants

New snow, strong winds, and warming temperatures will increase your chances of triggering an avalanche on any open slope greater than 35 degrees. Older weak snow still exists deep in the snowpack. You may trigger very large and deep avalanches from thinner spots in the snowpack or with larger triggers.

Problèmes avalancheux

  • Storm Slabs

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

    A density inversion within the new snow is expected to form as temperatures warm up during the snowfall. Hand shear tests can work well to test storm snow how the new snow bonds to the old snow, and within the storm layer itself. Avoid steep slopes and convex rolls if noticing that these "pop" easily, or other signs of instability such as shooting cracks.

    Moderate to strong winds with fresh snow available for transport could create wind slabs in exposed areas near and above treeline. Avoid slopes with recently wind driven snow such as cross loaded gullies and fresh cornices.

    Any shallow storm or wind slab has the potential to step down into deeper weak layers, turning a small soft slab avalanche into a deeper, hard slab.

  • Loose Wet

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

    As the temperatures warm up the snow surface will begin to get saturated by rain at lower elevations. Be observant for sticky, wet snow, rollerballs, and recent fan shaped avalanches. Don’t underestimate loose wet slides, as they can entrain a large amount of heavy, wet snow on steep slopes.

  • Deep Persistent Slabs

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

    Deep persistent slabs have the potential to wake up if you hit the wrong spot in the snowpack. Observers have noted that the December 9th interface has begun to round and gain strength, especially at lower elevations. However, on December 19th a pro observer performing tests on a Northwest aspect at 5500ft in the Stevens Pass zone found that the 12/9 layer still had the ability to propagate a fracture.

    Persistent weak layers are just that, persistent and weak. The snowpack may not speak to you with signs of instability, as they are becoming too far down in the snowpack to be very reactive while walking or riding on top. The recent near miss at Crystal Mountain and pertinent test results highlight that these may still occur. Safe travel through avalanche terrain will require conservative decision making, and the ability to seek out low angle slopes.

Discussion

Snow and Avalanche Discussion

Our hydrologic winter began in earnest on December 9th, when the storm track started pointing at the Pacific Northwest. The first week in December, little snow was on the ground, but temperatures were cold and the skies were clear. This formed a layer of surface hoar and/or facets on the surface of our then shallow snowpack. This is widespread throughout the Cascades. Nearly continuous storms have amounted to approximately 5 feet of snow over the top of this layer in the Stevens Pass area. Recent avalanches have failed on this layer at Crystal Mountain, where a natural cycle and controlled releases were observed. On December 18th, a ski patroller was caught and carried, but remarkably sustained very little injuries. The avalanche was large relative to the path, and large enough to bury several buildings. It occurred on a Northwest aspect at 6600ft.

Snowpack Discussion

Regional Synopsis: December 18, 2018

Why has the avalanche danger been so high for so long? Two reasons: A very active and wet weather pattern and widespread persistent weak layer.

For perspective, the approximate snow totals from 12/9-12/18 are:

  • Mt Baker: 102”

  • Washington Pass: 55”

  • Stevens Pass: 76” mid-mountain

  • Snoqualmie Pass: 68” mid-mountain

  • Crystal Mountain 70” Green Valley

  • Paradise: 78”

  • Mt Hood Meadows: 44” mid-mountain

  • Olympics: 48”

In many areas, a layer of buried surface hoar and/or weak sugary facets was buried on December 9th. This layer has been the cause in numerous natural, explosive, and skier triggered avalanches. As this layer gets deeper it gets harder to assess … in short, the scenario is tricky and getting more dangerous by the day.

Higher snow totals along the Hwy 542 corridor /Mt Baker area have driven several avalanche cycles during this period. Loading from recent storms has been more incremental to the east and south and we have not yet experienced a widespread cycle …

When will we reach the breaking point? It’s hard to say. What we do know is we have a deep weak layer, reports of very large explosives triggered slides at Mission Ridge and Crystal Mountain, and more storms on the way.

Be patient and continue to stick to lower angle slopes with nothing above you.  This is a good time to avoid areas where avalanches can start, run, and stop.

We’d like to thank all of you who have sent NWAC your observations. If you are out in the mountains, let us know what you see.

Régions couvertes