Lawinenlagebericht

Stevens Pass

Dec 18 – 19, 2018Northwest Avalanche Center
Auf der Karte ansehen
Archiviert · abgelaufen 8 years ago

Lawinengefahr

Thu
Hochgebirge
3 · Erheblich
Waldgrenze
3 · Erheblich
Unterhalb
2 · Mässig
Vollständige Karte öffnen

Kernpunkte

Very dangerous avalanche conditions have developed. Heavy snowfall and strong wind have added a substantial slab to a known weak layer. Safe travel through avalanche terrain will warrant cautious route finding.

Lawinenprobleme

  • Storm Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Likely
    Größe
    1

    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.

    A slight density inversion was noted on December 18th, which contributed to shallow storm slab instability. This problem will likely begin to heal during the day. Hand shear tests can work well to test storm snow instabilities. Avoid steep slopes and convex rolls if noticing that these "pop" easily, or other signs of instability.

    A bump in wind speeds and a slight cooling trend as snowfall tapers tonight could create wind slabs in exposed areas near and above treeline. Avoid slopes with recently wind driven snow such as gullies and fresh cornices. 

  • Deep Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    2

    Deep persistent slabs have the potential to wake up if you hit the wrong spot in the snowpack. In the past few days, observers have found that the December 9th interface has begun to round, and gain strength. This does not mean that we are out of the woods yet. 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 from the surface to be very reactive. Even though we don’t have signs of recent deep avalanches in this area, this weak interface is still there. The recent near miss at Crystal Mountain highlights this. 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 winter began approximately December 9th, when a series of storms rolled into the Pacific Northwest in earnest. 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 and is now buried by approximately 4 feet of snow in the Stevens Pass area. The most recent report of a slab failing on this layer was from December 18th at Crystal Mountain, when an small explosive released a very large avalanche during control work. 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. Though we don’t have any reports of slabs failing on this layer since December 13th in the Stevens Pass area, this is a very pertinent observation from nearby. It is also good to recognize that nearly continuous snowfall and poor visibility have severely limited our knowledge base.

Snowpack Discussion

Regional Synopsis: December 18, 2018

Why has the avalanche danger been so high for so long? Two reason: 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: 93”

  • Washington Pass: 45”

  • Stevens Pass: 66” mid-mountain

  • Snoqualmie Pass: 59” mid-mountain

  • Crystal Mountain 57” Green Valley

  • Paradise: 70”

  • Mt Hood Meadows: 37” mid-mountain

  • Olympics: 20”

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.

Abgedeckte Regionen