Avalanche Forecast

Archived

Dec 18th, 2018–Dec 19th, 2018

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

Stevens Pass.

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.

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.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.