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Avalanche Forecast

Archived

Dec 19th, 2018–Dec 20th, 2018

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

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Expect dangerous avalanche conditions. Heavy snowfall and strong winds will continue to create large avalanches. Cautious route-finding and conservative decision-making are essential Thursday.

Discussion

Discussion 

Persistent weak layers have been identified in the adjacent East Slopes Central forecast zone. Although we have no information about snow conditions from this zone, the Persistent layers are likely to exist here as well.

Avalanche danger may be slightly mitigated due to low snow conditions below treeline, but coverage is rapidly increasing. 

Check the adjacent zones for more information that may pertain to this area.

Forecast Schedule

General avalanche and snowpack information will be provided during the winter. However, at this time we do not have enough specific snowpack information to issue an avalanche hazard rating for the East Slopes South zone.  

Snowpack Discussion

Regional Synopsis: December 19, 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 12/9-12/19 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.

Problems

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

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.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.