Avalanche Forecast

Archived

Dec 22nd, 2018–Dec 23rd, 2018

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches possible, human triggered probable.

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches possible, human triggered probable.

Regions

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Very dangerous avalanche conditions exist. New snow and wind will stress an already weak and scary snowpack. Travel in avalanche terrain is not recommended.

Discussion

Snow and Avalanche Discussion

Reports of very large natural avalanches continue in the Washington Pass area (some could destroy a house). Many had very wide propagation (up to 1 mile wide) and ran far distances downslope. These avalanches continue to break near the ground where a weak layer was buried on December 9th. This layer has been identified as near-surface facets and/or surface hoar. It is even blended with basal facets and depth hoar in areas further east and at upper elevations. Heavy snowfall will test this layer again. The balance could be tipped, and another round of very large and destructive avalanches is possible. Give the snowpack time to adjust. Be very cautious if you decide to travel in the backcountry on Sunday.

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 combined with a 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 of 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 snowfall 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 of the Mt. Baker area, so we have not yet experienced a widespread cycle in other areas.

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.

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.