Avalanche Forecast

Archived

Dec 19th, 2018–Dec 20th, 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 avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches possible, human triggered probable.

Regions

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Expect very dangerous avalanche conditions. Heavy snowfall and strong winds will continue to create large avalanches. Travel in avalanche terrain is not recommended on Thursday.

Discussion

Snow and Avalanche Discussion:

We have an inherently weak snowpack structure in the East Cascades. The weak layers themselves have been identified as buried surface hoar and/or buried near surface facets, and appear to be blended into the form of basal facets and even depth hoar as one moves further east, or up in elevation. This layer is dated to December 9th, when it began snowing after a prolonged early season drought. Basal facets have been observed in the Wenatchee Mountains, the Stuart Range, the Chiwaukum Range, and near Washington Pass. In short, it is a poor structure and one in which it will be hard to develop confidence in for some time.

In the Icicle Creek on December 19th, I spotted a number of very large avalanches that likely occurred mid storm on the 18th. These were on a variety of aspects and elevations as low as the mid 5,000ft range. These slides were impressive, and filled in the tracks of some large slide paths. One recent explosive triggered slide at Mission Ridge stepped down to basal facets near the ground, resulting in a 20” to 40” deep crown that propagated approximately 120 feet wide. The continued loading from snowfall and strong winds will add to an already stressed lower snowpack. An old professor of mine used to say “Faceted grains are strong in compression, but weak in shear.” They can hold up to an enormous amount of weight, but when they do fail, wide propagations can be expected.

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 12/9-12/18 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.

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.