Avalanche Forecast

Archived

Dec 20th, 2018–Dec 21st, 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

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Avalanche conditions remain dangerous, especially on wind loaded slopes at mid and upper elevations. It is still possible to trigger very large avalanches that break widely across terrain features. Careful snowpack evaluation and cautious route finding are essential.

Discussion

Snow and Avalanche Discussion

Impressive avalanche cycles occurred recently in the adjacent Hwy 542 and Mt. Baker areas. Very large and destructive avalanches were reported (some could destroy a house). Many had very wide propagation (up to ½ mile). Several ran full track to valley bottom. The majority of avalanches failed a weak layer (surface hoar and facets) that was buried on December 9th.

Stormy conditions and poor visibility continue to limit our ability to understand the full extent of recent avalanche cycles. Many debris piles and crowns of even large avalanches are covered by recent snowfall. Observations from Wednesday indicate the most obvious signs of instability exist on recently wind loaded slopes. However, deeper buried layers within the snowpack continue to be stressed by new snow and wind. Although the likelihood of triggering a deep persistent slab avalanche decreased, the consequences of being caught in one remain high.

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

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.