Avalanche Forecast
Issued: Dec 20th, 2018 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Deep Persistent Slabs.
NWAC,
Northwest Avalanche Center
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.
Summary
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:
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Mt Baker: 102”
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Washington Pass: 55”
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Stevens Pass: 76” mid-mountain
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Snoqualmie Pass: 68” mid-mountain
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Crystal Mountain 70” Green Valley
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Paradise: 78”
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Mt Hood Meadows: 44” mid-mountain
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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
Strong and variable winds continue to redistribute snow near and above treeline. Human triggered wind slab avalanches are likely on unsupported wind loaded slopes and terrain features 35 degrees and steeper. These avalanches are dangerous by themselves and have the potential to step down and trigger even larger avalanches on deeper layers in the snowpack.
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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Deep Persistent Slabs
A persistent layer of buried surface hoar and/or facets is the culprit in many recent avalanches. This layer is deep in the snowpack. In many locations, it is 4-6ft below the snow surface. You are most likely to find the buried persistent weak layer above 5000ft in this zone.
Deep persistent slabs are tricky. They often do not show obvious signs of instability, are difficult to assess, and can be hard to trigger. Any avalanche failing on this layer is likely to be very large and destructive. This dangerous avalanche problem has not gone away and continues to deserve a high level of respect and caution.
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 or near the ground. 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. They commonly develop when Persistent Slabs become more deeply buried over time.
Deep Persistent Slabs avalanches can be destructive and deadly events that can take months to stabilize. You can trigger them from well down in the avalanche path, and after dozens of tracks have crossed the slope.
A snowboarder triggered this Deep Persistent Slab near treeline, well down in the path.
Deep, persistent slabs are destructive and deadly events that can take months to stabilize. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Give yourself a wide safety buffer to handle the uncertainty, potentially for the remainder of the season.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Dec 21st, 2018 10:00AM
