Avalanche Forecast
Issued: Dec 22nd, 2018 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Deep Persistent Slabs.
NWAC,
Northwest Avalanche Center
Very dangerous avalanche conditions exist. Avalanche danger will rise with heavy snow and wind beginning Saturday night. Travel in avalanche terrain is not recommended.
Summary
Discussion
Heavy snow and wind over the next 24 hours will test the snowpack once again. This round of precipitation may favor the Hwy 20 corridor, while areas further south in the zone will see less snow accumulation. You are most likely to trigger avalanches within new snow. However, deeply buried weak layers still lurk within the snowpack at mid and upper elevations. Any triggered avalanche within new snow has the potential to break further down in the snowpack creating a much larger, wider, more dangerous avalanche. Give the snowpack time to adjust. Be very cautious if you decide to travel in the backcountry on Sunday.
Impressive avalanche cycles occurred recently in neighboring zones. Reported avalanches were very large and destructive (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 on a weak layer (surface hoar and facets) that was buried on December 9th.
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
Storm Slabs
Heavy snowfall down to 3000ft accompanied by strong southerly winds will create storm slabs at all elevations. The incoming snow may not bond well to the old snow surface. Be especially careful of leeward areas below ridges and cross-loaded slopes. If triggered, these avalanches may step down to deeper layers resulting in very large avalanches. Watch for recent avalanches, shooting cracks, and collapses. Seek out low-angle supported terrain away from overhead hazard.
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. 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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Deep Persistent Slabs
A persistent layer of buried surface hoar and/or facets is the culprit in many recent avalanches throughout the region. This layer is deep in the snowpack. In many locations, it is 5-7ft below the snow surface. You are most likely to find the buried persistent weak layer above 5000ft.
Deep persistent slabs are tricky. They often do not show obvious signs of instability, are difficult to assess, and can be hard to trigger. Regardless of the uncertainty, heavy snowfall and will test this layer again. The balance could be tipped, and another round of very large and destructive avalanches is possible. 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 23rd, 2018 10:00AM
