Avalanche Forecast
Issued: Dec 22nd, 2018 10:54PM
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
Storm snow is likely to form slab avalanches, particularly at higher elevations where winds re-distribute the snow. Very large deep persistent slab avalanches are still possible and mostly unsurvivable and you will be most likely to trigger them at higher elevations.
Summary
Discussion
Forecast updated 12/23/2018 at 6:45 AM to reflect less-than-anticipated snowfall totals, particularly around Crystal and White Pass. Mount Rainier National Park is closed and avalanche danger may be higher than reflected in this forecast.
On Friday, professional observations from Tatoosh Range (south of Paradise) revealed no new significant avalanche activity following Thursday’s storm on the 12/9 layer. We don’t have any information about this layer in the White Pass area.
The 12/9 weak layer has been identifiable in snow pits in snowpits in the Crystal and Mt. Rainier areas, it was rounding and gaining strength to the point where it was dropped as a problem near and below treeline. Several intermittent crusts could be found above the 12/9 layer in the upper snowpack and the 12/21 crust extends up to 6000 ft. Recent winds have caused wide variations in snow depths - something worth noting when thinking about deep persistent slab avoidance.
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
Expect touchy avalanche conditions where heavy snow has fallen or wind has re-distributed the lower-density snow onto open slopes. Watch for storm snow greater than 8” deep as a sign that this problem will be present. You are most likely to trigger a storm slab on open and unsupported slopes steeper than 35 degrees. This problem could end up being a loose dry avalanche problem as the snow has fallen at lower density. Keep in mind, that if any avalanche triggers a persistent week layer, it will be deadly.
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: Alpine, Treeline.
Likelihood
Expected Size
Deep Persistent Slabs
A persistent weak layer which remains a concern above 6000 ft.
Thursday’s storm and wind event produced two widely propagating large avalanches with 3+ ft crowns. Several additional large or very large natural, skier, and explosive triggered avalanches occurred in the Crystal area Monday through Wednesday. Many of these avalanches acted in surprising ways including wrapping around terrain features, running long distances, and responding to relatively small triggers. A persistent weak layer consisting of buried surface hoar and/or facets has been the culprit in several very large avalanches in the Crystal area. This layer is 3-6 feet below the snow surface in many areas. You are most likely to find the buried persistent weak layer above 6000 ft in this region. While these avalanches are likely to start higher in the terrain, they are running down to much lower elevations. Any avalanche failing on this layer is likely to be very large, destructive and deadly.
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.
Likelihood
Expected Size
Valid until: Dec 23rd, 2018 10:54PM
