Avalanche Forecast
Regions: Snoqualmie Pass.
You are likely to trigger avalanches in the recent storm snow on any open slope greater than 35 degrees. Older weak snow still exists in the snowpack. You may trigger very large and deep avalanches from thinner spots in the snowpack or with larger triggers.
Discussion
We have reports of natural, skier, and explosive triggered avalanches in several areas over the last two days. These recent observations are coming from areas that have not seen widespread avalanche cycles on the 12/9 weak layer and are indicators that the backcountry snowpack is being stressed. Â
A very large avalanche was triggered by a single small explosive at Crystal Mountain Ski Resort. This avalanche failed on the buried persistent layer, took out several adjacent avalanche paths, and ran full track to the valley. The avalanche broke in surprising ways, pulling snow from up on the ridgeline. A ski patroller was caught, carried, and deployed his airbag. He survived with minor injuries despite the huge size of the avalanche.
Avalanche Details: NE aspect, 6600’, SS-AE-R4-D4-O
Large and surprising avalanches should give us pause. These are obvious signs that the persistent slab is still present and can produce avalanches.
Snowpack Discussion
Regional Synopsis: December 18, 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:
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Mt Baker: 93â€
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Washington Pass: 45â€
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Stevens Pass: 66†mid-mountain
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Snoqualmie Pass: 59†mid-mountain
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Crystal Mountain 57†Green Valley
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Paradise: 70â€
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Mt Hood Meadows: 37†mid-mountain
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Olympics: 20â€
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. Â 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.
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Avalanche Problems
Storm Slabs
You are most likely to encounter unstable storm slabs Wednesday. The thickness of these slabs will increase with elevation and in areas where the wind has deposited snow. Storm slabs need time to heal. Unless the slope is scoured, stay off of all steep open slopes. Avalanches could start in the new snow and step do to deep persistent layers below.
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: Likely
Expected Size: 1 - 1
Deep Persistent Slabs
Deep persistent slabs are tricky. They often don’t show obvious signs of instability, are difficult to assess, and can be hard to trigger. You are most likely to trigger a deep persistent slab from thinner spots in the snowpack or if involved with a smaller avalanche. Any avalanche failing on this layer is likely to be very large, and destructive. Avalanches may cross terrain features, encompass numerous aspects, and run far into their runouts. Give this troublesome avalanche problem a wide berth.
In the Snoqualmie Pass area you are most likely to find a buried persistent weaklayer of surface hoar and/or facets above 4500 feet and under at least 3-4 feet of snow.
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: All elevations.
Likelihood: Possible
Expected Size: 2 - 2