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.
NWAC,
Northwest Avalanche Center
Avoid all avalanche terrain as heavy snow and strong wind create increasing and very dangerous avalanche conditions.
Summary
Discussion
Snow and Avalanche Discussion
Thursday's storm system brought warmth and moderate to strong W-SW winds during the day, followed by a few inches of snow and more wind Thursday night. A strengthening rain crust should be easily discernible near and below treeline, with just a few inches of recent snow sitting on top in wind sheltered areas.
Weak snow formed and buried earlier in the month was identified in the upper snowpack earlier in the week; Mt. Hood Meadows pro-patrol found buried surface hoar mostly intact in sheltered locations from 5500 to 6500 ft. We continue to seek more information about this layer following Thursday’s storm.
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 a widespread avalanche cycle due to rapid snow and wind loading. These slabs will be large and increasing in size as the slab depth increases throughout the day. Avoid areas where avalanches may start, stop, and run.
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
Valid until: Dec 23rd, 2018 10:54PM
