Avalanche Forecast
Issued: Dec 18th, 2018 10:24AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Elevated avalanche conditions linger after an intense storm. You can trigger a very large slab avalanche today, particularly in high elevation start zones. These start zones endanger lower-elevation paths and runouts.
Summary
Discussion
On Tuesday morning, Mt. Hood Pro Patrol found high avalanche danger at all elevations. One notable slide occurred naturally on a path that normally only goes in east wind loading events. Given our predominant southerly and southwesterly winds during the recent storm, this event was surprising and suggests the possible reactivity of persistent grain types (although the bed-surface of this slide was not identified due to poor visibility).
Weak snow formed and buried earlier in the month can still be found in the upper snowpack. On Monday, Mt. Hood Meadows pro-patrol found buried surface hoar most intact in sheltered locations from 5500 to 6500 ft with the surface hoar layer below 5500 ft. While this layer was preserved intact 16-20" below the snow surface, it was neither reactive in snowpack tests nor involved in any recent avalanche activity. Several rain crusts were found above this layer. We will continue to track this layer and there is a small chance that with loading or rain it could produce a wet slab.
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: 93”
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Washington Pass: 45”
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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: 70”
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Mt Hood Meadows: 37” mid-mountain
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Olympics: 20”
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. 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 winds have created dangerous and very large wind slabs that will be reactive to human trigger. Natural avalanches are possible and may come down from the higher slopes on Mt. Hood. Consider very carefully before stepping into large avalanche paths. Watch for scoured ridge-lines, rounded snow pillows, and sastrugi as confirmation that wind slabs are creating land-mines in the terrain.
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
Storm Slabs
Approximately 4" of heavy, saturated snow sits atop 6-10" of dry snow in the upper BTL and NTL bands where rainfall early Tuesday morning created a slab. Tuesday Mt. Hood pro patrol produced 50 large to very large natural and explosive-triggered avalanches and numerous small to large skier-triggered avalanches below treeline on this layer. Loading of new wind-blown snow will continue to stress this poorly supported layer and it will need another day to gain strength.
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.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Dec 19th, 2018 10:24AM
