Avalanche Forecast
Issued: Dec 21st, 2018 10:27AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs.
NWAC,
Northwest Avalanche Center
We have limited information from above treeline on Mt. Hood. Recently formed shallow wind slabs may linger on lee aspects near and especially above treeline. Use caution on steep wind loaded slopes.
Summary
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
Wind Slabs
Watch for evidence of recent wind transported snow, such as firm surface snow, deeper drifts, and wind scoured areas. Human triggered wind slab avalanches are possible on unsupported wind loaded slopes and terrain features 35 degrees and steeper.
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
Valid until: Dec 22nd, 2018 10:27AM
