Avalanche Forecast
Issued: Dec 13th, 2018 10:12AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs.
NWAC,
Northwest Avalanche Center
Strong storms earlier this week have formed unstable wind slabs. These conditions are beginning to improve, however, continue to watch for recently wind-drifted or actively wind-loading slopes, primarily near and above treeline.
Summary
Snowpack Discussion
Regional Synopsis 20181213
Winter is here.
A productive winter storm pattern has elevated the avalanche danger and provided an interesting start to the season.
A quick breakdown:
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Tuesday and Wednesday saw a significant localized avalanche cycles.
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We have a lot of new snow...2’ to 5’ above 4500ft.
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We’ve gotten a lot of wind.
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We have weak layers near the ground.
Dangerous avalanche conditions will persist over the next couple of days. That said, it is early season and the flavor of avalanche you may run into is likely a function of elevation, timing, and dumb luck.
Here are some basic emerging patterns:
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Snowfall totals so far this week (Monday morning to Thursday evening) illustrate more recent snowfall in the north than the south:
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Mt. Baker: 55”
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Washington Pass: 29”
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Stevens Pass: 37”
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Snoqualmie Pass: 28”
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Paradise: 38”
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Mt. Hood Meadows: 13”
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Upper versus Lower Elevations: The change in the snowpack is still pretty dramatic with elevation. Height of snow decreases rapidly below 4500’ at Baker and Washington Pass, 5500’ at Crystal/Rainier. The Passes have better low elevation coverage, but it's still pretty thin below 4000’. With additional warm storms in the forecast, this pattern is expected to continue for awhile.
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East versus West: Loading along the East slope has been more incremental, and a variety of buried facet and surface hoar layers may be found. This is most pronounced near WA Pass. Although significant snowfalls and precip totals have resulted in thicker, more homogeneous snowpack in the western zones, lingering weak layers near the ground will persist through the week.
The uncertainty of how reactive our buried weak layers are will carry into the weekend. Be cautious and get home safe.
Problems
Wind Slabs
Fresh and recently formed small winds slabs will be found on specific terrain features and you will still be able to trigger them Friday. Rainfall into the above treeline terrain late Tuesday helped to mitigate larger, older wind slabs. Soft wind slabs should be forming over a crust in most areas. The presence of wind-scoured and textured snow surfaces or wind-scoured ridgeline indicate that wind slabs are likely to have formed nearby.
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: All elevations.
Likelihood
Expected Size
Valid until: Dec 14th, 2018 10:12AM
