Avalanche Forecast
Issued: Dec 17th, 2018 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Very stormy conditions and high snow levels will create dangerous avalanche conditions, particularly at higher elevation. Avoid large avalanche paths where avalanches may start, stop, and run.
Summary
Discussion
Above 7000 ft, wind slabs will be growing and rapidly becoming very large on Tuesday and may release naturally or with a human trigger. This creates very dangerous avalanche conditions and wind slabs that release at higher elevation will be deadly, so avoid all large avalanche paths on Tuesday.
Loose wet avalanche conditions will develop near treeline and below.
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 snowpact 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
Updated Regional Synopsis 20181216
Dangerous and fickle avalanche conditions remain as active weather continues into the week. Thick slabs of new snow (2-5 feet) are perched above a weak layer of faceted crystals. Avalanches triggered on this layer could be very large and life threatening.
Reports continue to come in of very large natural and triggered avalanches in the northern and eastern zones. For perspective, several of these slides have been classified as D3, or large enough to destroy a house. You can find similar snowpack structure responsible for these avalanches in many other locations throughout the Cascades, including Stevens and Snoqualmie Passes.
Anyone accessing alpine areas should limit their exposure to areas where avalanches start, run and stop. In some places the weak snow will talk to you by whumpfing and cracking. In other places, the heavy blanket of new snow is thick enough that it can give a false sense of stability while it masks the dangerous layering below.
Approximate snow totals from 12/10 - 12/16:
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Mt. Baker: 75”
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Washington Pass: 35”
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Stevens Pass: 42”
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Snoqualmie Pass: 36”
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Paradise: 51”
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Crystal Mt Base: 29”
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Mt. Hood Meadows: 21”
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Olympics: mix of rain and snow, no net gain
The change in the snowpack is 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. Mt. Hood and Hurricane Ridge have low snow coverage below 5000’.
If you're out in the mountains, please let us know what you see.
Be cautious and get home safe.
Problems
Wind Slabs
Snow and wind will combine to create very large wind slabs as heavy, wet snow gets blown into higher elevation avalanche start zones. Avoid all terrain exposed to these areas.
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.
Likelihood
Expected Size
Loose Wet
Snow will change to rain at lower to mid elevation creating unconsolidated snow conditions. These will be limited in size due to the multiple crusts within the current snowpack. Watch for heavy rain or ankle deep boot penetration as signs that you should avoid all terrain 30 degrees or steeper.
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Dec 18th, 2018 10:00AM
