Avalanche Forecast
Issued: Dec 12th, 2018 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
We have more than doubled our snowpack height over the past few days, and the heavy snowfall continues. The deep, new snow is sitting on a weak layer. This is a very dangerous recipe for large avalanches, and it is not a good time to enter avalanche terrain.
Summary
Discussion
Since December 10, the total snowpack depth has more than doubled on Washington Pass! The first 9 days in December were clear and cold. Surface hoar grew large during this time, and facets developed near the surface of the cold, low density snow. On December 9th this layer was covered up and preserved on many aspects and elevations. On December 11-12th, a substantial storm deposited more than 22” of snow with over 1.5” of snow water equivalent. On the 11th observers identified the December 9th layer as the layer that was failing, and was found to be from one to two feet down in the snowpack.
During the day on the 11th a professional observer reported a large natural avalanche in the Spire Gully on Washington Pass with an impressive powder cloud. He triggered avalanches remotely (from a distance), and reported widespread collapsing near the hairpin on Hwy 20. Test results confirmed the potential for propagation on buried surface hoar and near surface facets. These slabs are sitting on old weak snow and are likely to be found from two to four feet down today.
Snowpack Discussion
Regional Synopsis 20181211
Welcome winter!!!
The NW snowpack is evolving quickly and a near constant pattern of winter storms with fluctuating freezing levels is making things interesting for our 1st week of forecasting.
Here’s what we know:
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Tuesday saw a significant avalanche cycle.
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We have a lot of new snow...2-3’ above 4500ft.
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We’ve gotten a lot of wind.
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We have weak layers near the ground.
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We are going to get more snow before the weekend.
All in all that’s enough to indicate that 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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Storm total (Monday morning to Wednesday evening):
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Mt. Baker: 24”
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Washington Pass: 22”
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Snoqualmie Pass: 10”
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Paradise: 17”
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Mt. Hood Meadows: 10”
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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 hoard 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.
Problems
Persistent Slabs
With another round of heavy snowfall, a warming trend and strong winds, we may tip the scales for very large avalanches. Slabs from two to three feet thick are sitting on old, weak snow. This storm may be the straw that breaks the camel's back, overloading this layer to a breaking point. Any failure on this deep, weak layer would be surprising and scary. It is recommended to avoid avalanche terrain and avoid entering areas with overhead hazard.
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Storm Slabs
Natural activity within the new snow is likely to spike with the heaviest precipitation rates on Thursday morning. This will coincide with a rapid warmup. Look for clues such as shooting cracks and natural slabs on road cuts.
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 13th, 2018 10:00AM
