Avalanche Forecast
Issued: Dec 13th, 2018 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs.
NWAC,
Northwest Avalanche Center
Recent new snow and winds have covered a pronounced weak layer, creating a recipe for dangerous avalanches conditions. Safe travel through avalanche terrain will require a very cautious approach and conservative decision making.
Summary
Discussion
The total snowpack depth has more than doubled on Washington Pass during the week. We’ve received 29” of snow with over 2.2” of water equivalent. 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, a substantial storm resulted in a widespread natural avalanche cycle. A natural out of Spire Gully near the hairpin on Hwy 20 was witnessed along with an impressive powder cloud. Observers identified the December 9th layer as the layer that was likely failing, and was found to be from one to two feet down in the snowpack. Test results confirmed the potential for propagation on buried surface hoar and near surface facets.
Observations from East North Forecast Zone have been very limited, as it is early season and not many folks have been up yet. Please share your observations if getting out, thanks!
Snowpack Discussion
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
Persistent Slabs
We may not have received enough new snow and water from this latest storm to tip the scales for a widespread natural avalanche cycle on deeply buried weak layers. These slabs are likely from two to three feet thick. The telltale signs of instability with this layer are whumphing and collapsing. These signs may or may not present itself, but any failure on this deep, weak layer would be surprising and scary. It is recommended to dig down to look for this layer. Enter avalanche terrain with respect, as safe travel through it will require a very cautious approach.
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: All elevations.
Likelihood
Expected Size
Valid until: Dec 14th, 2018 10:00AM
