Avalanche Forecast
Issued: Dec 15th, 2018 10:13AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs.
NWAC,
Northwest Avalanche Center
You can trigger dangerous and surprising avalanches that could break widely across terrain features. Avalanche danger will rise with continued snow, wind, and a warming trend on Sunday. Careful route finding and cautious decision-making are essential.
Summary
Discussion
A series of storms over the past week delivered over two feet of new snow to the Washington Pass area. This added load doubled the height of the snowpack and tested weak layers of buried surface hoar and facets. The result was a widespread natural avalanche cycle on December 11th. Recent observations confirm the potential for propagation on these weak layers buried 2-3ft below the snow surface.
Limited information exists for the East North Forecast Zone. If you head out in this area, please send us your observations.
Snowpack Discussion
We made it through our first strong winter storm and are headed into a weekend with dangerous and fickle avalanche conditions. 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 explosives 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.
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 - 12/14:
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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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Olympics: mostly rain
The change in the snowpack is pretty dramatic with elevation. The 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 a while.
Be cautious and get home safe.
Problems
Persistent Slabs
You may be able to trigger large avalanches that can bury and kill you. Continued snowfall will stress the snowpack and keep skier triggering likely. These avalanches could break near the ground and act in surprising ways. You may be able to trigger these avalanches remotely, from low-angle terrain or adjacent slopes. Give yourself a wide safety buffer to address the uncertainty. Watch for shooting cracks, collapses, and recent avalanches. Choose moderately angled and supported terrain away from overhead hazards.
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 16th, 2018 10:13AM
