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 Storm Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Heavy precipitation, warming temperatures, and strong winds are the perfect set-up for avalanches Tuesday. When conditions are like this, it is time to stay away from areas where avalanches can start, run, and stop.
Summary
Discussion
Reports of natural and triggered avalanches continue in the adjacent Mt. Baker area. Periods of clear skies over the weekend allowed observers to view evidence of a unique and impressive avalanche cycle. Numerous avalanches were very large and destructive (some could destroy a house). Several ran full track to valley bottom. Many had very wide propagation (up to ¼ mile). The majority of avalanches failed on a weak layer (surface hoar and/or facets) buried 4-6 ft deep.
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
Storm Slabs
Heavy snow and rising freezing levels along with heavy wind-loading will create very large slabs at mid and upper elevations. Avoid all avalanche terrain where you see heavy snow loading.
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
Persistent Slabs
A concerning weak layer is buried under 2+ feet of new snow at mid and upper elevations. You or storm-related snow loading may trigger the avalanche, which will be very large and destructive. 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 and avoid large avalanche paths at all elevations. 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 18th, 2018 10:00AM
