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 Persistent Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Very dangerous avalanche conditions are developing. Heavy snowfall and strong wind is likely to push our weak snowpack over the edge and cause a widespread avalanche cycle. Avoid travel in avalanche terrain, including runout zones in valley bottoms.
Summary
Discussion
The storms of the past week have been incrementally loading an inherently weak snowpack structure. The weak layers themselves have been identified as buried surface hoar and/or buried near surface facets, and appear to be blended into the form of basal facets and even depth hoar as one moves further east, or up in elevation. Basal facets have been observed in the Wenatchee Mountains, the Chiwaukum Range, and above 5500ft in the East-North Zone.
Many areas within the zone saw a widespread cycle on December 11th. That said, there are some areas further east or where the snowpack is thinner that likely did not have a cycle at all.
For many areas this storm could be what pushes the stress vs. strength ratio of the slab vs. weak layer over the edge, resulting in large and widespread natural avalanching.
Snowpack Discussion
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
Persistent Slabs
In recent days, there have been ski triggered, explosive triggered, and remotely triggered avalanches on Mission Ridge. These could all be harbingers to what is to come today in the mountains. Shooting cracks, whumpfing, and recent avalanches are obvious clues to instability. Avalanches may be very large and will certainly be life threatening.
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
Storm Slabs
Signs of instability for storm slabs are shooting cracks within the new snow, and recent slab avalanches. Any thin, soft storm slab has the potential to step down into deeper weak layers, turning a small soft slab avalanche into a much more dangerous hard slab.
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 18th, 2018 10:00AM
