Avalanche Forecast
Issued: Dec 15th, 2018 10:10AM
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. Careful route-finding and cautious decision-making are essential to travel near avalanche terrain. If you experience collapsing, cracking in the snow, or see recent avalanches, avoid open slopes 35 degrees or steeper.
Summary
Discussion
New snow and wind will maintain dangerous avalanche conditions. You will find highly variable conditions and snow depths throughout the region. We have limited observations for this large area. Locations like Mission Ridge have seen very dangerous avalanche conditions, where travelers continue to trigger avalanches. Other parts of the region may have less snow and less reactive conditions.
Snowpack Discussion
Regional Synopsis 20181214
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. 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.
Be cautious and get home safe.
Problems
Persistent Slabs
There is a lot of uncertainty about where you can trigger these avalanches. Persistent slabs are difficult to predict and often break much larger and wider than you expect. The best way to stay safe is to avoid open slopes 35 degrees and steeper where persistent slabs are suspected. Use extra caution around slopes where you see thickly pillowed wind features and variable height of snow. Put an extra buffer of terrain between where you travel and anywhere avalanches could start. Stay well out from under large avalanche paths and do not stop near these features.
You will find a layer of weak, sugar-like, faceted snow down 2-3 feet from the surface. Where this snowpack structure exists, you may see signs of instability such as collapsing (a whumpfing sound), recent avalanches, snowpack tests that show propagation, or cracks that travel away from where you impact the snow. These are all indicators that you should avoid avalanche terrain.
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 10:10AM
