Avalanche Forecast
Issued: Dec 14th, 2018 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
We have not seen the avalanche activity in this zone that we have seen in the northern and eastern areas. That said, there are certainly places where the new snow is not bonded to older, weak snow at the ground. Additionally, strong storms earlier this week formed a variety of wind slabs near and above treeline.
Summary
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 a while.
Be cautious and get home safe.
Problems
Wind Slabs
Rainfall into the above treeline terrain late Tuesday helped to lock down larger, older wind slabs. New softer slabs and drifts may be forming over the crust along the lee side of exposed ridge-lines and terrain features.
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Persistent Slabs
We have a high degree of uncertainty concerning the distribution and reactivity of buried persistent layers in this region. There are reports of deeper faceted layers in the crystal backcountry and on Chinook Pass, but we have few reports of avalanches. The best bet today is to be very cautious in and around start zones in areas above 6000 feet where you are more likely to find deeper slabs and old weak snow.
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 15th, 2018 10:00AM
