Avalanche Forecast
Regions: Olympics.
The 12/17 persistent slab remains the main avalanche problem in the Hurricane Ridge area but this layer should be less likely to trigger Saturday except in isolated areas. Continue to avoid steeper slopes of consequence especially in less skied areas at Hurricane Ridge until we can confirm this layer is un-reactive to human triggering. While wind affected snow should be widespread, wind slab will likely be stubborn to trigger Saturday.
Detailed Forecast
After a mostly clear start to Saturday, a fast moving frontal system will bring light snow to the Olympics by late morning which should continue through the day. New snow amounts through 4 pm will generally be light, but westerly winds, especially near and above treeline, are expected spike up by mid-day.Â
The 12/17 persistent slab remains the main avalanche problem in the Hurricane Ridge area but this layer should be less likely to trigger Saturday except in isolated areas. Continue to avoid steeper slopes of consequence especially in less skied areas at Hurricane Ridge until we can confirm this layer is un-reactive to human triggering.
While wind affected snow should be widespread, wind slab will likely be stubborn to trigger Saturday.Â
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Snowpack Discussion
Weather and Snowpack
A strong front and strong west flow aloft was seen over the Olympics and Cascades on Monday and Tuesday. The NWAC station indicated strong south to southwest winds Monday and Tuesday with almost 2 feet of new snow for the 48 hours ending Wednesday morning with a cooling trend.
A warm front caused moderate to strong winds and periods of light to moderate snow Thursday with about half an inch of water through Thursday evening at the Hurricane Ridge station. New snow initially fell during a warming trend with moderate S-SW winds but stormy conditions quickly subsided behind the front Thursday evening.Â
Recent Observations
NWAC pro-observer, Matt Schonwald was at Hurricane Ridge on Wednesday and reported the 12/17 PWL is still causing collapsing on all aspects especially in less skied areas below ridge lines. Recent natural wind slab releases of 10-12 inches where also seen on N-NE slopes off ridges at about 6000 feet. A 2-4 foot x 150 foot wide wind slab crown on the convex north slope below the visitor overlook was seen which may have released on a buried surface hoar layer from around Christmas Eve. The road to Hurricane was closed on Thursday.Â
On Friday, NPS rangers reported wind affected snow surfaces well below treeline with about 4" of dense new storm snow. No natural avalanches from Thursday or Thursday night were observed with good visibility Friday morning. Â Little loose surface snow was available for future transport.Â
Avalanche Problems
Persistent Slabs
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: Possible
Expected Size: 1 - 1
Wind Slabs
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: North, North East, East, South East, South, North West.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1