Avalanche Forecast
Regions: Olympics.
Watch for wind transported snow on a wide variety of non-traditional aspects and features Monday. Building wind layers may be touchy and cautious travel conservative decision making is urged. The 12/17 persistent slab has become less likely to trigger. Avoid travel in open terrain and especially with large groups as these slides may require a larger load to trigger.
Detailed Forecast
Snow showers should end overnight Sunday, giving way to clear skies and cold temperatures. Strong N-NE ridgetop winds should continue to transport the ample loose surface snow and build touchy wind slabs on more non-traditional or unusual SE-SW aspects.
Deep, low density snow will make sluffs or loose slides likely on steep wind sheltered slopes. Watch for travel on steep slopes above terrain traps, such as cliffs or creeks where the entertainment of loose snow may bury someone caught more deeply.Â
New and lingering wind slabs will be the primary problem on Monday. The 12/17 persistent slab remains a concern in the Hurricane Ridge area, but is becoming less sensitive to trigger.
While triggering a slab to the 12/17 interface has become less likely, the travel advice remains conservative. Avoid travel in open terrain and especially with large groups as these slides may require a larger load to trigger.Â
Wind affected snow should be widespread Monday, especially with strong shifting winds, watch for wind transported snow on a variety of aspects and cross loaded features. 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 Thursday, deposited about 0.5 inches of water equivalent with a warming trend and moderate S-SW winds ending Thursday evening.Â
A break Friday allowed for some snowpack consolidation.
A cold low pressure system renewed snowfall at low snow levels Saturday afternoon with snowfall continuing New Year's Day. Â Moist N-NE winds on New Year's helped orographic lift along the north slope areas, depositing about 15 inches of new snow at Hurricane Ridge by Sunday afternoon!Â
Recent Observations
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.
There were two triggered avalanches Friday in the Hurricane area, including a solo traveler who was partially buried after triggering a soft slab avalanche and was fortunately able to self rescue. Â
NWAC pro-observer, Matt Schonwald was back in the field in the Hurricane Ridge area Saturday, 12/31. Matt found evidence of widespread wind transported snow near ridges and cross loaded features, cornices on multiple ridges and evidence of naturally triggered storm slab avalanches, likely during recent storms late last week.Â
In multiple test pits Saturday, the 12/17 PWL was found still intact buried consistently 60-80 cm below the surface. However, strong, well settled snow above and good bonding is indicating that triggering this layer is becoming unlikely. Â
Avalanche Problems
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: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
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: Unlikely
Expected Size: 1 - 1