Avalanche Forecast
Issued: Jan 2nd, 2018 12:45PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Shallow wind slabs may linger in wind exposed terrain above treeline while small loose wet avalanches are possible on steep solar slopes. In some areas, firm surface crusts will make for difficult travel conditions so be prepared to self-arrest and think about the sliding hazard before crossing steeper slopes.
Summary
Detailed Forecast
After a cloudy start, partly to mostly sunny skies are expected Wednesday afternoon with continued mild temperatures. The exception once again will apply to the Cascade Passes with cooler temperatures and areas of low clouds due to continued offshore flow.
Lingering wind slabs should be far less sensitive to triggering on Wednesday and confined to higher terrain. Continue to watch for areas of recently wind transported snow such as fresh cornices, snow drifts, and uneven snow surfaces. Identify and avoid locations where recent wind loading occurred.
Wind slabs can be deceptively difficult to manage in the terrain. Take a moment and read our recent blog post by NWAC Pro Observer Jeremy Allyn on wind slabs.
Small loose wet avalanches are possible on steep solar slopes in areas that experience warm temperatures and afternoon sunshine on Wednesday. Be aware of the consequence of even a small loose wet avalanche around terrain traps.
Cornices along ridge crest in the west slopes of the Cascades and Passes have grown quite large especially at higher elevations. Remember to give these features a wide berth. Cornices can break much farther back from the ridge than expected.
In some areas, firm surface crusts will make for difficult travel conditions so be prepared to self-arrest and think about the sliding hazard before crossing steep slopes.
Despite all this new snow, early season hazards still exist. Many creek beds have still not filled in for the winter.
Snowpack Discussion
Mild weather seen Sunday through Tuesday has allowed lingering wind slabs to gain strength. Temperatures have been cooler in the Passes due to easterly flow, initially slowing the pace of stabilization in these areas.
Regional temperature and wind variability during recent high pressure and offshore flow
Winds Saturday formed shallow slabs on a variety of aspects near and above treeline. Depending on elevation these wind slabs sit on soft snow storm snow or firm rain crust. Shallow wind slabs were triggered by recreational skiers on Sunday in the Snoqualmie and Mt. Baker backcountry.
The Dec 28th/29th storm cycle formed a rain and/or freezing rain crust throughout the west slopes of the Cascades including the Passes that reached at least into the near treeline elevation band. A few inches to a foot of snow fell at the tail end, with the most snow at Mt. Baker.
These firm crusts capped storm snow which fell earlier during the storm cycle. Below the 12/30 crust (date the crust was buried), a generally strengthening snowpack can be found. Weather stations from across the region confirm the upper snowpack is settling.
Observations
North
On Saturday and Sunday Mt Baker Pro Patrol reported wind transportation of the new snow at the upper elevations forming small shallow slabs. Evidence of rain was found to the top of the ski area.
NWAC pro observer Lee Lazzara was on Bear Paw Mountain Saturday. Lee reported a variety of snow surfaces conditions due to recent wind transportation of snow. He observed wind slabs up to 2 ft thick sitting on a firm rain crust formed during the overnight rain event.
Central
On Sunday, Stevens Pass Patrol described a freezing rain event to the top of the ski area that occurred Friday night. Winds Saturday formed shallow wind slabs within the area. Approximately 4” of soft snow sits above the freezing rain crust in wind sheltered areas.
Also on Sunday, Alpental Patrol reported a ½” freezing rain crust to the top of the ski area.
South
On Sunday, Crystal Mt Ski Patrol saw pockets of wind slabs at higher elevations. Friday’s rain crust extended to 6500’ in the ski area.
On Saturday NWAC Pro Observer Jeremy Allyn was in the Crystal backcountry. He observed evidence of rain in the form of a breakable rain crust all the way to ridge crest (6500’). Recent winds had redistributed the overnight snow forming pockets of wind slabs on lee slopes. Jeremy noted that variable snow surface conditions exist due to the recent wind event.
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.
Likelihood
Expected Size
Loose Wet
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Aspects: South East, South, South West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Jan 3rd, 2018 12:45PM
