Avalanche Forecast
Issued: Jan 11th, 2018 12:02PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Dangerous avalanche conditions will persist on Friday. Time and patience is required to let recent storm snow instabilities heal. Recent and fresh wind slab, potentially deep, will exist near and above treeline and may build onto open slopes below treeline. Loose wet avalanches are possible below treeline. All types of avalanches may become large by entraining recent storm snow. Travel conservatively Friday and avoid consequential terrain.
Summary
Detailed Forecast
Snow levels should eventually settle around 4000-4500 feet later Thursday night and Friday across the region. Light to moderate showers seen through Friday morning should taper down in the afternoon. Post-frontal W-SW winds should be moderate for the central and southwest Cascades, with lighter winds expected in the northwest Cascades.
Dangerous avalanche conditions will persist on Friday. Time and patience is required to let recent storm snow instabilities heal. Recent and fresh wind slab, potentially deep, will exist near and above treeline and may build onto open slopes below treeline. Loose wet avalanches are possible below treeline. All types of avalanches may become large by entraining recent storm snow. Travel conservatively Friday and avoid consequential terrain.
Tree well/snow immersion suffocation hazards are present due to all of storm snow received this week. Keep eyes on your partner at all times. Despite the recent snow, early season hazards still exist. Many creek beds have still not filled in for the winter.
Snowpack Discussion
A strong storm system Wednesday night and Thursday brought 10" to up to 2 or more feet of snow (through 5 pm Thursday) across the west slopes of the Cascades and Passes with the highest storm totals at Stevens and Snoqualmie Passes and at Mt. Rainier. Winds along with a warming trend were strongest in the southwest zone including Crystal, Paradise and White Pass while areas further north had a more muted warming trend during the day. Storm snow piled up quickly and many areas noted natural storm slab avalanches primarily releasing within the new storm snow. Only in one area (Stevens Pass) were avalanches recorded as stepping down to storm layers from Wednesday 1/10.
This system follows on the heels of 10-12" of snow received Tuesday night/Wednesday at Snoqualmie Pass/Stevens Pass and with lesser amounts in other areas. Moderate westerly winds late Tuesday through Wednesday re-distributed snow below treeline in exposed areas and particularly near and above treeline where it exposed the most recent crust in some locations or formed pockets of deep wind slab nearby.
The recent storm snow sits on a thin 1/9 crust found up into the near treeline elevation band in most areas. A more supportable and thicker 1/5 crust from rain or freezing rain is easily identifiable in the upper snowpack.
Below the 1/5 interface, observations continue to indicate a strong snowpack with no notable layers of concern.
Observations
North
On Thursday, a skier was caught and carried in a small avalanche he triggered in a closed area of the Mt. Baker ski area. Details were sparse but he was uninjured. Also at Mt. Baker Ski area, patrollers heard a natural avalanche release in the vicinity of Shuskan Arm but visibility was poor.
On Wednesday a skier triggered a slab avalanche in the Baker backcountry on a north aspect at 5500'. Also noted on the NWAC observation page from Wednesday, a snowboarder in the Bagley Lakes/Table Mt area was caught and carried in a small slab avalanche and buried waist deep before self extracting.
Central
Thursday was a very touchy and active avalanche day at Stevens Pass and Snoqualmie Pass. Forecaster Dallas Glass observed numerous natural storm slab avalanches on all aspects near and below treeline in around the immediate Stevens Pass backcountry (see Instagram later this evening). Storm slabs released within new storm layers, occasionally stepping down deeper to a storm layer from Wednesday 1/10. He also observed shooting cracks and remotely triggered slides throughout the day. Over the last two days, tree wells have become increasingly hazardous. Pro-patrol reported natural storm slabs released in the Alpental Back Bowls Thursday. Storm slabs were very touchy during control work at Alpental throughout the day with paths reloading quickly during heavy snowfall. Snoqualmie DOT reported a natural avalanche in one of their Denny Mt paths.
On Wednesday afternoon, Stevens DOT reported a large natural slab avalanche that crossed the railroad tressel near Windy Ridge, west of Stevens Pass. Numerous professionals at the Stevens Pass ski area or in nearby backcountry terrain reported sensitive wind slab building near treeline with active wind transport occurring into the below treeline band.
South
On Thursday, NPS park rangers at Mt. Rainier noted natural loose wet avalanches occurring below tree-line mid-day.
On Wednesday NWAC Pro Observer Ian Nicholson was in the Crystal Backcountry. He observed sensitive wind slab 6-10" in thickness in specific terrain near ridge lines. From the ridgeline he was able to produce shooting cracks off his ski tip. He also observed recent snow forming shallow 4" storm slabs during the day and about 4" above the 1/5 crust.
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
Expected Size
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
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.
Elevations: Below Treeline.
Likelihood
Expected Size
Valid until: Jan 12th, 2018 12:02PM
