Avalanche Forecast
Regions: Cascades - East.
New storm and wind slab may become increasingly sensitive and widespread Saturday due to additional light loading and a continued slow warming trend especially in the Washington Pass area. Slabs may fail on weak persistent grain types formed prior to this storm cycle. There is a large amount of uncertainty so it's imperative you assess snowpack stability throughout the day throughout the terrain and quickly dial back your plans if you experience signs of increasing instability.
Detailed Forecast
Another round of light snow showers, except locally moderate closer to the Cascade crest Friday night should be followed by increasing shower activity late Saturday afternoon and evening. Look for a relative break in precipitation from Saturday mid-morning through early afternoon. The slow warming trend seen along the east slopes should accelerate a bit on Saturday as arctic air currently in place is continuously eroded by westerly flow.  Â
Dangerous avalanche conditions may develop on Saturday as we look to tip the scales with additional light loading particularly in the Washington Pass area. Gradual warming will help new snowfall settle and develop more of a slab structure. Slabs may fail on weak persistent grain types formed prior to this storm cycle. There is a large amount of uncertainty so it's imperative you assess snowpack stability throughout the day throughout the terrain and quickly dial back your plans if you experience signs of increasing instability.
Look for new wind slab near and above treeline particularly in the Mission Ridge area where the greatest danger will be on lee easterly aspects. The avalanche forecast will reflect a greater likelihood of a wind slab problem in the central-east and southeast zones than the Washington Pass area. Due to ongoing easterly winds closer to the Cascade Passes, wind slab will be listed on all aspects. Â
In steep sheltered terrain continue to watch for small loose dry avalanches near terrain traps like cliff bands where even small avalanches can have big consequences.Â
Even though the lower part of the below treeline band is filling in, watch for early season travel hazards such as barely covered rocks and open creeks.
Snowpack Discussion
Weather and Snowpack
An active and cool weather pattern has resulted in a snowy December thus far with NWAC stations along the east slopes recording about 1-2 feet of snow.
Cold and fair weather took hold midweek. During this time, there have been many reports of right side up, favorable density profiles with lower density snow nearer the surface. The Washington Pass area in particular has enjoyed excellent skiing and riding conditions with good stability and light winds. Widespread near-surface faceting and surface hoar formation had been observed particularly in this area of the east slopes. The Mission Ridge area experienced moderate easterly winds through Thursday night before switching to moderate westerly winds early Friday morning. Generally light snowfall accumulated Thursday night and Friday along the east slopes.Â
Recent Observations
Pro-observer Jeff Ward was in the Washington Pass area Friday and reported up to 25 cm (10") of new snowfall at Pass level by Friday afternoon. The new snow was of lower density but cohesive enough that easily and remotely triggered soft slabs were possible on all aspects with rapidly decreasing stability.
Mission Ridge pro-patrol reported the switch to westerly winds had loaded NE aspects below ridgetop. Easy to trigger wind slabs of 40 cm deep were observed on NE aspects around 6700 feet. Just a few hundred feet below in less wind affected terrain, the snowpack was generally right side up and lacked a cohesive slab. A shallow snowpack still locally limits the avalanche danger to the higher elevations and more wind loaded aspects.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
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: Likely
Expected Size: 1 - 1
Loose Dry
Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose Dry avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose Dry avalanches can trigger slab avalanches that break into deeper snow layers.
Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.
Loose Dry avalanche with the characteristic point initiation and fan shape.
Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1