Avalanche Forecast
Regions: Snoqualmie Pass.
The avalanche danger will peak Sunday night through Monday morning during the period of heaviest loading, slightly decreasing later in the day. The snow-line through this period will determine the sensitivity and depth of storm and wind slabs. Expect rising avalanche danger with an increase in elevation, and avoid traveling below paths connected to higher start zones.
Detailed Forecast
The avalanche danger will peak Sunday night through Monday morning during the period of heaviest loading, slightly decreasing later in the day and tapering from north to south across the Cascades. The snow-line through this period will determine the sensitivity and depth of storm and wind slabs. Expect rising avalanche danger with an increase in elevation, and avoid traveling below paths connected to higher start zones. The most significant snowfall is expected at higher elevations of the near and above elevation bands, and moderate to strong W-SW winds will easily transport new snow to lee slopes. A locally lower snow level at Mt. Baker will increase the likelihood of storm slabs developing below treeline. Storm slabs may develop in the upper portion of the below treeline band from Stevens Pass and south to Mt. Rainier.Â
Generally small loose wet avalanches are possible at lower elevations in steep terrain. Be especially careful of even small loose wet avalanches around terrain traps.Â
Remember that beneath any new snow received during this storm, a myriad of early season terrain hazards exist, especially at lower elevations. Expect terrain hazards with poorly covered rocks, vegetation and creeks, particularly on south-facing terrain, at lower elevations, and on exposed ridges where wind events have stripped much of the seasons snowcover.
Snowpack Discussion
After 12 long days of high pressure, the first frontal system in quite awhile crossed the Cascades on Friday. New snow accumulations Saturday morning ranged from 2-5 inches along the west slopes of the Cascades with a local maximum at the top of Alpental with 8 inches of new snow. Below this storm snow, a highly varied array of snow conditions exist, including melt-freeze crusts, settled old storm snow, and a mix of recently formed surface hoar or near surface facets (sugary snow) at lower elevations and some shaded terrain.
Warm frontal moisture that arrived on Sunday brought light rain to 4000-4500 feet in the north Cascades, a battle between rain and wet snow at Stevens and Snoqualmie Passes with light snowfall accumulating at the top of Alpental, and a mix up to about 5500 feet in the south Cascades (Paradise, Crystal) through 4 pm Sunday. No significant avalanche activity was reported. The warmer temperatures and light rain likely helped wet or potentially destroy recently buried persistent weak layers at lower elevations, especially in the Stevens Pass area.  Â
The overall snowpack of 2-5 feet along the west slopes is stable with settled old snow sitting over the strong Thanksgiving rain crust, buried about 1.5-2.5 feet.
Observations
North
Baker pro-patrol reported no avalanche concerns in their area Sunday morning. Â
Central
Both Stevens and Alpental pro-patrol reported a few small loose wet avalanches localized to very steep terrain features, but otherwise no avalanche activity in area.Â
Alpental pro-patrol reported sensitive but shallow 4-6" storm slabs during Saturday morning's avalanche mitigation. The storm slabs were easy to trigger but did not run far. Wind transported snow was noted near-treeline from N-NW winds Friday night.
There is a pertinent observation from the Yodelin area of Stevens Pass from Friday detailing recently buried surface hoar in an open location below treeline on the NWAC observations page.
South
Professional observer Jeremy Allyn was in the Crystal backcountry on Saturday. He found variable snow with pockets of enjoyable dust on crust in smoother terrain. Very localized pockets of soft wind slab were unreactive. Plenty of early season terrain hazards exist on solar aspects, wind scoured terrain and at lower elevations. Â
Avalanche Problems
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
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, North West.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
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: Possible
Expected Size: 1 - 1