Avalanche Forecast
Regions: Stevens Pass.
Very dangerous avalanche conditions should persist Thursday, especially near and above treeline where the strongest winds are expected and where weaker bonds to the early March crust should remain.
Detailed Forecast
Another strong warm front should cause more moderate to heavy rain or snow at higher snow levels Wednesday night with a bit of a break Thursday morning.  Another front should renew moderate rain or snow at lower snow levels during the day Thursday along with strong winds.Â
Five day water equivalent nearing 4-7 inches and snowfall of about 3-6 feet should be the expected storm totals along the west slope Cascade zones ending Thursday afternoon.
Due to rain continuing to reach higher elevations Wednesday night before gradually cooling Thursday, continued  wet loose avalanches remain a concern at mid and lower elevations.  This should be at least up to the near treeline zones. Steep slopes that recently received heavy snowfall should be primed for wet loose avalanches. Natural snowballing and natural releases are usually precursors to this type of avalanche.
Wind slab will be a likely concern mainly on lee slopes above treeline but gradually lowering to near treeline as cooling occurs Thursday. This should be mainly northwest to southeast aspects. Watch for signs of wind transported snow, such as trees void of snow and sculpted surface snow patterns and deposited pillows of denser wind slab.  Recall a weak bond may remain to the early March crust and may lead to some areas of very sensitive avalanches.Â
Storm slab will also be listed as a concern above treeline on a wider variety of aspects. Wetter denser new snow accumulating over lower density snow in the above treeline zone will contribute to this concern.
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Snowpack Discussion
Recent Weather
An extended mild and dry period occurred the latter half of January generally forming a crust in all areas.
Very cold weather followed in early February which produced light low density snowfall and facets just above the late January crust.
A two week storm cycle ended a week ago causing 7-12 feet of snowfall and many avalanche cycles near and west of the crest.
Warm dry weather was seen late last week from Wednesday Through Saturday causing wet snow avalanches and melt/freeze crusts throughout the region. Surface cold air also redeveloped east of the crest last weekend.Â
Temperatures dropped significantly by Sunday prior to the onset of a new storm cycle.  This caused poor bonding of the initial snowfall to the newly formed crust layer in most areas.Â
A series of fronts have moved across the area since Sunday at the rate of at least one per day. Freezing levels have been generally on the rise through these events causing rain or wet snow conditions to about 5500 ft in the north, 6000 ft central and 7000 ft or higher in the south.
Washington Cascades near and west of the crest
The latest storm cycle began on Sunday. An avalanche cycle was seen on Sunday night and Monday morning. DOT crews at Stevens and Snoqualmie reported widespread natural and triggered 1-2 foot avalanches Sunday night and Monday morning which closed Snoqualmie Pass. Poor bonds to the melt-freeze crust from late last week have been reported with the crust likely providing bed surfaces.
Snow levels have been rising and temperatures warming Tuesday afternoon through Wednesday with heavy rain or wet snow producing several avalanche cycles with the latest reports from the Alpental ski patrol where another round of natural avalanches were seen Wednesday morning with mostly loose wet snow avalanches.  However some larger natural releases were noted above cliff bands where unsupported slabs released above rock faces slopes.  Some of these releases deposited significant snow on the slopes below.Â
There have been no recent reports above treeline as to the extent of dry wind slab and storm slab development.  However, with the significant precipitation received and very strong winds, direct observations are not needed to determine that very dangerous avalanche conditions are likely near and above treeline. Â
The late January crust and facet layers remain a concern and are now very deeply buried and have been getting tested by the recent very heavy snowfall. Large avalanches have been seen at Stevens and Snoqualmie the past week. The Stevens Pass ski patrol produced this video of explosive control producing a 10-15 foot avalanche, you might want to turn down your speaker volume first. A large natural hard slab avalanche with a 10 foot crown released on a steep north slope on Chair Peak near Alpental Saturday 22 February and caught 3 skiers. Here is a link to more information about deep persistent slabs which are unusual in our area and not predictable.
Avalanche Problems
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: Treeline, Below Treeline.
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.
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: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Deep Persistent Slabs
Release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer, deep in the snowpack or near the ground. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage. They commonly develop when Persistent Slabs become more deeply buried over time.
Deep Persistent Slabs avalanches can be destructive and deadly events that can take months to stabilize. You can trigger them from well down in the avalanche path, and after dozens of tracks have crossed the slope.
A snowboarder triggered this Deep Persistent Slab near treeline, well down in the path.
Deep, persistent slabs are destructive and deadly events that can take months to stabilize. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Give yourself a wide safety buffer to handle the uncertainty, potentially for the remainder of the season.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Unlikely
Expected Size: 1 - 2