Avalanche Forecast
Regions: Stevens Pass.
Avoid avalanche terrain especially at higher elevations on Tuesday.
Detailed Forecast
Three day snowfall ending Tuesday morning should range from 1-4 feet near and west of the crest. West southwest flow aloft should continue to carry warm front moisture to the Northwest. This should cause rain or snow showers with rising snow levels on Tuesday.
Wind slab will be a likely concern mainly on lee slopes at higher elevations. This should be mainly north to southeast aspects. Watch for signs of firmer wind transported snow.
Storm slab should also remain likely above the snow line on Tuesday on a wider variety of aspects. Wetter denser new snow accumulating over lower density snow will contribute to this concern.
Below the snow line wet loose avalanches will be the primary concern. Steep slopes that received heavy snowfall and then receive rain will be primed for wet-loose avalanches. Natural snowballing and smaller natural releases are usually precursors to this type of avalanche.
Deep persistent slab will remain an unpredictable concern and snowpit tests are not practical. Triggering such an avalanche is unlikely but the consequences could be catastrophic. The best way to deal with this concern for the time being would be to avoid large open avalanche terrain.Â
Snowpack Discussion
Recent Northwest 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 2 week storm cycle ended a week ago causing 7-12 feet of snowfall and avalanche cycles near and west of the crest.
Warm dry weather was seen late last week Wednesday to Saturday causing wet snow avalanches and melt/freeze crusts throughout the region. Surface cold air also redeveloped east of the crest last weekend.
Washington Cascades near and west of the crest
Another 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 1-2 foot avalanches Sunday night and Monday morning which closed Snoqualmie Pass. Poor bonds to the melf-freeze crust from late last week have been reported with the crust probably providing bed surfaces. Our NWAC observer Jeff Hambelton reported less active recent snow layers from Mt Baker Monday afternoon.
The late January crust layer and overlying facets are now very deeply buried and have been getting tested by the recent very heavy snowfall. Large avalanches in the Stevens and Snoqualmie areas have been seen over the past week. The Stevens Pass ski patrol produced this impressive 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 off of steep north facing terrain of Chair Peak near Alpental Saturday 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
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
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
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: 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