Avalanche Forecast
Regions: Stevens Pass.
You may be able to trigger avalanches in specific locations Thursday around Stevens Pass. You are most likely to trigger an avalanche on steep sunny slopes with wet surface snow, on wind loaded features above treeline, and on slopes where older weak persistent layers still exist.
Detailed Forecast
A weak layer of surface hoar and/or facets was buried on March 8th in the Stevens Pass. This resulted in several natural avalanches on the 9th. Observations continue to find this layer 12-18 inches below the snow surface in this region. Snow profiles and snowpack tests can be used to identify this layer but cannot prove its absences. Avoid triggering a persistent slab by staying off of large open slopes greater than 35 degrees.
Expect new roller balls, pinwheels, and loose wet avalanches Thursday. The sun will create unstable wet surface snow conditions. Expect these conditions to develop first on steep sunny slopes and move from eastern to southern to western aspects. Stay away from steep slopes where you see signs of wet surface snow. Loose wet avalanches can seem common and are often small, however they can have large consequences if they carry you into gullies, over cliffs, or through trees.
If you travel above treeline, use visual clues to identify and avoid areas where recent winds have deposited snow. Snow drifts, wind textured snow, and uneven snow surfaces with cracking all indicate that wind slabs may exist nearby. You are most likely to trigger a wind slab on steep convex rollovers, unsupported slopes, or cross-loaded features.
Even though it is not listed as a problem, Deep Persistent Slabs may still occur in the Stevens area. This is a classic low likelihood-high consequence scenario. Remember tracks on a slope do not mean it is safe. You can avoid this difficult to predict weak layer by simply staying out of large avalanche startzones.
Cornices may grow weak and fail during day time warming and periods of direct sunshine. Select travel routes and techniques that limit your exposure to cornices above you.
Snowpack Discussion
Six inches of snow fell in the Stevens area Tuesday night and Wednesday. At higher elevations new shallow wind slabs formed on lee slopes. The new snow bonded well to the old snow surface. There are no reports of newly buried weak layers at this time.
Avalanche activity Wednsday was limited to very small loose wet avalanches and rollerballs that occurred with day time heating. One skier triggered loose wet avalanche was observed on an east facing test slope at 5200’.
Below the recent snow a new crust is strengthening as it freezes and grows. On Wednesday this new crust was generally not supportable.  It ranged in thickness from 1-4†depending on location.
A layer of weak surface hoar and facets buried on March 8th can be found 12-18 inches below the snow surface in some locations. While this persistent weak layer has been observed on most aspects near and below treeline.
An older deeper persistent weak layer can be found 3 to 5 feet below the snow surface. Weak sugary facets are located just above a very firm and widespread crust (2/8).
There are no significant layers of concern below the 2/8 crust.
Observations
NWAC forecaster Dallas Glass was in the Stevens backcountry Wednesday. Dallas found 6 inches of new snow well bonded to the old snow surface. Snowpack tests in several locations continued to identify the 3/8 persistent weak layer.
Several professional avalanche observations from all around the Stevens Pass area Friday through Sunday reported avalanches and snowpack tests confirming the presence of a buried persistent weak layer (3/8) on most aspects near and below treeline.
Numerous snowpack tests from the Stevens Pass area gathered over the last several weeks by multiple avalanche professionals continue to suggest that the 2/13 facet layer can produce avalanches. The most consistent test has been the Propagation Saw Test.Â
Avalanche Problems
Persistent Slabs
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Elevations: Treeline, Below Treeline.
Likelihood: Possible
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.
Aspects: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood: Unlikely
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, West, North West.
Elevations: Alpine.
Likelihood: Possible
Expected Size: 1 - 1