Avalanche Forecast
Issued: Mar 10th, 2018 10:33AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet, Wind Slabs, Persistent Slabs and Deep Persistent Slabs.
NWAC,
Northwest Avalanche Center
Avalanche fatalities continue to occur in the Cascades. Do not drop your guard Sunday. Dangerous and complex avalanche conditions still exist in the mountains. Avoid avalanches by staying out of start zones. You will be more likely to trigger avalanches with daytime warming and settling of the snowpack.
Summary
Detailed Forecast
Changes in the weather will continue to drive changes in the avalanche hazards Sunday. Avoid avalanches by staying off of slopes where avalanches may start. New persistent slab problems have been reported around Steven Pass, the Eastern Cascades, and Southern British Columbia. Currently we do not have any observations of newly buried weak layers in other Western Cascade Regions. Use caution Sunday. If you experience a sudden collapse, shooting cracks, or whumphing sounds, stay out of nearby avalanche terrain.
You will be able to trigger wet snow avalanches Sunday as warm temperatures and sunny skies create wet snow conditions. Expect new rollerballs, pinwheels, and loose wet avalanches to occur. These conditions will develop first on steep rocky slopes receiving direct sunshine. Unstable wet snow will spread to other aspects as the day warms. As water reaches deeper weak layers in the snowpack, wet slab avalanches may occur. If you see any evidence of new wet slab avalanches dial back your terrain use and avoid all avalanche terrain.
You are most likely to find and trigger lingering wind slab avalanches on shaded slopes where winds recently deposited snow. Use visual clues to identify and avoid slopes where wind loading has occurred.
Several older deeper persistent weak layers exist in the snowpack. This is a classic low likelihood-high consequence scenario. Just because you see tracks on a slope does not mean it is safe. While it may be very difficult to trigger these deeper layers, any avalanche failing on them will be large and destructive.
Limit your exposure to overhead cornices as you travel. They may fail without warning due to daytime heating and the sun.
Snowpack Discussion
Two new natural avalanches were reported in the Crystal backcountry Saturday. Both occurred on NW aspects near ridgeline around mid-day.
Mild temperatures and sunny skies effected snow surfaces Saturday. Partly cloudy skies and warm temperatures will likely cause a weak refreeze Saturday night.
New snow fell across the western regions of the Cascades Thursday night and Friday. Significant changes in snow totals were experienced with elevation. Wet heavy snow and rain was observed in up to 5500’ at Crystal, 4400’ at Snoqualmie, and 4300’ farther north. Above these elevations, 12-20 inches of new storm snow accumulated.
Generally SW winds transported the snow in all regions forming new wind slabs on lee slopes and cross-loaded mid-slope features.
This new snow has fallen on a variety of old snow surfaces including settled cold snow and new melt-freeze crusts. There is potential in some locations that surface hoar and/or near surface facets were buried.
Several older persistent weak layers exist within the snowpack. On E-S-W aspects a thin facet-crust combo (2/23) can be found. Snowpack test results show this layer healing but it has been reactive in some snowpack tests. An older deeper and more widespread persistent weak layer has been observed for several weeks. Weak sugary facets (2/13) sit just above a firm crust formed and buried in early February (2/8). This crust is generally found about 3-4 feet below the snow surface.
There are no other significant layers of concern below the 2/8 crust.
Observations
Baker
Mt Baker ski patrol reported well bonded new snow Saturday. Mt Baker received rain up to 5400’ on Friday.
An avalanche professional at Heather Meadows Friday reported significant wind transportation of snow. He did not find a buried persistent weak layer below the recent storm snow in the locations he observed.
Snoqualmie
An avalanche professional at Alpental reported rain up to 4400 feet Thursday night. Two new natural wind slabs were observed in steep NE facing terrain.
South
Crystal Mt Ski Patrol reported two new natural avalanches on NW slopes around 6000'. These D1.5 avalanches occurred mid-day.
NWAC professional observer Jeremy Allyn traveled in the Crystal Backcountry Friday. He found significant wind transportation of the new snow above 6000’. A cornice failure triggered a widely propagating wind slab 2 feet deep on an E aspect of East Peak. Observations generally demonstrated the new snow was bonding well to the old snow surface.
Also Friday, Crystal Mountain ski patrol reported several new large crowns seen across the White River valley in the Sourdough Mountains. Avalanches occurred on NE aspects and were estimated to be 4-5 feet deep. Wind transported snow was observed on the upper mountain.
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.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
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
Expected Size
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
Expected Size
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
Expected Size
Valid until: Mar 11th, 2018 11:33AM
