Avalanche Forecast
Issued: Mar 9th, 2018 10:45AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Loose Wet, Persistent Slabs and Deep Persistent Slabs.
NWAC,
Northwest Avalanche Center
Several overlapping avalanche problems will create dangerous avalanche conditions Saturday. Avoid steep slopes over 35 degrees where avalanches start. Expect changing conditions through the day as warming and a strong sun effect the snow surface making it easier to trigger all avalanches.
Summary
Detailed Forecast
A new persistent slab avalanche problem has been observed in the Stevens Pass area and in locations along the Eastern Cascades. Newly buried surface hoar and facets have resulted in several avalanches on a variety of aspects (E, SE, S, NW) between 4400-5400 feet. Current observations have not found this layer in other Western Regions. However, use caution. If you experience sudden collapses, shooting cracks, or whumping noises, avoid all nearby avalanche terrain.
You will be able to trigger wind slabs on lee slopes and cross-loaded features. Use visual clues such as snow drifts, uneven snow surfaces, and fresh cornices to identify and avoid nearby slopes where the wind has deposited snow.
Expect new rollerballs, pinwheels, and small loose wet avalanches to occur on sunny aspects throughout the day. These conditions will occur first on steep rocky slopes receiving direct sunshine. Stay off of steep slopes where you see signs of wet surface snow conditions.
Persistent and deep persistent slab avalanche may still occur. To avoid these low likelihood - high consequence situations, stay out of large open slopes that may harbor this difficult to predict and manage avalanche problem. While it is difficult to trigger these types of avalanches, smaller surface avalanches such as loose wet and wind slabs may step down, failing into deeper older layers.
Snowpack Discussion
Several new avalanches were reported Friday. Small natural wind slab avalanches were observed near Alpental Ski area on steep NE facing terrain at 5400’. A cornice triggered wind slab was observed in the Crystal Backcountry on a E slope at 6400’. Several very large new crowns estimated to be 4-5' tall were observed in the Sourdough Mountains NE of Mt Rainier. These avalanches were seen on NE facing slopes around 6500’. Most of these avalanches occurred Thursday night into Friday morning.
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’ Crystal, 4400’ 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
An avalanche professional at Heather Meadows 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.
SouthNWAC 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.
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
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
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
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 10th, 2018 10:45AM
