Avalanche Forecast
Issued: Mar 17th, 2018 11:12AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet and Deep Persistent Slabs.
NWAC,
Northwest Avalanche Center
If the sun comes out, watch for Loose Wet avalanches on steep sunny slopes. While Deep Persistent Slab avalanches are now very difficult to trigger, the potential for high consequences lurks deep in the snowpack. Weak layers buried deep in old snow have been responsible for fatalities and serious injuries each of the past four weekends.
Summary
Detailed Forecast
The threat of Loose Wet avalanches lingers today. If the skies clear and the sun makes an appearance, watch for uncohesive, wet snow on steep slopes. While Loose Wet avalanches are easier to predict and avoid, they can still be powerful enough to push you into trees, over cliffs, and through rocks. Use caution near these terrain traps on steep sun-exposed slopes. Until southerly slopes get softened by the sun they will remain hard and slick. If you travel on these slopes while they are still firm it may be difficult to stop a fall.
Recent warm temperatures and sun have brought up a host of other concerns. Avoid standing on or under cornices as they can easily break and fall. Avoid glide cracks and stay out from under steep rocky terrain that is holding snow.
Deep Persistent Slab avalanches are now unlikely and difficult to trigger. If you manage to trigger one, the avalanche will be so large and destructive that you will have little chance of surviving. Persistent weak layers have been the cause avalanche fatalities or serious injuries every weekend for the past 4 weeks. Currently, you will see significant backcountry traffic in avalanche terrain, but little signs of avalanches. Test results and avalanches show that these layers can still be triggered. The best way to stay safe from the potential consequences is to limit your exposure to large avalanche paths. Stay out of start zones and complex terrain in large avalanche paths.
Snowpack Discussion
Light snow accumulations on Saturday and Sunday followed a warm sunny stretch at the end of the work-week. Many sunny slopes have very hard crusts in the top foot of the snowpack. Most areas picked up 4-6 inches of new snow accumulated Wednesday, which quickly melted on sunny slopes. High elevation, northerly slopes are staying soft and dry.
Below this most recent crust a variety of snowpack structures have been reported. Most observations indicate strong layers of dry snow intermixed with various melt freeze crusts. In isolated locations, thin buried persistent weak layers have been reported. You are most likely to find a buried persistent weak layer in shaded locations, at higher elevations, and/or in terrain slightly east of the Cascade Crest.
A more widespread persistent layer continues to be found 3-6 feet below the snow surface. A thin layer of weak sugary facets have been observed just above a stout crust (2/8). While some Compression Test and Extended Column Tests may show results on this deep weak layer, many will not. Deep Tap tests and Propagation Saw tests will yield the most reliable results. While snowpack tests may demonstrate the presence of a weak layer, they are difficult to interpret and are not a good tool for determining whether or not a slope is safe.
Observations
Stevens
NWAC forecaster Dallas Glass was in the Stevens backcountry Wednesday. Dallas observed 6 inches of new snow bonding well to a semi-supportable and strengthening crust. He observed the 2/8 layer down 4 feet in most locations.
Snoqualmie
NWAC professional observer Jeremy Allyn traveled in the Alpental Valley Saturday. Jeremy found unsupportive snow on steep sunny slopes by afternoon. He observed many loose wet avalanches up to D2 from the past few days, as well as glide cracks opening on low elevations rocky slopes.
South
NWAC Forecaster Dallas Glass was in the Paradise area of Mt Rainier on Saturday. Dallas found a very hard, frozen crust about 1 foot thick on sunny slopes. He also reported sudden test on the 2/13 facets about 4 feet below the surface.
NWAC professional observer Jeremy Allyn traveled in the Crystal backcountry Thursday. Jeremy found 6 inches of new dry snow moderately bonded to a newly forming crust. He observed limited new loose surface snow concerns on steep sunny slopes. The 2/8 layer was found down 4 feet.
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: East, South East, South, South West, West.
Elevations: All elevations.
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 18th, 2018 11:12AM
