Avalanche Forecast

Issued: Mar 19th, 2018 11:00AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is low. Known problems include Deep Persistent Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

While avalanche conditions are slowly easing, Low Danger does not mean no danger. Deep Persistent Slab avalanches are very difficult to trigger, but 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

While Deep Persistent Slabs are very difficult to trigger, the resulting avalanche could leave little chance of survival. Persistent weak layers have been the cause of avalanche fatalities or serious injuries for the past month. Recent test results and avalanches show that these layers are still present, though getting harder to trigger. The best way to stay safe is to limit your exposure to large avalanche paths. Stay out of start large zones and complex avalanche paths.

You may see roller balls and small Loose Wet avalanches running on steep sunny slopes near rocks and trees. Until solar exposed slopes get softened by the sun they will remain hard and slick. It may be difficult to stop a fall on these slopes while they are still firm. Avoid traveling near or under areas with glide cracks and stay away from large cornices.

Snowpack Discussion

Expect warming temperatures and sun for Tuesday. While there may be some roller balls or small point releases in areas that received a couple inches or more of recent snow, Loose Wet avalanches should not pose significant hazard. A dusting of light snow on Saturday and Sunday followed a warm, sunny stretch at the end of the work-week. Many sunny slopes have very hard stout crusts in the top foot of the snowpack. Northerly, shaded slopes at pass elevations and above are staying soft and dry.

Immediately below this most recent crust you may find unfrozen wet grains. Most observations indicate strong layers of dry snow intermixed with various melt freeze crusts throughout the upper snowpack. 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 are good for demonstrating the presence of a weak layer, they are not a decision-making tool to determining whether or not a slope is safe.

Observations

Stevens

NWAC forecaster Josh Hirshberg reported up to 4 inches of new snow in the Stevens backcountry on Monday. On sun-exposed slopes, he observed melt forms in the top 2 feet of the snowpack. The 2/13 layer (about 3 feet below the surface) had received liquid water and transitioned to melt forms, as well.

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.  

On Saturday, observers reported a large wet natural avalanche that stepped into older layers on a SW aspect at 4500ft near Red Mountain.

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

Deep Persistent Slabs

An icon showing 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

Valid until: Mar 20th, 2018 11:00AM