Avalanche Forecast

There are subtle weaknesses in the recent storm snow that are still capable of producing avalanches. Areas such as Mt Rainier and Chinnook Pass, that accumulated more snow Sunday,  will see higher avalanche danger. If you are planning to travel near and above treeline or in areas where the wind as affected the snow, you will still be able to trigger these soft slabs. At lower elevations and in wind protected areas, you are most likely to trigger a storm slab on convex rollovers, in terrain steeper than 40 degrees, or on unsupported slopes. You can use smaller inconsequential test slopes to check how the storm snow is bonding to the old snow as you travel.

When the sun comes out, expect small loose avalanches to occur on steep, rocky, sunny slopes. Don’t let them catch you off guard, especially if you are on slopes that could have dangerous consequences.

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Observations such as the recent natural avalanches in the Crystal backcountry are keeping this layer on our minds. It’s tough to say when layers like this are finally healed. The depth to the layer and recent poor weather have limited observations. As a result, we have a high degree of uncertainty about this avalanche problem.

Here’s what we know. A layer of buried surface hoar and facets is generally found 3-6 feet below the snow surface at elevations above 6000 feet. You would be most likely to trigger a deep avalanche from thin spots in the snowpack, such as around rocks, or if a smaller, new snow avalanche stepped down to the old weak snow. Any resulting avalanche would be very large and destructive. Hopefully this layer is healing, but until we can confirm that, we want you to keep thinking about this low likelihood, but high consequence avalanche problem.

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.