Avalanche Forecast

A density inversion within the new snow is expected to form as temperatures warm up during the snowfall. Hand shear tests can work well to test storm snow how the new snow bonds to the old snow, and within the storm layer itself. Avoid steep slopes and convex rolls if noticing that these "pop" easily, or other signs of instability such as shooting cracks.

Moderate to strong winds with fresh snow available for transport could create wind slabs in exposed areas near and above treeline. Avoid slopes with recently wind driven snow such as cross loaded gullies and fresh cornices.

Any shallow storm or wind slab has the potential to step down into deeper weak layers, turning a small soft slab avalanche into a deeper, hard slab.

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.

Deep persistent slabs have the potential to wake up if you hit the wrong spot in the snowpack. Observers have noted that the December 9th interface has begun to round and gain strength, especially at lower elevations. However, on December 19th a pro observer performing tests on a Northwest aspect at 5500ft in the Stevens Pass zone found that the 12/9 layer still had the ability to propagate a fracture.

Persistent weak layers are just that, persistent and weak. The snowpack may not speak to you with signs of instability, as they are becoming too far down in the snowpack to be very reactive while walking or riding on top. The recent near miss at Crystal Mountain and pertinent test results highlight that these may still occur. Safe travel through avalanche terrain will require conservative decision making, and the ability to seek out low angle slopes.

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.

As the temperatures warm up the snow surface will begin to get saturated by rain at lower elevations. Be observant for sticky, wet snow, rollerballs, and recent fan shaped avalanches. Don’t underestimate loose wet slides, as they can entrain a large amount of heavy, wet snow on steep slopes.

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.