Avalanche Forecast

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.

A slight density inversion was noted on December 18th, which contributed to shallow storm slab instability. This problem will likely begin to heal during the day. Hand shear tests can work well to test storm snow instabilities. Avoid steep slopes and convex rolls if noticing that these "pop" easily, or other signs of instability.

A bump in wind speeds and a slight cooling trend as snowfall tapers tonight could create wind slabs in exposed areas near and above treeline. Avoid slopes with recently wind driven snow such as gullies and fresh cornices. 

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. In the past few days, observers have found that the December 9th interface has begun to round, and gain strength. This does not mean that we are out of the woods yet. 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 from the surface to be very reactive. Even though we don’t have signs of recent deep avalanches in this area, this weak interface is still there. The recent near miss at Crystal Mountain highlights this. 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.