Avalanche Forecast

You can trigger avalanches where about a foot of fresh snow accumulates. These avalanches will generally be soft and confined to steep slopes. Moderate to strong winds have created stiffer slabs at upper elevations and in wind-exposed terrain that could break more widely. Use extra caution on slopes over 35 degrees and avoid deeper drifts of snow near and above treeline. Some drifts may get covered up and disguised by fresh snow without wind. Use small, inconsequential test slopes to check and see how the new snow is bonding. A general cooling trend will help these bond, but with continued snowfall this will take time. 

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.

On the eastern edge of the zone, very large avalanches remain possible within an old, faceted layer near the ground. Deep persistent slabs are very difficult to predict. You may not get direct signs of instability with this layer that is likely down from 3 to 5 feet from the surface. What we do know is that after a significant loading event such as this one, deep persistent slabs may become easier to trigger. The snowpack needs time to adjust to the new load.

Reduce the risk of this low likelihood, high consequence situation by avoiding particular types of steep slopes. Things to look out for are wind stiffened slabs, and where they may overly shallow, rocky areas near and above treeline. Avoid steep, unsupported slopes with recent wind loading. Don’t underestimate how far and wide a slab failing in old snow could run when identifying safer areas to stop and regroup. Be intentional about putting a significant distance in between yourself, and where avalanches start, run, and stop.

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.