Avalanche Forecast

The most dangerous slopes will be at upper elevations where thick slabs may have formed from deeper accumulations and wind. Stay off of steep slopes near and above treeline if finding these slabs. The storm snow is sitting on a thin crust buried on December 20th up to 5,500ft, and low density old snow above that. This weak interface was inhibiting the bonding between the storm snow and the old snow surface. Carefully evaluate the new snow depth, it’s cohesiveness, and how it is bonding before choosing to travel in avalanche terrain. Use small, inconsequential slopes and tests to check if the storm snow will slide.

Small loose wet avalanches may become likely in the afternoon on steep, southerly aspects. Limit your time spent on steep slopes during the middle of the day, and if noticing roller balls or recent loose avalanches. 

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.

You may still be able to trigger Deep Persistent Slabs in the eastern portion of the zone near and above treeline. While these avalanches may be difficult to trigger, they are also very difficult to predict. If you hit the wrong place in the snowpack, it could result in a very large and dangerous avalanche. Large triggers are more likely to initiate a deep slab. Steep, unsupported slopes are the most likely terrain where these can occur.

The best way to reduce the risk of this low likelihood, but high consequence situation is to minimize your exposure to slopes capable of producing large avalanches.

-Attempt to put a significant distance in between where you are traveling and where avalanches start, run, and stop.

-Limit the amount of time you spend on or near slopes 35 degrees and steeper.

-Don’t underestimate how far and wide these could run when identifying safer areas to stop and regroup.

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.