Avalanche Forecast

The likelihood, size, and distribution of storm slabs will be a very dependant on elevation and how much snow falls at Snoqualmie Pass after temperatures drop below freezing. Soft snow will be sitting on top of a newly freezing crust formed from Saturday’s rain. While you are traveling, you can monitor unstable storm snow with several simple observations. Is there more than 6 inches of new snow? Is the new snow poorly bonded to the underlying crust? Has the wind transported snow into thicker, firmer slabs? Answering “yes” to any of these questions should cause you to suspect storm slabs are present on nearby slopes. You are most likely to trigger an avalanche on open slopes greater than 35 degrees.

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.

It’s been more than a week since we have heard any reports of avalanches on the deep persistent weak layer. That’s a good sign. We think this layer is gaining strength, however we also know that deep persistent layers like this are very difficult to assess and predict. You are most likely to encounter this problem above 5000 ft where a layer of surface hoar and facets are buried 3-6 ft deep. If you plan on traveling into larger avalanche terrain near and above treeline, you should take time to consider and discuss 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.