Avalanche Forecast

You may be able to trigger storm slab avalanches near the Cascade Crest. Check the interface between the new and old snow and look for any weak layers just below the old snow surface. Gusty winds could form deeper drifts on leeward sides of ridges and terrain features at upper elevations. Use tests and small test slopes to check how easily the snow can slide. Watch for cracking in the snow as a sign of instability. Expect heavy precipitation and rain overnight up to 5,500ft making snow surfaces wet.

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.

While persistent slab avalanches has been unlikely, the coming storm could be enough to activate some weak layers. Weak layers are most prominent on slopes facing toward the north half of the compass, in the Wenatchee Mountains, and where the snowpack is shallower than 3 feet deep. Persistent slab avalanches can be surprising and can break widely across terrain. The best way to deal with this low-likelihood, high consequence scenario is to pick conservative terrain options. Specific features to avoid are where stiff drifted slabs overlie the facets, steep convex rollovers, areas of variable height of snow, shallow and rocky features, and unsupported slopes. 

A layer of facets, formed in late November, can be found near the bottom of the snowpack, often resting on a stout crust and sometimes capped with a thin crust. The layer is typically about 1-3 feet below the surface. It was responsible for numerous large avalanches and signs of instability during the storm and avalanche cycle around December 20th. Other weak layers, such as facets in the lower third of the snowpack and recently buried facets and surface hoar, (found in this observation) could become the source of avalanches with the coming storm.

Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.

The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.

This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.

Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.