Avalanche Forecast

New snow and wind will increase the likelihood of very large avalanches failing on buried weak layers near or above 6000' over the next few days. Reports continue to trickle in of remotely triggered large or very large avalanches over the last few days, releasing roughly 2-4 ft below the snow surface. The most recent remotely triggered avalanche was in Cutthroat Creek on Sat 1/4. Professional observer Mark Allen recorded a crown profile on Sunday and found the slab failed on buried surface hoar 2 ft below the snow surface. In a different drainage, this layer was present on north and south aspects at similar depths around 5900'. 

If you observe whumps and or shooting cracks, these are obvious signs that you can trigger a persistent slab avalanche. They can break surprisingly wide, connecting slope features and can be triggered from much lower angled terrain. Places that may be more likely to avalanche are thin spots in the snowpack, large alpine bowls, and steep convex rollovers. 

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.

West winds have been loading the same aspects over the last few days with more snow falling near the Cascade crest. These same aspects and areas will be continually loaded by a stronger storm producing more snowfall through Monday. As the chances of natural or human triggered avalanches increases, avoid exposing yourself to any terrain capable of producing large avalanches. 

Significant warming looks likely to hold off in the Washington Pass area until Monday night, making loose dry avalanches more likely in less wind affected terrain at mid and lower elevations. Be wary of fast-moving loose dry avalanches in very steep terrain above terrain traps like cliff bands. If snowfall rates keep up, watch for a transition to soft storm slabs by keying into clear signs of instability like shooting cracks or evidence of natural avalanches in similar terrain.

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.