Avalanche Forecast

The new snow will be piling up very quickly with these temperatures, and the winds will easily drift it into slabs. Storm slabs are thickening already this evening and will continue to get deeper and more reactive into the night. Shooting cracks, recent avalanches, and heavy snowfall rates are all red flag signs for avalanches. Avoid entering or going below slopes steeper than 30 degrees. Make sure you don't travel in potential avalanche runout zones, as slides could be triggered from a distance and may travel very far with large powder clouds. 

Loose dry avalanches may be a concern at lower elevations. These may be large by Tuesday. Watch for fan shaped avalanches, and avoid hanging out in places where snow normally sheds off steep slopes from above. 

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.

Persistent slabs are getting deeper and nearing a tipping point at Stevens Pass. A layer of facets rest on top of a crust beneath the weekends storm snow and can now be found from 2 to 3 feet deep. With the heavy storm and snowfall into Tuesday, we believe this layer may begin to fail in a widespread fashion. Will this storm be the one? Signs appear to be hinting in that direction. If you go into the mountains you may hear huge whumphs, and or see deep shooting cracks. Watch out, because slabs may be triggered from a distance. In the few places out of wind effected locations, the low density of the snow may not have enough cohesion to act as a slab, yet.

This new snow will mask the old, hollow slabs that rest above this layer. If you trigger one of these slabs they may be big enough to destroy a large semi-truck.

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.