Avalanche Forecast

Explosive triggered slide (12/18/2018)

We are entering dangerous times because many slopes could be close to failure, and all that is needed is a trigger. Avalanches may be very wide and will certainly be life-threatening.

In recent days, there have been reports of skier triggered, explosive triggered, and remotely triggered avalanches on Mission Ridge on a layer of buried facets above a late November crust near the ground. These were on Northwest to East aspects, near and above treeline. With the recent accumulation and wind, we have likely added a slab to areas where previously the upper snowpack wasn’t cohesive enough to communicate a fracture with the weak layer. Expect slides to fail near the ground and may propagate long distances. Stick to slopes of less than 30 degrees. Avoid terrain where slides can start, run, and stop, especially if you notice any whumpfing, shooting cracks, or recent deep avalanches.  

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.

Explosive triggered slab that stepped down. (12/18/2018)

Any shallow storm or wind slab has the potential to step down into deeper weak layers near the ground, turning a shallow slab avalanche into a much wider, life-threatening one. Look for signs of instability like shooting cracks within the new snow, and recent slab avalanches. A bump in wind speeds and a slight cooling trend as snowfall tapers could create wind slabs in exposed areas near and above treeline. Avoid slopes with recently wind driven snow such as cross-loaded gullies. Give cornices a healthy amount of respect if nearing them on ridgelines.  

At lower elevations, the rain is percolating through the snowpack. Be leery of saturated snow, as any wet loose slides could turn into wet slabs. Stay off of steep, saturated slopes if they haven't refrozen yet.

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.

Rain at lower elevations may trigger loose-wet avalanches. Even small loose-wet avalanches can be dangerous, especially if they carry you into a terrain trap, such as a creek bed, over cliffs or into trees.

Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.

Several loose wet avalanches, and lots of pinwheels and roller balls.

Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.