Avalanche Forecast

Persistent Slabs can be difficult to assess and predict. We have a high degree of uncertainty concerning the exact distribution of buried persistent layers in this region. This is a time to stay out of startzone in areas above 4500 feet where you are more likely to find old weak snow unaffected by recent rains. Around Snoqualmie Pass 2-3 feet of settled snow now sits atop a layer of weak sugar facets and/or surface hoar. If you trigger an avalanche on this layer it could be large and deadly.

You may encounter lingering wind and storm snow instabilities from this week. These are gaining strength, but could cause a problem for travelers above 4000 feet. Keep your eyes open and use visual clues such as snow drifts and poorly bonded surface snow to identify and avoid these healing storm slabs.

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.

Below 4000 feet, wet surface snow will linger into Friday. Expect loose wet avalanches on slopes greater than 35 degrees. Don’t underestimate these small avalanches. They can be dangerous if they carry you into terrain traps. Loose wet avalanche danger will decrease as temperatures cool and the snowpack begins to freeze.

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.