Avalanche Forecast

Heavy precipitation will build fresh storm slabs over the next 24 hours. Avalanches are very likely within new snow. Wind-loaded terrain at upper elevations will develop thicker and more reactive slabs. Below 5000ft, a mix of rain and snow will create the potential for wet loose avalanches. Avalanches involving new snow could break widely across terrain features. They may also step down and trigger deeper weak layers in the snowpack. Watch for shooting cracks, collapses, and recent avalanches. Give the storm snow time to stabilize.

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.

New snow will thicken existing slabs resting on a layer of feathery surface hoar that formed around Christmas. This low friction layer has been observed to be sitting on a firm, old snow surface above 5500ft. Very large avalanches are likely above this elevation over the next 24 hours. Be especially careful of wind loaded areas where stiffer slabs rest atop this layer.  These avalanches may act in surprising ways, may initiate on low angle slopes, and may run far distances into valley bottoms. This significant loading event may also wake up deeper persistent weak layers near the ground capable of producing unsurvivable 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.