Avalanche Forecast

Give the snowpack time to adjust to the new load. Settlement and southwest winds will allow the new snow to form a more cohesive slab. Use extra caution around any slope steeper than 35 degrees. Avoid drifts and steer around areas where the wind has stiffened the snow on leeward slopes at mid and upper elevations. Wind loaded slopes could produce larger and more dangerous avalanches. Look for shooting cracks, recent avalanches, and use hand shears to check how the new snow is bonding to the underlying surface. Is there strong over weak, or are you seeing shooting cracks? Then there is slab structure. Avoid unsupported convex rolls and recently wind loaded locations.

Loose dry avalanches may be the main concern on steep slopes at low elevations. Watch for fan-shaped avalanches, and avoid hanging out in places where snow normally sheds off steep slopes from above. If the skies clear Wednesday afternoon, expect loose wet activity on steep sun-exposed slopes.

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.

Near Washington Pass, a recently buried layer of facets may be found underneath the new snow on a variety of aspects. If the facets lay over a firm wind board or stout melt freeze crust, it may become a lasting weak layer. Look out for slab structure, especially in recently wind loaded areas, and check for weak snow underneath the slab. This may be a good time to step back and give the snowpack some time to adjust while we collectively discover more about this recently buried layer.

We have less information about areas near, east, and south of Mazama. Persistent slab avalanches may be in play in these areas where recent snow may have fallen on a weaker snowpack. Watch for cracking, listen for collapses, or dig down to look for these layers of concern. If you can find layers of facets or surface hoar, all you need is a slab on top, and you have a recipe for dangerous 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.