Avalanche Forecast

The new snow could pile up very quickly with these temperatures. All we need to form storm slabs is for the new snow to be ever so slightly different than the previous snow surface. This may or may not happen, but chances are it will be. Look for shooting cracks, recent avalanches on small cutbanks, 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. 

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 that may be a lasting weak layer. Look out for slab structure, such as recently wind loaded areas and check for weak snow underneath the slab. This may be a good time to step it 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, but persistent slab avalanches may be in play there, where more new 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 of surface hoar, all you need is a slab on top (which we may or may not have with the recent storm snow), and you’ve got a recipe for dangerous avalanches. This structure may be most dangerous in the near treeline band, and on west to north to east aspects where weak snow is well preserved, but also on southerly aspects, where facets may be over a thin sun crust.

Be careful especially on steep, convex rolls, in terrain traps such as gullies, and on any recently wind loaded slopes.  

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.