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. 

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.

These are dangerous times in the mountains. The most recently buried layer of facets over a crust is the main concern in the area. This ugly setup can be found from 18" to 26" down, depending where you look. This structure may be most dangerous in above 5,000 where the new snow is becoming more of a slab. As time goes on, the fluffy storm snow will continue to pile up, and it will continue to become a more of a cohesive slab. It is only a matter of time before the weight of this slab overcomes the strength of the weak layers deeper in the snowpack. If you can find layers of facets of surface hoar, all you need is a slab on top (which we have in most places with the recent storm), and you’ve got a recipe for dangerous avalanches. Travel techniques to stay safe in the mountains require skill in navigation to find slopes less than 30 degrees, and well away from the runout of steeper slopes above. 

Deeper persistent slab avalanches are becoming possible in the Wenatchee and Entiat Mountains, where they could break deep, or all the way to the ground. Watch for cracking, listen for collapses, or dig down to look for these layers of concern.

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.