The Bottom Line: Very dangerous conditions have developed in the East Central Cascades. Incremental loading is beginning to stress multiple persistent weak layers to their breaking point. This is the time to step back and avoid recreating in avalanche terrain.
Snow and Avalanche Discussion
The snowpack near and east of Highway 97 in the Wenatchee Mountains may become extremely dangerous this weekend. Old persistent weak layers appear to be groaning from a mere 7†of snow that fell on the 17th, and will continue to get more heavily loaded into Saturday. A recently buried layer of surface hoar and facets may produce widely propagating avalanches throughout Eastern Cascades.
Toward the end of the cold and dry period, shaded aspects harbored surface hoar and facets east of the crest. One observer reports skiing on a ‘glass carpet’ of surface hoar between 4,000 and 5,500ft. Another noted ‘dramatic faceting’ near and below treeline on shaded aspects. The new snow doesn't appear to be bonding very well, especially where it sits over facets and surface hoar. A few natural avalanches occurred overnight on the 18th at Mission Ridge, these ran on a crust from early January. Observers have noted loud whumphs and collapses near Blewett Pass the past few days. As the snow accumulates with warming temperatures and the winds blow into Saturday, we will create a cohesive slab and further stress these layers. More on the recently buried snow surface in this observation here.
Recently buried layer of large surface hoar (1/17). Where a slab sits over this layer, expect dangerous conditions to exist.
Avalanche Problems
Persistent Slabs
The snowpack has become ripe for large, human triggered avalanches. There are multiple persistent weak layers that all have the potential to fracture and avalanche with this incoming storm. These layers are tricky because they’ve sat around for weeks not producing avalanches. Just the past few days they have been incrementally loaded and appear to be ‘waking up’ with large whumphs being reported near Blewett Pass -even as low as 5,000ft. They are likely to be doing the same in other areas on the eastern edge of the range where less than 3 or 4 feet of snow is on the ground. Slides could fail a few feet down, or on the ground, taking the entire seasons snowpack with it. Slides could be very wide and encompass entire terrain features. Don't get lured onto slopes steep enough to avalanche, don't be the trigger. Avoid runout zones below steep slopes, as it may be possible to trigger slides from below as well.
Further west, the main layer of concern will be surface hoar that has been recently buried by new snow in the past few days. Slides may wrap around terrain features and surprise you and your group.
Aspects:
All aspects.
Elevations:
All elevations.
Likelihood: Likely
Expected Size: 2 - 3
Wind Slabs
The winds will rip near ridgelines on Saturday morning. These, combined with a burst of new snow will form dense slabs, increasing the avalanche danger. Anticipate that these slabs won’t bond well with the old, weak snow surface that recently got buried. Look for shooting cracks within the new snow. Does the snow sound hollow? Are there deep dunes of drifted snow? What is underneath that slab, is there strong over weak? If so, it is time to avoid slopes steep enough to avalanche.
