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Avalanche Forecast

Archived

Dec 23rd, 2018–Dec 24th, 2018

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Snoqualmie Pass.

Hello soft snow! Recent snow from Sunday left unstable storm slabs in the Snoqualmie area, however, they aren’t everywhere. You are most likely to trigger an avalanche as you go up in elevation, into areas affected by the wind, or on convex rollovers.

Discussion

Avalanche Summary

We were able to observe several small natural and human triggered storm slabs Sunday. Most of these occurred in very steep terrain, on convex rollovers, or near cliffs. That’s no surprise, since Snoqualmie received 8-10 inches of snow, and counting, as of Sunday afternoon. What did catch our eye was how widely these avalanches propagated.

Snowpack Discussion

New Regional Synopsis Coming December 24. 

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). 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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.