Avalanche Forecast

Archived

Dec 29th, 2018–Dec 30th, 2018

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

Stevens Pass.

Continued snowfall will keep the avalanche danger elevated for another day. Evaluate the bond of the new snow carefully. Moderate to strong winds have created deep drifts to avoid near and above treeline.

Discussion

Snow and Avalanche Discussion:

On the 29th the temperatures warmed up significantly. Temperatures hit 34 degrees at 5200ft on the pass, and a rain crust likely formed up to at least 6,000ft. A widespread cycle of loose wet avalanches were observed, some were large enough to injure, bury, or kill a person. The cold front came in and the precipitation turned back to snow in the evening. Winds were moderate to strong at upper elevations.

Snowpack Discussion

Coming December 30th

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.