Avalanche Forecast

Archived

Dec 23rd, 2018–Dec 24th, 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 possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

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Sunday’s storm brought new snow and wind forming unstable snow. You are most likely to trigger an avalanche as you go up in elevation, into areas affected by the wind, or on convex rollover. A little bit of patience and some cautious terrain travel will help keep you out of avalanches Monday.

Discussion

Avalanche Summary

We received two very notable avalanche observations yesterday in the West South zone. An avalanche near White Pass demonstrates the potential for new snow avalanches especially at higher elevations and in areas where the wind deposited snow.

In the Crystal backcountry, two large natural avalanches failed on a persistent weak layer. These observations are the latest in a string of avalanches on this layer in the Crystal area. Both of these avalanches acted in surprising ways, propagating widely and, in one case, crossing terrain features.

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.