Avalanche Forecast

Archived

Dec 22nd, 2019–Dec 23rd, 2019

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely.

Regions

Sea To Sky.

Clear skies may lure you into bigger terrain, but this is not the time to expose yourself. The recent snow is resting on a weak layer ready to fail with the weight of a human. Read more here. Best to avoid alpine avalanche terrain and travel conservatively at treeline.

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength.

Weather Forecast

SUNDAY NIGHT: Clear skies, light northwest wind, alpine temperature -6 C, freezing level below valley bottom.

MONDAY: Clear skies, light southwest wind, alpine temperature -7 C, freezing level below valley bottom.

TUESDAY: Cloudy with light snowfall, accumulation 1 to 3 cm, moderate south wind, alpine temperature -8 C, freezing level below valley bottom.

WEDNESDAY: Cloudy with light snowfall, accumulation 1 to 3 cm, light northeast wind, alpine temperature -7 C, freezing level below valley bottom.

Avalanche Summary

Many large persistent slab avalanches were triggered by explosives on Saturday and Sunday. The avalanches failed on the mid-November weak layer described in the Snowpack Summary. Some of the avalanches were remotely triggered, meaning the weak layer was triggered from one spot and propagated through the snowpack to reach the steeper terrain where the avalanche released.

Human-triggered avalanches are expected to be very likely to occur on Monday at higher elevations. We recommend avoidance of alpine avalanche terrain and very conservative decision-making at treeline, given this weak snowpack.

Snowpack Summary

Around 70 to 120 cm of recent snow fell with very strong southwest wind. All of this snow overlies a weak layer of surface hoar as well as sugary faceted grains and hard melt-freeze crust buried mid-November, which is a recipe for large and destructive avalanches. These weak layers may be found anywhere from about 80 to 150 cm deep.

This weak and touchy snowpack is highly atypical for the region and may persist for some time. Conservative terrain travel along with periods of avoidance of avalanche terrain will be imperative to manage your risk until the weak snow gains strength.

Terrain and Travel

Travel in alpine terrain is not recommended.
Don't let the desire for deep powder pull you into high consequence terrain.
Persistent slabs have potential to pull back to lower angle terrain.
Make conservative terrain choices and avoid overhead hazard.

Problems

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of 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 Slab.

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.