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Avalanche Forecast

Archived

Dec 19th, 2019–Dec 20th, 2019

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

Regions

Sea To Sky.

Another 20 to 40 cm of snow and strong wind is forecast for Thursday night into Friday. This will continue to load a buried weak layer and may trigger a natural avalanche cycle. Best to avoid avalanche terrain on Friday and give the snowpack time to stabilize.

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

THURSDAY NIGHT: Cloudy with heavy snowfall, accumulation 20 to 40 cm, strong southwest wind, alpine temperature -4 C, freezing level 1200 m.

FRIDAY: Cloudy with snowfall, accumulation 5 to 10 cm, moderate to strong southwest wind, alpine temperature -5 C, freezing level 1100 m.

SATURDAY: Cloudy with snowfall, accumulation 5 to 10 cm, moderate southwest wind, alpine temperature -6 C, freezing level 900 m.

SUNDAY: Cloudy with snowfall, accumulation 10 to 15 cm, light south wind, alpine temperature -6 C, freezing level 900 m.

Avalanche Summary

Numerous large slab avalanches were triggered naturally and by explosives on Thursday. The avalanches failed in the recent storm snow and some of them stepped down to the mid-November weak layer described in the Snowpack Summary.

A natural avalanche cycle may occur Thursday night during the brunt of the storm. Both natural and human-triggered avalanches are expected to be very likely to occur on Friday, failing both in the storm snow as well as the mid-November weak layer.

Snowpack Summary

Another 20 to 40 cm of snow is expected to fall Thursday night, which will add to the 20 to 30 cm that fell on Wednesday and Thursday. 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. A natural avalanche cycle may occur Thursday night as this weak layer gets loaded. Skier triggering will be very likely to occur on Friday.

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 will be imperative to manage your risk until the weak snow gains strength. Read this informative Mountain Conditions Report for more information.

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.

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.