Avalanche Forecast

Archived

Dec 21st, 2021–Dec 22nd, 2021

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Below Threshold.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Below Threshold.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Below Threshold.

Regions

Yukon.

New snow and wind have formed fresh windslabs. Avoid wind loaded terrain which will appear deep and styrofoam-like. Small avalanches in motion may be able to trigger large avalanches. Limit exposure to overhead avalanche terrain.

Confidence

Moderate - Uncertainty is due to the extreme variability of wind effect on the snowpack.

Weather Forecast

Winds increasing overnight and remaining strong from the north accompanied by cold temperatures.

TUESDAY NIGHT: Overnight low temperature around -10, moderate to strong north/ northeast wind. No new snow expected.

WEDNESDAY: Mostly clear, daytime high temperature around -12, strong north/northwest wind, no snow expected.

THURSDAY: Broken cloud cover, daytime high temperature around -21, moderate to strong north/northwest wind, possible 2 cm of snow.

FRIDAY: Broken cloud cover, daytime high temperature around -27, moderate north wind, no snow expected.

Avalanche Summary

We expect a natural avalanche cycle is happening, and we will be on the lookout for evidence of this during the week ahead.

This MIN from Saturday shows a small wind slab in a terrain feature known for producing these kinds of avalanches. We expect more of them are happening during this recent storm.

On December 1, a size 2.5 avalanche was remotely triggered by a recreational group on Log Cabin. This avalanche likely failed on the weak, sugary snow at the bottom of the snowpack.

Snowpack Summary

10cm of new snow is now being blown around by strong north winds. All of it will come to rest on a variety of old snow surfaces. Like wind pressed sastrugi in the alpine, and some low density snow in sheltered treed areas.

On the weekend two MIN reports here & here talked about some surface hoar at and below 1300m and we'll be looking for it over the next few days.

The middle snowpack is supportive to travel but snow depth still varies greatly with aspect and elevation.

The lower snowpack is made of weak sugary snow. An avalanche at this interface is most likely to be triggered in shallow areas or with a large impact like a cornice fall or surface avalanche.

Terrain and Travel

Wind slabs are most reactive during their formation.
Be aware of highly variable recent wind loading patterns.
If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.