Avalanche Forecast

Archived

Nov 28th, 2021–Nov 29th, 2021

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.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Below Threshold.

Regions

Yukon.

Current conditions are challenging and require slope by slope evaluation. If in doubt, choose simple terrain.

Confidence

Low - Uncertainty is due to the limited number of field observations.

Weather Forecast

The snow will taper off as outflow winds push drier and colder air across White Pass as the Arctic front finally moves further south. Flurries will be on and off until the next significant snowfall late Tuesday.

SUNDAY NIGHT: Snow 5-10 cm / Strong, southwesterly winds / Low of -12

MONDAY: Cloudy with flurries, 3-5 cm / Strong, southerly winds / High of -10

TUESDAY: Cloudy with flurries; 3-5 cm / Moderate, southerly winds / High of -10

WEDNESDAY: Snow 10-15 cm / Strong, southwesterly winds / High of -8

Avalanche Summary

Few natural avalanches (size 1) were observed on cross-loaded features at treeline on Saturday near Powder Valley after the recent strong winds formed reactive slabs. Deeper in the snowpack, a weak layer combining facets and crusts has also produced isolated, but surprisingly large avalanches.

If you go out in the mountains, please post your observations and/or photos to the Mountain Information Network!

With uncertainty about the snowpack structure, it may also be possible to trigger large avalanches from thin areas on big alpine slopes. Until there is more information about the snowpack, an extra-cautious approach and careful snowpack evaluation is recommended, especially in high-consequence terrain.

Snowpack Summary

30+ cm of recent storm snow and strong southwesterly winds have formed thick wind slabs reactive to human triggers.

A persistent weak layer of facets over a crust near the bottom of the snowpack has been reported as being reactive to human triggers near Fraser. Read about it in the Mountain Information network HERE.

Terrain and Travel

Avoid freshly wind loaded terrain features.
Use ridges or ribs to avoid areas of wind loaded snow.
Be aware of the potential for large avalanches due to the presence of a persistent slab.
Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
Avoid shallow snowpack areas, rock outcroppings and steep convex terrain where triggering is most likely.

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.

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.