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Avalanche Forecast

Archived

Feb 7th, 2020–Feb 8th, 2020

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

Little Yoho.

With incoming snow and variable winds that are expected to increase through Sunday, it will be important to make conservative terrain choices this weekend. The possibility of avalanches on the deep persistent layer should be at front of mind. 

Weather Forecast

A low pressure system will cross the region Friday night bringing moderate SW winds to the alpine, 5 to 10 cm of snow and a slight up-tick in alpine temperatures to -12C. The temperatures will drop back to -15C values in the alpine early Saturday as flurries diminish midday. Expect winds to shift east in the AM before diminishing in the PM.

Snowpack Summary

Up to10cm of snow Wednesday gives 40-80cm since February. Wind slabs exist in the alpine. A rain crust up to1900m is now buried by 10-20cm of snow. The Dec 31 layer of facets, surface hoar and sun crust is down 50-100cm. The deep persistent basal layer is ~30cm above the ground and is more developed in shallow snowpack areas.

Avalanche Summary

Skier triggered sz 2.5 in the Lipalian 3 in the Lake Louise backcountry Thursday, no injuries

Explosive work at local ski areas has seen the deep persistent layer fail, up to sz 2, to ground under light loads (1kg explosive = human load).

Last weekend's natural avalanche cycle saw large avalanches up to size 3.5 running to valley bottoms.

Confidence

Intensity of incoming weather systems is uncertain on Friday

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.

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.