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Avalanche Forecast

Archived

Dec 16th, 2018–Dec 17th, 2018

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

Regions

South Rockies.

Triggering large avalanches remains possible, especially on wind loaded slopes.

Confidence

Moderate -

Weather Forecast

SUNDAY NIGHT: A weak front passes overnight bringing 3-10 cm of snow, strong to extreme wind from the southwest, alpine temperatures around -2 C.MONDAY: Light flurries, strong wind from the southwest, freezing level rising to 1700 m, alpine high temperatures around -5 C.TUESDAY: 5-20 cm of snow, extreme wind from the southwest, freezing level around 1700 m.WEDNESDAY: Scattered flurries with 4-8 cm of snow, strong wind from the southwest, freezing level around 1500 m.

Avalanche Summary

Explosive control produced numerous large (size 2-3) wind slab and deep persistent slab avalanches on Friday and Saturday, primarily on north and east aspects above 2000 m.Natural activity was also reported on Friday, primarily size 1-2 wind slab avalanches on east-facing lee terrain in the alpine. Some appear to have stepped down to the deep persistent weak layer near the bottom of the snowpack. See some photos of these from north of Crowsnest Pass. On Thursday, the storm snow was also very reactive to skier and snowmobilie traffic, producing widespread cracking, whumpfing, and some small avalanches.

Snowpack Summary

Strong winds have affected all open terrain, scouring windward slopes and forming wind deposits on lee slopes. Currently only 30-120 cm of snow can be found in alpine areas and much less at lower elevations.Roughly 20-30 cm of recent snow has buried old wind slabs and sun crusts. The lower snowpack has a weak structure composed of facets and crust that has been reactive to heavy triggers. Terrain features like smooth alpine bowls with variable snowpack depths are suspect for human triggering.

Problems

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.

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.