Avalanche Forecast

Issued: Dec 16th, 2016 3:19PM

The alpine rating is considerable, the treeline rating is moderate, and the below treeline rating is low. Known problems include Wind Slabs and Persistent Slabs.

rbuhler, Avalanche Canada

Northwest Coastal.

The avalanche danger will increase over the weekend with the progression of the incoming storms systems. Wind slabs are the primary concern for Saturday and a more widespread storm slab problem may develop by Sunday.

Summary

Confidence

Moderate - Timing, track, or intensity of incoming weather system is uncertain

Weather Forecast

The ridge of high pressure that has been producing cold, dry conditions is breaking down and allowing active weather systems into the region starting Saturday. Around 5 cm of snowfall is expected on Saturday with strong northwest winds and treeline temperatures around -10C. Heavy snowfall is forecast to start Saturday night and up to 40 cm is possible by Sunday afternoon. Alpine winds should remain strong on Sunday but are forecast to shift to the southwest. Freezing levels are forecast to rise to around 800 m on Sunday. Another period of heavy snowfall with strong winds is currently forecast for Sunday overnight and into Monday.

Avalanche Summary

No recent avalanches have been reported in this region. Ongoing strong winds have formed hard wind slabs in exposed terrain which may still be reactive to human triggering in isolated areas. Touchy new winds slabs are expected to form on Saturday with the forecast for light snowfall and strong alpine winds. As the storm ramps up Saturday night and Sunday, a widespread storm slab problem is expected to develop. The surface hoar layer from mid-November may still be reactive in isolated areas and is creating a low probability, high consequence problem. As new load accumulates over the weekend, this layer is expected to increase in reactivity where is still exists.

Snowpack Summary

Clear skies and strong outflow winds have created a variety of snow surfaces including scouring and hard wind slabs in exposed areas. Faceting of the upper snowpack and surface hoar up to 12 mm has in reported in sheltered areas. The new snow is expected to bond poorly to this interface. A layer of surface hoar which was buried mid-November is down 60-120 cm in many parts of the region. This layer has been dormant recently but may increase in reactivity as the storm adds new load to the snowpack. Treeline snow depths are around 140-200 cm in the Terrace and Stewart areas, but substantially less further north. Concerns in the north are basal weaknesses that may exist in the shallower snowpack areas especially on smooth alpine features like glaciers.

Problems

Wind Slabs

Recent strong outflow winds have formed hard wind slabs. On Saturday, strong winds from the opposite direction with new snowfall will form new wind slabs. Use caution on all aspects in wind affected terrain.

Use ridges or ribs to avoid pockets of wind loaded snow.Recent wind has varied in direction so watch for wind slabs on all aspects.Avoid slopes where the snow feels stiff or slabby.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 2

Persistent Slabs

A persistent weak layer of surface hoar is buried 60-120 cm deep throughout the region, and may be reactive to human triggers in isolated locations. Watch for increasing reactivity of this layer as the storm progresses over the weekend.

Dig down to find and test weak layers before committing to a line.Be aware of the potential for large avalanches due to the presence of buried surface hoar.Use conservative route selection, choose moderate angled and supported terrain with low consequence.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

2 - 3

Valid until: Dec 17th, 2016 2:00PM