Avalanche Forecast

Issued: Dec 24th, 2016 3:23PM

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

shorton, Avalanche Canada

Northwest Coastal.

The incoming storm will build touchy wind slabs and possibly wake up weak layers buried by last week's storm.

Summary

Confidence

Moderate - Due to the number and quality of field observations

Weather Forecast

SUNDAY: Cloudy with afternoon flurries, southwest winds picking up throughout the day reaching 60 km/h by the afternoon and 100 km/h overnight, temperatures around -10C.MONDAY: Stormy with 20-40 cm of new snow, 40-60 km/h southwest winds, temperatures around -5C.TUESDAY: Stormy with 10-20 cm of new snow, 20-40 km/h southwest winds, temperatures around -5C.

Avalanche Summary

Wind slabs continued to be reactive on Friday, as reports included a rider-triggered size 2 soft slab on a north aspect at treeline and numerous explosive-triggered size 1.5-2.5 avalanches on east and northeast aspects. More reports confirm a large natural avalanche cycle producing up to size 3 avalanches occurred during last week's storm. In the northern part of the region where the snowpack is thinner, skiers have remotely triggered several size 1.5-2 deep persistent slab avalanches in steep shallow start zones (failing on weak sugary facets near the ground). With winds picking up on Sunday, expect wind slabs to remain touchy.

Snowpack Summary

Clear cold weather has limited settlement of the recent storm snow. New surface hoar (up to 15 mm) and sun crusts have been reported forming on the snow surface, and intense snow transport has been observed in the alpine. The region received up to 125 cm of new snow between 17 and 21 December. The new snow arrived with extreme winds, which has left widespread hard wind slabs in exposed terrain. All this new snow sits over a variable interface composed of hard wind slabs in wind exposed terrain, weak faceted (sugary) snow, and 5-10 mm feathery surface hoar in sheltered areas. The bond between the newer storm snow and this interface is a concern and could develop into a persistent slab problem. In areas with less snow, there are two additional layers that should be on your radar. The first is a layer of surface hoar that was buried in mid-November and may now be down around a metre in thinner spots. The second is an old rain crust from November that may have developed weak facets, particularly in the north of the region.

Problems

Wind Slabs

Large amounts of recent snow coupled with extreme winds built reactive slabs behind exposed terrain features. Winds slabs exist on all aspects, as recent winds have come from the northeast.

Use caution in lee areas. Recent wind loading has created wind slabs.Be aware of the potential for wide propagations due to the presence of hard windslabs.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

2 - 3

Persistent Slabs

The early December cold snap has left several weak layers composed of surface hoar and facets buried about 1 metre deep. Additional loading from new snow and wind could potentially release large avalanches on these layers.

Avoid exposure to overhead avalanche terrain, large avalanches may reach the end of run out zones.If triggered the wind slabs may step down to deeper layers resulting in large avalanches.Be aware of the potential for large avalanches due to the presence of buried surface hoar.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Unlikely - Possible

Expected Size

2 - 4

Valid until: Dec 25th, 2016 2:00PM