Avalanche Forecast

Issued: Dec 22nd, 2016 4:29PM

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.

Avalanche Canada jfloyer, Avalanche Canada

Significant amounts of new snow with extreme winds have set up touchy hard wind slabs in many areas that will stay reactive to human triggering for a while. There is a possibility of avalanches stepping down to lower layers in isolated spots.

Summary

Confidence

Moderate - Wind effect is extremely variable

Weather Forecast

Friday: Dry and sunny. Light northeasterly winds. Alpine temperatures around -10C. Saturday: Dry and sunny. Light easterly winds. Alpine temperatures around -13C. Sunday: Dry and cloudy. Winds starting to increase during the day reaching 50 km/h from the south at ridgetop by the afternoon. Temperatures around -13C.

Avalanche Summary

We're starting to get a bit of a picture that there was a reasonable natural avalanche cycle (as expected) up to size 3 that occurred during and immediately after the storm that ended on Wednesday. West of Terrace there was evidence of avalanches running in the storm snow up to size 3 on both north and south facing slopes. Storm slab avalanches up to size 2 had previously been reported on Sunday through Tuesday. I would anticipate wind slabs will remain touchy for a few more days, especially where they have formed into hard slabs in the alpine.

Snowpack Summary

Up to 125 cm new snow was recorded in this region between 17 and 21 December. Winds were often extreme, and fresh hard wind slabs are reported to be widespread. All this new snow sits over a highly variable interface comprising of hard wind slabs in wind exposed terrain, widespread faceting of the upper snowpack, and 5-10 mm surface hoar in sheltered areas. The new snow was expected to bond poorly to this interface, but it's been notable how few avalanches have been reported, and the ones that have have been on the smaller side (typically size 2). Did the outflow winds scour the surface of any surface hoar before the new snow arrived? Has the upper snowpack remained soft preventing large slab avalanches? Is nobody out there looking because the storm has been so severe? In areas with less snow, there are two additional layers that should be on your radar. The first is a layer of surface hoar which was buried 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

An icon showing Wind Slabs
Large amounts of recent snow coupled with extreme winds have built reactive slabs behind many exposed terrain features. Allow several days of calmer weather for these to stabilize.
Good day to make conservative terrain choices.The new snow buries a weak layer which may increase the reactivity of new slabs.The new snow may require several days to settle and stabilize.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 3

Persistent Slabs

An icon showing Persistent Slabs
A layer of surface hoar down 80-120 cm and/or a weak crust layer near the bottom of the snowpack may wake-up as a result of storm loading. Storm slab avalanches may step down to these layers resulting in very large and destructive avalanches.
Storm slab avalanches may step down to deeper layers resulting in very large avalanches.Avoid exposure to overhead avalanche terrain, large avalanches may reach the end of run out zones.Be aware of the potential for large avalanches due to the presence of buried surface hoar.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

2 - 4

Valid until: Dec 23rd, 2016 2:00PM