Avalanche Forecast

Issued: Dec 29th, 2017 4:15PM

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

ghelgeson, Avalanche Canada

Kootenay Boundary.

It's getting trickier, new snow and wind Friday night will likely form sensitive storm slabs and could push the persistent slab problem to natural failure. With all this uncertainty its best to stay conservative with terrain selection this weekend.

Summary

Confidence

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

Weather Forecast

One last blast of Pacific moisture is set to clash with the Arctic air mass over the province as a low pressure system moves towards the Lower Mainland late Friday. This storm is expected to deliver significant snow and wind all of Friday night, exiting to the east Saturday morning. Look for cold dry weather on Sunday and Monday.FRIDAY NIGHT: Freezing level at valley bottom, 5 to 25 cm of snow possible, moderate to strong southwest wind through the night.SATURDAY: Skies clearing throughout the day, freezing level at valley bottom, moderate to strong northwest wind, 1 to 10 cm of snow possible.SUNDAY: Clear skies, freezing level at valley bottom, light variable wind, no snow expected.MONDAY: Clear skies, freezing level at valley bottom, light variable wind, no snow expected.

Avalanche Summary

On Thursday control work produced storm slabs on southeast through southwest facing slopes up to size 2.5. Loose dry avalanches to size 2.5 were also reported. On Tuesday a skier triggered a size 1.5 wind slab on a north-facing alpine slope in Kokanee Glacier Provincial Park. The skier went for a short ride, but was uninjured in the event.

Snowpack Summary

Thursday's storm produced 10 to 25 cm of low density snow which was accompanied by light to moderate wind out of south, southwest and even a bit of of southeast. The new snow overlies recently buried surface hoar, although at this point not much is known about the size or distribution of this potential weak layer.Between 25 and 80 cm below the surface you'll find the December 15th interface which consists of a melt-freeze crust on steep, solar, higher elevation slopes and well-developed surface hoar which seems most pronounced in sheltered terrain at and below treeline. The bond at this variable interface is of critical importance, especially in areas where the overlying slab is deep and well-consolidated.The late November crust is now buried 60 to 100 cm below the surface and has been producing hard, resistant results in recent snowpack tests. Beneath this crust the lower snowpack is strong and well-settled.

Problems

Storm Slabs

Continued snowfall and strong wind Friday night and Saturday morning will continue to form storm slabs that will be most problematic in wind exposed terrain. If the snow feels hollow or stiff, storm slabs may be larger than expected.

Be careful with wind loaded pockets, especially near ridge crests and roll-overs.Be alert to conditions that change with elevation and wind exposure.Good day to choose conservative lines and carefully watch for clues of instability.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 2

Persistent Slabs

A well-developed layer of surface hoar is buried up to 75 cm below the surface. The new storm snow may promote increased settlement and cohesion of the overlying slab which would make this interface more sensitive to human triggering.

Avoid steep slopes and convexities at and below treeline where buried surface hoar may be preserved.Use conservative route selection, choose moderate angled and supported terrain with low consequence.Storm slab avalanches in motion could step down and trigger larger persistent slab avalanches.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

2 - 3

Valid until: Dec 30th, 2017 2:00PM