Avalanche Forecast

Issued: Dec 4th, 2012 9:04AM

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

ccampbell, Avalanche Canada

South Columbia.

Summary

Confidence

Fair - Due to limited field observations for the entire period

Weather Forecast

Wednesday: Moderate snowfall in the morning decreasing throughout the day with 5-10cm of accumulation, freezing levels around 1000m but cooling rapidly throughout the day, strong to extreme westerly winds easing throughout the day. Thursday and Friday: Light to moderate snowfall with 5-10cm of accumulation each day, freezing levels at valley bottoms and generally light southwesterly winds.

Avalanche Summary

Several 60-70cm deep storm slab avalanches up to Size 2.0 were ski cut on north facing unsupported treeline rolls on Monday. These all ran on surface hoar buried late last week. In the alpine, explosive controlled cornices triggered several 20-40cm thick Size 1.0-2.0 soft wind slab avalanches on west through northeast aspects, and many natural avalanches were heard running in very steep high alpine areas on north aspects.

Snowpack Summary

Another 20cm of snow in the Selkirks and Eastern Monashees brings the total snowpack depth to as much as 270cm at treeline. 3-10mm surface hoar buried late last week can be found down 50-80cm and has been reactive to light triggers on unsupported features at treeline elevations. The well-settled and strong mid-pack may be bridging deeper weaknesses; however, recent snowpack tests produced occasional hard but sudden results on the early November facet/crust combination down 100-190cm in treeline and alpine areas. Furthermore, favorable slab structure for step-down fractures can contribute to the persistence of this deep weakness. Meanwhile in the Okanagan, relatively warm temperatures are promoting a well consolidated 70-80cm treeline snowpack with isolated small thin wind slabs.

Problems

Wind Slabs

Fresh wind slabs can be found well below ridge crests, behind terrain features and in cross-loaded gullies in any wind-exposed terrain. Expect these deep drifts to be very touchy.

Avoid freshly wind loaded features.>Use ridges or ribs to avoid pockets of wind loaded snow.>

Aspects: North, North East, East, South East.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

2 - 5

Storm Slabs

Weaknesses within and under the recent storm snow are susceptible to human triggers, primarily on unsupported convex treeline roll-overs, and could easily produce avalanches large enough to bury a person.

Avoid open slopes and convex rolls at and below treeline where buried surface hoar may be preserved.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

2 - 5

Persistent Slabs

An old facet/crust combination deep in the snowpack may wake up with heavy triggers, smaller avalanches stepping down, or triggering from thin-spots. Although unlikely, deep persistent slab avalanches can be massive.

Be aware of thin areas that may propagate to deeper instabilites.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

3 - 6

Valid until: Dec 5th, 2012 2:00PM