Avalanche Forecast

Issued: Dec 4th, 2012 9:02AM

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

ccampbell, Avalanche Canada

Sea To Sky.

Summary

Confidence

Fair - Due to limited field observations for the entire period

Weather Forecast

Wednesday: Moderate snowfall in the morning and afternoon 5-10cm of accumulation for the day, freezing levels around 700m, and strong southwesterly mountaintop winds. Thursday: Moderate snowfall with 10-20cm of accumulation, freezing levels around 700m, and strong southwesterly winds becoming moderate westerlies. Friday: Light snowfall with a couple of centimeters of accumulation, freezing levels around 600m and moderate westerly winds.

Avalanche Summary

Reports from the Whistler area on Monday include numerous explosive-triggered and ski cut 20-100cm deep Size 1.0-2.0 storm slab and wind slab avalanches on northwest through east facing treeline and alpine slopes. One 50-150cm deep Size 2 slab avalanche may have stepped-down to the early November deep persistent weakness.

Snowpack Summary

Treeline and alpine areas have received well over a metre of new snow since late last week with weaknesses lingering within and under this storm snow. Of particular note was preserved 8mm surface hoar found on a sheltered open slope just below treeline, which gave moderate but energetic sudden collapse results in repeated compression tests. The mid-pack seems to be fairly settled, strong, and possibly bridging instabilities that may exist deeper. However, snowpack tests continue to produce occasional hard but sudden results on the early November facet/crust combination down as deep as 200cm on leeward slopes. Furthermore, favorable slab structure for step-down fractures can contribute to the persistence of this deep weakness.

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.>Give cornices a wide berth when travelling on or below ridges.>Use ridges or ribs to avoid pockets of wind loaded snow.>

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

Elevations: Alpine, Treeline.

Likelihood

Likely - Very Likely

Expected Size

2 - 5

Storm Slabs

Weaknesses within and under the recent storm snow are susceptible to human triggers, particularly in sheltered areas treeline and below, 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, particularly on slopes with smooth ground cover.

Be aware of thin areas that may propagate to deeper instabilites.>Avoid convexities or areas with a thin or variable snowpack.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

2 - 6

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