Avalanche Forecast

Issued: Mar 10th, 2012 10:01AM

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

pgoddard, Avalanche Canada

Sea To Sky.

Summary

Confidence

Fair - Timing, track, or intensity of incoming weather is uncertain

Weather Forecast

Sunday: 10-15cm overnight and a further 10-15cm through the day. Freezing level around 600m. Strong south-westerly winds.Monday: 10-20cm snow, mainly in the afternoon/evening. Strong to gale southerly winds. Freezing level near 600m.Tuesday: Light snow. Light south-westerly winds. Freezing level near 600m.

Avalanche Summary

There have been at least two close calls involving snowmobile-triggered avalanches in the region over the last couple of days, one of which was triggered from 200m below the fracture line. A size 2 natural avalanche was reported from Garibaldi Park at 1800m on a north aspect on Friday. There was an avalanche fatality at Grizzly Lake on Powder Mountain on Tuesday. The avalanche occurred on an east aspect near 2000m. The slab was 80cm-150cm thick and 525m wide and failed on the mid-February persistent weakness. More reports of last weekend's widespread large natural avalanche cycle are coming in with observations of size 4.5 avalanches. Some of the larger slabs propagated 2-3km along ridgelines and stepped down to the mid-February persistent weakness, and there's no reason why similar avalanches can't happen with this next round of loading.

Snowpack Summary

New snow, accompanied with strong southerly winds, is setting up new storm slabs and wind slabs overlying a variable previous snow surface comprising crusts, moist snow and old wind slabs. Previous storm snow amounts were in the region of 50+ cm and this snow bonded poorly to the older surfaces below. The instabilities associated with both these storm slabs as well as deeper weak layers buried within the snowpack have given us a situation where avalanches could initiate in the upper storm snow and easily propagate to lower weaknesses in the snowpack. This dramatically increases the potential size of avalanche occurrences, which is pushing up into the size 3-4 category. There is also the possibility for avalanches to be remote-triggered, triggered mid-slope, and/or propagate into low-angled terrain.

Problems

Wind Slabs

Strong winds continue to create wind slabs below ridges, behind terrain features and in gullies. These are becoming deeper and heavier by the day, and are expected to cause large natural avalanches.

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

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 6

Storm Slabs

Storm slab problems are likely to get worse as the storm progresses. Touchy weaknesses within and under the recent storm snow have been reported. Cohesionless low-density snow overlying a crust can also produce very large loose-snow avalanches.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely - Very Likely

Expected Size

2 - 6

Persistent Slabs

Heavy loading is expected to reawaken a persistent weakness in the upper snowpack. The potential for remote triggering, step down avalanches, and wide propagations makes this persistent slab problem particularly tricky to manage.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

4 - 8

Valid until: Mar 11th, 2012 9:00AM