Avalanche Forecast

Issued: Mar 12th, 2012 10:20AM

The alpine rating is high, the treeline rating is high, and the below treeline rating is considerable. 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

Tuesday: Light to moderate snowfall continues. Moderate south-westerly winds. Freezing level around 500m.Wednesday: The next storm pulse arrives with moderate to heavy snow. Freezing level rising to 1000m. Strong south to south-westerly winds.Thursday: Heavy snow. Freezing level rising to 1500m. Strong south-westerly winds.

Avalanche Summary

It's been an active week for avalanche activity and a natural cycle is expected to continue through the early part of the week. On Sunday, a size 3 step-down slab was triggered by a cornice on a north-west aspect, with a crown depth up to 1.5m. Explosives triggered several size 2 storm slabs on a variety of aspects. Many occurred on wind-loaded north-east to north-west aspects on Saturday. Two skier involvements were reported on north/north-west aspects at 2100m. Earlier in the week, snowmobilers remotely triggered avalanches from 200m below the fracture line. There was an avalanche fatality at Grizzly Lake on Powder Mountain on Tuesday involving a snowmobiler. 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. An unnervingly large natural avalanche cycle occurred last weekend, with slabs up to size 4.5 propagating 2-3km along ridgelines and stepping down to the mid-February layer. Similar avalanches are possible as storm loading continues to overload deeper weaknesses this week.

Snowpack Summary

Stormy conditions are creating ever-deeper wind slabs and storm slabs. Extreme winds are leading to the development of wind slabs unusually low on the slope and perhaps even in openings below treeline. Various old surfaces including crusts, moist snow, old wind slabs and old storm slabs sit below this. A key concern is the potential for a storm- or wind-slab to step down and trigger deeper weaknesses, formed in mid-February, which are still producing clean and fast shears in snowpack tests. Very large avalanches are possible, which could be remote-triggered, triggered mid-slope, and/or propagate into low-angled terrain.

Problems

Wind Slabs

Wind slabs lurk below ridges, behind terrain features and in gullies. These are becoming deeper and heavier by the day. Extreme wind speeds on Monday may have left slabs unusually low on the slope or in openings below treeline.

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

Elevations: Alpine, Treeline.

Likelihood

Very Likely - Certain

Expected Size

1 - 6

Storm Slabs

Storm slabs are building deep with successive weather systems. Touchy weaknesses within and under the recent storm snow have been reported. Cohesionless low-density snow overlying a crust can also produce large loose-snow avalanches.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Very Likely - Certain

Expected Size

1 - 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

Likely

Expected Size

4 - 8

Valid until: Mar 13th, 2012 9:00AM