Avalanche Forecast

Issued: Mar 7th, 2012 10:02AM

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

ccampbell, Avalanche Canada

Northwest Coastal.

Summary

Confidence

Fair - Timing, track, or intensity of incoming weather is uncertain for the entire period

Weather Forecast

A series of frontal systems is expected to affect the region throughout the forecast period with at least 20-30cm each day for Thursday, Friday, and Saturday, with Thursday looking like the warmest and wettest. Strong southerly winds on Thursday should shift to southwesterlies by Friday. Freezing levels around 1000m on Thursday should drop to or near valley bottoms by the weekend.

Avalanche Summary

Recent reports include more evidence of a large natural cycle from earlier in the week, but natural slab activity seems to have tapered off for the time being with only isolated relatively small events observed yesterday. Two slope-cut slab avalanches on a convex roll sliding fast on surface hoar down 40-50cm. Other slope cuts triggered Size 1 wind slabs in the immediate lee of ridgecrests and exposed rolls and explosive control in the Shames area triggered Size 1.5 to 2.0, 20-50cm thick slabs on east aspects. Several small natural dry sluffs involving the recent storm snow and surface facets were reported as travelling a fair distance for their size.

Snowpack Summary

Moderate amounts of new snow and strong southwesterly and down flow (katabatic) winds are keeping wind slabs fresh and weak on lee slopes and cornices looming from ridgecrests. It seems as though surface hoar buried at the beginning of March, now down 25-50cm under the recent storm snow, has a sufficiently cohesive slab for avalanches in some areas. The early February persistent weak layers down around 80-100cm at treeline seem to be bonding, but remain a concern at higher elevations with heavy triggers such as cornice falls. While below treeline these persistent slabs are reactive to human triggers on isolated sheltered steep terrain where buried surface hoar is preserved. Recent compression tests on a southwest aspect at 1000 m gave moderate sudden collapse results down 60cm where this weakness was found to be facets with an associated crust.

Problems

Wind Slabs

Generally lurking below ridgecrests, behind terrain breaks, and in gullies. Wind slabs are very touchy and there is still a lot of snow available for transport.

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

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 4

Cornices

A hazard in themselves but more of a concern as a heavy trigger for large slab avalanches on the slope below.

Aspects: North, North East, East.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 3

Persistent Slabs

A persistent weak layer of crusts, facets, and surface hoar is buried down close to metre. The likelihood of triggering this layer is decreasing, but the consequences of an avalanche failing on this layer are high.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

2 - 5

Valid until: Mar 8th, 2012 8:00AM