Avalanche Forecast

Issued: Mar 17th, 2013 10:39AM

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

jfloyer, Avalanche Canada

Northwest Coastal.

Summary

Confidence

Fair - Timing or intensity of solar radiation is uncertain

Weather Forecast

Synopsis: Northwest flow gives way to a frontal system that barely brushes the region on Tuesday night/Wednesday.Monday: Flurries, with some breaks in the cloud giving daytime warming. Afternoon freezing level around 500 m. Winds light northwesterly.Tuesday: Light snow starting late in the day. Freezing level 500 m. Southerly ridgetop winds increasing to around 50 km/h.Wednesday: 5-10 cm new snow. Freezing level 500 m. Winds light southeasterly.

Avalanche Summary

On Saturday, natural slab avalanches were reported up to size 3 on all aspects in the alpine. Solar aspects were particularly reactive and daytime warming triggered slides on the surface hoar layer from March 9th buried approximately 45 cm. Cornice releases were also numerous and triggered slab avalanches on the slopes below. On Friday, a skier triggered a size 2 avalanche on the March 9th surface hoar layer with a crown depth of 45 cm. Several other skier-triggered avalanches occurred on north and west aspects on the same weak layer.

Snowpack Summary

25-50 cm of recent storm snow rests on a variety of old snow surfaces, including crusts, previous wind slabs and surface hoar. Warmer temperatures and recent winds have now set this new snow into a reactive slab, with wind slabs building in exposed lee areas. The surface hoar (buried March 9th) is reported to be well-developed and fairly widespread, at treeline and alpine elevations. Recent snowpack tests have produced sudden failures with moderate loading force at this interface. The mid snowpack is generally well settled and strong. Cornices are large and untrustworthy, especially when the sun is out.

Problems

Storm Slabs

Recent new snow has consolidated into a slab and is reacting on steep terrain. Outflow winds have reverse-loaded slopes in some areas. The underlying weak layer of surface hoar is quite touchy.

Choose conservative lines and watch for clues of instability.>Avoid freshly wind loaded features.>Avoid open slopes and convex rolls where buried surface hoar may be preserved.>Choose well supported terrain without convexities.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 5

Cornices

Cornices are reported to be large and unstable, especially when temperatures become warm or the sun is out. Watch for reverse loading from outflow winds.

Do not travel on slopes that are exposed to cornices overhead.>Stay well to the windward side of corniced ridges.>

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

Elevations: Alpine.

Likelihood

Likely

Expected Size

2 - 6

Valid until: Mar 18th, 2013 2:00PM