Avalanche Forecast

Issued: Mar 18th, 2013 11:16AM

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

jfloyer, Avalanche Canada

Northwest Coastal.

An already touchy snowpack will likely produce natural avalanches on Tuesday as the wind picks up.

Summary

Confidence

Poor - Intensity of incoming weather is uncertain

Weather Forecast

Synopsis: A Frontal system passes mostly to the south of this region on Tuesday night/Wednesday, bringing generally light precipitation.Tuesday: 5-10 cm new snow. Freezing level 500 m. Southwesterly ridgetop winds increasing to around 50 km/h.Wednesday: 5-10 cm new snow. Freezing level 500 m. Winds light southeasterly.Thursday: Mostly dry with some good sunny breaks. Generally light westerly winds.

Avalanche Summary

A very large (size 3.5) avalanche was triggered in Cherry Bowl in the Shames backcountry on Sunday. Click here for a full report. This slide illustrates clearly the potential for deep and destructive releases on the March 9th surface hoar layer. 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

30-60 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.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

2 - 6

Persistent Slabs

A weak layer comprising surface hoar crystals is buried around 50 cm below the surface and is reacting readily to natural and human triggers. This layer appears to have the ability to propagate over large areas of terrain.

Avoid alpine terrain.>Choose well supported terrain without convexities.>Whumpfing, shooting cracks and recent avalanches are all strong indicators of an unstable snowpack.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

3 - 7

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.>

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

Elevations: Alpine.

Likelihood

Likely

Expected Size

2 - 6

Valid until: Mar 19th, 2013 2:00PM