Avalanche Forecast

Issued: Mar 20th, 2013 9:27AM

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

Peter, Avalanche Canada

Northwest Coastal.

Summary

Confidence

Fair

Weather Forecast

Thursday: Mainly cloudy and unsettled. Expect periods of convective flurries (5-10 cm) and sunny breaks. The freezing level is around 400 m and winds are moderate from the west-northwest. Friday: A ridge of high pressure brings a mix of sun and cloud. The freezing level drops to valley bottom overnight and rises to 500 m during the day. Winds are light to moderate from the northwest. Saturday: Increasing cloud with light snow developing. The freezing level rises to 600 m and winds are moderate from the southwest.

Avalanche Summary

Recent observations are limited but there are a couple new reports of rider triggered avalanches up to size 1.5 releasing on the March 9th layer down around 50 cm. 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.

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 strong winds have now set this new snow into a reactive slab, with wind slabs building in exposed areas on a variety of aspects. 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 (pops) 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

Wind Slabs

Watch for deep and weak wind slabs behind ridges and terrain features and in cross-loaded gullies.

Use ridges or ribs to avoid pockets of wind loaded snow.>Be cautious as you transition into wind affected terrain.>

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

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 4

Persistent Slabs

A weak layer comprising surface hoar crystals is buried around 60 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.

Be aware of the potential for large, deep avalanches due to the presence of buried surface hoar.>Whumpfing is direct evidence of a buried instability.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

2 - 7

Cornices

Cornices are reported to be large and unstable, especially when temperatures become warm or the sun is out. A cornice fall could trigger a large slab avalanche on the slope below.

Give cornices a wide berth when travelling on or below ridges.>Do not travel on slopes that are exposed to cornices overhead.>

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

Elevations: Alpine.

Likelihood

Possible

Expected Size

2 - 6

Valid until: Mar 21st, 2013 2:00PM