Avalanche Forecast

Issued: Feb 20th, 2013 9:54AM

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

jlammers, Avalanche Canada

Purcells.

Summary

Confidence

Fair - Intensity of incoming weather is uncertain on Friday

Weather Forecast

Thursday: Light snowfall intensifying in the evening / Moderate southwest winds / Freezing level at 900mFriday: Moderate snowfall / Strong southwest winds / Freezing level at 1000mSaturday: Light snowfall / Light northwest winds / Freezing level at 700m

Avalanche Summary

On Monday a size 2 skier-triggered avalanche occurred near Invermere which resulted in a fatality. The avalanche ran on a south/southeast aspect at 2700m and is thought to have failed at the February 11/12 interface. More recently, avalanche control in the region avalanches to size 3. Most of the control work involved cornice control shots which, in one case, triggered a large slab on the slope below.

Snowpack Summary

Very light amounts of low density snow add to the recently developed storm slab that is between 15-60 cms across the region. The recent storm slab is expected to remain reactive where it is sitting on the weak surface hoar layer that was buried on February 12th. The recent storm slab has also been reactive on Southerly aspects where a sun crust formed during the period from February 8th-11th. Future loading may cause another cycle of natural activity, or an increased sensitivity to human triggers. There are older weak layers that are now buried down around 70-80 cms and also at about 110 cms. These layers have been less likely to trigger by skiers, but they may still be sensitive to large loads like avalanches in motion or cornice fall.

Problems

Storm Slabs

Continued reactivity is quite possible from the buried February 12th surface hoar layer. The amount of snow above the weak layer is variable across the region. This layer may be particularly reactive where it coexists with a crust on solar aspects.

Dig down to find and test weak layers before committing to a line.>Make observations and assess conditions continually as you travel.>The storm slab will require several days to settle and stabilize.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 4

Wind Slabs

Light amounts of new snow and moderate winds are likely to create fresh wind slabs at higher elevations. Watch for triggering in the lee of ridge crests and in gullies.

Be cautious as you transition into wind affected terrain.>

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

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 3

Persistent Slabs

There are a couple of older layers of buried surface hoar and crust combinations that are buried down about 70 cms and 100 cms, that may be triggered by large additional loads.

Be aware of the potential for large, deep avalanches due to the presence of buried surface hoar.>Choose well supported terrain without convexities.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Unlikely - Possible

Expected Size

2 - 5

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