Avalanche Forecast

Issued: Feb 14th, 2013 9:57AM

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

triley, Avalanche Canada

Purcells.

Summary

Confidence

Fair - Due to variable snowpack conditions for the entire period

Weather Forecast

Overnight and Friday: Strong Northwesterly winds overnight are expected to diminish to moderate during the day. There may be a period of broken skies or mostly clear skies in the afternoon. Alpine temperatures are expected to be about -5.0 and the freezing level should be about 1600 metres.Saturday: The moist air on the South side of the trough is expected to bring 10 cms of snow to the region as it passes through from the Northwest. Expect strong Westerly winds with very strong gusts. Alpine temperatures should be about -7.0Sunday: The next weak ridge of high pressure is expected to develop over the interior, bringing cooler temperatures and drier conditions.

Avalanche Summary

Explosives control resulted in slab avalanches up to size 2.0 on most aspects that were about 30 cms deep. Natural avalanches were reported up to size 2.0 that were 20-30 cms deep, and a previous cycle that likely ran during the storm resulted in avalanches up to size 2.5. Most areas reported that the new storm slab and fresh wind slabs were easy to trigger. There was one report from the Saint Mary's drainage of a windloaded storm slab that stepped down to a sliding layer about 60 cms down that was probably the January 23rd weak layer; this resulted in an avalanche size 2.5.

Snowpack Summary

There is a lot of variability in the depth of the new snow slab and the new wind slabs. Areas that received 30-40 cms of new snow have developed thick new wind slabs at higher elevations that may be triggered by light additional loads. The storm slab in these areas has been reported to be reactive to skiers, as it slides easily on a layer of surface hoar that developed just previous to the latest storm. In some areas there is a sun crust that developed during the sunny weather last weekend that is a smooth and fast sliding layer for the new storm slab. Deeper weak layers continue to show planar results in snow profile tests down about 70 cms and also at about 100 cms. These deeper layers are less likely to be triggered by skiers, but cornice falls or other large triggers may step down and result in large avalanches.

Problems

Wind Slabs

Wind slabs may be reactive to light loads like skiers or snowmobiles. Areas of concern are steep open slopes immediately lee of ridges and terrain features.

Avoid freshly wind loaded features.>Be cautious as you transition into wind affected terrain.>

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

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 4

Storm Slabs

The new storm slab may be easy to trigger where it is sitting on a layer of buried surface hoar and/or a sun crust.

The new snow will require several days to settle and stabilize.>Choose conservative lines and watch for clues of instability.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 4

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.>Avoid open slopes and convex rolls where buried surface hoar may be preserved.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

2 - 5

Valid until: Feb 15th, 2013 2:00PM