Avalanche Forecast

Issued: Jan 21st, 2016 8:45AM

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

jlammers, Avalanche Canada

Kootenay Boundary.

A warm, wet and windy storm forecast for Thursday will cause the Avalanche Danger to rise throughout the day. Loading from rain and snow will add stress to a touchy layer buried deep in the snowpack.

Summary

Confidence

Moderate - Intensity of incoming weather systems is uncertain on Friday

Weather Forecast

Around 20cm of new snow is expected to fall between Thursday night and Friday afternoon, although localized accumulations of up to 30cm are possible along the US boarder. Continued snowfall on Saturday may bring up to 10cm of new snow while a developing ridge should allow for mainly sunny skies on Sunday. Ridgetop winds will be mainly strong and southwesterly on Friday becoming light by the weekend. Freezing levels may climb as high as 1700m by late Friday, and then drop to about 1000m by the weekend.

Avalanche Summary

Recent storm loading promoted explosive and human-triggered storm slab activity to size 2.5 on a variety of aspects in higher elevation terrain. In some cases, avalanches were triggered within the storm snow and then stepped down to the early January surface hoar layer. With forecast snow and wind, storm instabilities are expected to continue. Continued loading will also add mass and sensitivity to the deeper early January persistent weak layer. As the overlying slab gets thicker, avalanches at this interface will become larger and more destructive.

Snowpack Summary

Continued moderate snowfall and strong ridgetop winds are expected to form new storm slabs at all elevations on Friday. 50-85 cm below the surface you'll find a weak layer of surface hoar and a hard crust on steep solar aspects. This layer, which formed at the beginning of January, seems variably reactive. Some test results suggest an improving bond at this interface while remote triggering and reports of whumpfing show this layer is still very sensitive to light loads in many areas. I would be increasingly wary of this potentially destructive layer at treeline and below where it has been reactive on all aspects. Up to 120cm below the surface, a melt freeze crust from December is still producing sudden results in snowpack tests. No recent avalanches have been reported at this interface; however, it is worth keeping an eye on with forecast warming and continued storm loading. Below this, the snowpack is generally well settled and strong.

Problems

Storm Slabs

Latest weather models indicate up to 30cm of new snow may fall on Thursday night and Friday. Warm temperatures and strong winds will form dense storm slabs at all elevation bands. A storm slab in motion can also trigger deeper persistent weaknesses.

Avoid all avalanche terrain during periods of heavy loading from new snow, wind, or rain.>Be cautious as you transition into wind affected terrain.>Stay off recent wind loaded areas until the slope has had a chance to stabilize.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely - Very Likely

Expected Size

1 - 4

Persistent Slabs

A warm, wet and windy storm forecast for Thursday will likely overload persistent weaknesses which lie up to 85cm below the surface. Destructive persistent slab avalanches will be particularly touchy at elevations where precipitation falls as rain.

Be aware of the potential for large avalanches due to the presence of buried surface hoar.>Use conservative route selection, stick to moderate angled terrain with low consequence.>