Avalanche Forecast

Issued: Apr 5th, 2017 3:27PM

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

triley, Avalanche Canada

South Columbia.

Cornice falls have been responsible for most of the avalanches reported in the past few days. Storm slabs or cornices may trigger persistent weak layers resulting in large avalanches.

Summary

Confidence

Moderate - Forecast snowfall amounts are uncertain on Thursday

Weather Forecast

Overnight: 5-8 cm of new snow with moderate-strong southwest winds and freezing down to 1500 metres. Thursday: 10-15 cm of new snow above 1500 metres with strong southwest winds, and daytime freezing up to 1800 metres. Friday: Freezing down to 1000 metres before a mix of sun and cloud brings the freezing level back up to 2000 metres with light winds. Saturday: Overcast with 5-8 cm of new snow and light winds. A cooling trend should start to drop the daytime freezing levels.

Avalanche Summary

Several natural cornice falls up to size 3.5 were reported on Tuesday in the Monashees and the Selkikrks. Natural cornice and one persistent slab avalanche up to size 3.5 were reported from the Monashees on Monday. The persistent slab avalanche was triggered by a wind slab in steep terrain on a southwest aspect at 2400 metres. On Sunday natural cornice falls released up to size 2.5, and one pulled an unsupported slope resulting in a deep slab release. There were also several reports of skier controlled and accidentally triggered storm slabs up to size 1.0.

Snowpack Summary

New snow is forecast to develop new storm slabs in the alpine and at treeline. These new storm slabs are developing above a mix of old surfaces that include melt-freeze crusts on solar aspects in the alpine, and all aspects at treeline. In some areas the new snow may hide some lingering wind slabs that are left from the last stormy period. The biggest problem lately has been large and fragile cornices that have regularly fallen off naturally with loading from wind, or due to warming from direct sun or daytime heating. The February weak layers are down about 160-200 cm and the deep mid-December facet layer and November rain crust both still linger near the bottom of the snowpack. These deep weak layers have been released with large triggers like cornice falls, and they may be more likely to fail on southerly aspects during periods of strong solar radiation or from heavy loading from storm snow. Expect cornices to experience new fragile growth during the next few stormy days.

Problems

Storm Slabs

New storm slabs are expected to develop due to forecast new snow and wind. These storm slabs may be very touchy where they are sitting on a recently buried layer of surface hoar. This is more likely on shaded slopes in the alpine.

Use ridges or ribs to avoid pockets of wind loaded snow.Be especially careful with wind loaded pockets near ridge crests and roll-overs.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

2 - 3

Cornices

Cornices are large and may fall off naturally due to loading from new snow, wind, or daytime warming. Cornice falls in motion may release persistent weak layers resulting in large avalanches.

Do not travel on slopes that are exposed to cornices overhead.Pay attention to overhead hazards like cornices which could easily trigger persistent slabs.

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

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

2 - 3

Persistent Slabs

A cornice fall or smaller slab avalanche could trigger large, destructive avalanches on deeply buried weak layers. The likelihood of this happening will increase as solar warming weakens unstable cornices and storm slabs over the course of the day.

Recognize and avoid runout zones.Cornices or smaller slab avalanches could step down to deeper layers resulting in large avalanches.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely

Expected Size

2 - 4

Valid until: Apr 6th, 2017 2:00PM