Avalanche Forecast

Issued: Dec 5th, 2017 3:49PM

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

cgarritty, Avalanche Canada

Northwest Inland.

Rapid warming will deliver a stress test to the snowpack on Wednesday. Dial back your terrain selection and be keenly aware of overhead hazards.

Summary

Confidence

Moderate -

Weather Forecast

Wednesday: A mix of sun and cloud with thicker valley cloud due to an alpine temperature inversion. Light to moderate southwest winds. Freezing level rising from 2200 to 3400 metres over the day with alpine temperatures rising from around 0 to +5. Cooler temperatures at valley bottom.Thursday: Mainly sunny with some valley cloud due to lingering temperature inversion. Light to moderate southwest winds. Freezing level to 3500 metres with alpine temperatures around +7. Cooler temperatures at valley bottom.Friday: A mix of sun and cloud. Strong southwest winds. Freezing level around 2500 metres with alpine temperatures around +5. Freezing level and temperatures will drop over the day as the temperature inversion breaks down.

Avalanche Summary

No recent avalanche activity has been reported. Stiff wind slabs now exist at treeline and in the alpine on leeward slopes and cross-loaded features. If triggered, these could step down to trigger deeper weak layers and initiate large avalanches. The last significant avalanche observation was submitted on November 28th from Skilokis Creek. Here, a size 2.5 persistent slab was triggered from 150 m away. The possibility for further persistent slab releases will increase with forecast warming.

Snowpack Summary

Average snowpack depths now total 80-140 cm at treeline elevations and up to 160 cm in the alpine. (Depths of up to 210 cm have been reported from the Howsons.) Wind effect from recent strong southwest to northwest winds extended down into treeline elevations and resulted in fairly extensive wind slab formation. Below the wind-affected surface, approximately 30-50 cm of recent snow now overlies two crusts that were buried near the end of November. These crusts are widespread and extend into the alpine. Recent snowpack tests have shown sudden results at these crust interfaces. Below these crusts, we have a well settled midpack overlying the late October crust near the base of the snowpack. This "crust/facet combo" is widespread and has been reactive to rider and remote triggers.

Problems

Deep Persistent Slabs

The late October crust lies near the base of the snowpack with a thin layer of weak, sugary snow above it. This layer has already produced very large avalanches with both rider and remote triggers. Wednesday's warming will test this layer's strength.

Use conservative route selection, choose moderate angled and supported terrain with low consequence.Be especially cautious in shallow snowpack areas where triggering a deeper layer may be easier.Minimize or avoid exposure to overhead hazard.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

2 - 3

Wind Slabs

Recent snow and wind have formed touchy wind slabs. Expect these slabs to be more pronounced at higher elevations and on leeward aspects. Solar radiation and warming will set up touchier conditions on Wednesday.

Increase your caution around convexities and areas with a thin or variable snowpack.If triggered, wind slabs may step down to deeper layers and result in large avalanches.Minimize your exposure to sun-exposed slopes.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 2

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