Avalanche Forecast

Issued: Dec 17th, 2018 5:13PM

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

shorton, Avalanche Canada

South Rockies.

Dial back your terrain choices if slopes are getting loaded by new snow and wind on Tuesday.

Summary

Confidence

Moderate - Intensity of incoming weather systems is uncertain on Tuesday

Weather Forecast

MONDAY NIGHT: 5-10cm of snow as a front approaches, strong wind from the southwest, freezing level around 1300m. TUESDAY: 5-10cm of snow, extreme wind from the southwest, freezing level up to 1600m, alpine high temperature around -3 C. WEDNESDAY: Mix of sun and cloud, strong wind from the west, freezing level up to 1500m, alpine high temperature around -5c. THURSDAY: Mix of sun and cloud, moderate wind from the west, freezing level up to 1500m, alpine high temperature around -5c.

Avalanche Summary

On Monday, a skier triggered a large slab avalanche (size 2) on a east aspect around 1800 m. The avalanche failed on a persistent weak layer 40 cm below the surface.On Friday and Saturday, explosive control produced numerous large (size 2-3) wind slab and deep persistent slab avalanches on north and east aspects above 2000 m.Natural activity was also reported on Friday, primarily size 1-2 wind slab avalanches on east-facing lee terrain in the alpine. Some appear to have stepped down to the deep persistent weak layers near the bottom of the snowpack. See some photos of these from north of Crowsnest Pass.

Snowpack Summary

Tuesday's storm will build fresh wind slabs at higher elevations. Strong wind has affected all open terrain over the past few days, scouring windward slopes and forming wind deposits on lee slopes. Currently only 30-120 cm of snow can be found in alpine areas and much less at lower elevations.The lower snowpack has a weak structure composed of facets and crusts. Heavy explosive triggers have produced large avalanches on these layers, and human triggering is suspect on steep terrain features and slopes with variable snowpack depth.

Problems

Wind Slabs

10-20 cm of new snow with strong wind will form reactive wind slabs in open terrain.

If triggered the wind slabs may step down to deeper layers resulting in large avalanches.Be aware of the potential for wide propagations due to the presence of hard windslabs.Use ridges or ribs to avoid pockets of wind loaded snow.

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

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 2

Deep Persistent Slabs

The weak snow in the lower snowpack could produce large avalanches. Be cautions in areas where the surface feels stiff or slabby, such as around thick wind deposits.

Back off if you encounter signs of instability like whumphing, shooting cracks or recent avalanches.Use conservative route selection, choose moderate angled and supported terrain with low consequence.Avoid shallow snowpack areas where triggering is more likely.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1.5 - 2.5

Valid until: Dec 18th, 2018 2:00PM