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Avalanche Forecast

Mar 21st, 2012–Mar 22nd, 2012
Alpine
4: High
Treeline
3: Considerable
Below Treeline
2: Moderate
Alpine
3: Considerable
Treeline
3: Considerable
Below Treeline
2: Moderate
Alpine
3: Considerable
Treeline
3: Considerable
Below Treeline
2: Moderate

Regions: South Rockies.

New snow amounts are expected to vary across the region. Bump up the treeline rating to HIGH if there's more than 25 cm new snow; bump the alpine rating down to Considerable if there's less than 15 cm.

Confidence

Fair - Intensity of incoming weather is uncertain on Thursday

Weather Forecast

20-30 cm new snow overlies a crust that exists on south aspects to around 1900 m and on other aspects to around 1700 m. Recent winds have blown the new snow into wind slabs in exposed lee terrain. Below the crust, two persistent weak layers from mid-February are still being tracked by professionals. The upper one, buried around 16th Feb, is buried approximately 150 cm. The lower one, buried around 9th Feb, is buried typically 180 cm in high snow areas, but as little as 90 cm in low snow areas. Snowpack tests indicate, while both surface hoar layers are becoming more difficult to trigger, they still have the potential to release avalanches, with regular sudden, 'pop' results still being observed.

Avalanche Summary

On Tuesday, wind slabs in the size 1-1.5 range were triggered naturally and by skiers at treeline and above on slopes loaded by south-westerly winds. On Sunday, a size 1.5 avalanche released naturally on a north aspect. On Saturday, a recent avalanche was observed on a north aspect in the southern Elk Valley, suspected to have started as a wind slab and ending in moist snow. Evidence of a previous natural cycle was also reported. On Friday, explosives produced size 1-2 avalanches.

Snowpack Summary

20-30 cm new snow overlies a crust that exists on south aspects to around 1900 m and on other aspects to around 1700 m. Recent winds have blown the new snow into wind slabs in exposed lee terrain. Below the crust, two persistent weak layers from mid-February are still being tracked by professionals. The upper one, buried around 16th Feb, is buried approximately 150 cm. The lower one, buried around 9th Feb, is buried typically 180 cm in high snow areas, but as little as 90 cm in low snow areas. Snowpack tests indicate, while both surface hoar layers are becoming more difficult to trigger, they still have the potential to release avalanches, with regular sudden, 'pop' results still being observed. If avalanches do release on this layer, they will go big. The most likely triggering mechanisms are a person traveling over a thin snowpack area, or a storm slab, wind slab or cornice stepping down. Basal facets may still exist, particularly in shallow snowpack areas with steep, rocky start zones. Cornices have grown large and threaten slopes below.

Avalanche Problems

Wind Slabs

New snow and wind continue to set up wind slabs behind ridges and terrain breaks. Large cornices also threaten slopes below.

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

Elevations: Alpine, Treeline.

Likelihood: Likely - Very Likely

Expected Size: 1 - 4

Storm Slabs

New snow will continue to build storm slabs. It's possible for surface avalanches to step down to a weak persistent layer, initiating large destructive avalanches.

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Likely

Expected Size: 1 - 5

Deep Persistent Slabs

Persistent weak layers, now buried 1-2m deep, still have the potential to produce large and dangerous avalanches. They could be triggered by cornice fall, smaller avalanches stepping down, and under the weight of a snowmobile and rider.

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Unlikely - Possible

Expected Size: 4 - 8