Avalanche Forecast

Issued: Dec 5th, 2012 10:19AM

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

jlammers, Avalanche Canada

Purcells.

Summary

Confidence

Fair - Due to variable snowpack conditions

Weather Forecast

Overnight and Thursday: Light snowfall / moderate to strong west winds/ Freezing level at 800mFriday: Light snowfall / moderate to strong west winds / Freezing level at surfaceSaturday: Trace amounts of snow / Light west winds / Freezing level at surface

Avalanche Summary

On Tuesday, releases on the early November crust to size 1.5 were observed on a variety of aspects in the Invermere area between 2300m and 1800m. Triggering has been by explosives use and ski cutting, while one event was a skier accidental. I expect significant wind slab activity in the wake of strong winds and snowfall on Tuesday/Wednesday.

Snowpack Summary

In the upper snowpack, strong winds and light to locally heavy snowfall are responsible for newly formed wind slabs which are likely to exist on north to northeast aspects at treeline and above. Winds have also shaped new cornices which could fail and act as a trigger for the slope belowUp to 75cm ofÂ snow that fell over the past week may sit above a surface hoar layer that was buried on the 28th. Although not widespread in the region, releases on this layer are quite likely if you have it in your area. Another surface hoar layer that developed during mid-november may exist up to 100cm down, and is still on the radar of professionals in the Golden area. At the base of the snowpack, there is a rain crust near the ground that we're calling the early November crust. There are some facets below it, or it may appear as a sandwich of crusts and facets. Although we haven't seen much activity on this layer in deeper snowpack areas, it is still reactive in the southeast corner of the region. In general, significant variations in snowpack structure exist across the region. Digging down and making your own observations will be critical to safe slope selection.

Problems

Wind Slabs

Recent snowfall and strong winds are responsible for widespread windslab development. With higher wind values, loading may have occurred lower on the slope than normal.

Be cautious as you transition into wind affected terrain.>The new snow may require several days to settle and stabilize.>

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

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 4

Persistent Slabs

Two buried spotty surface hoar layers exist up to a metre down and seem to be most reactive in deeper snowpack areas. Where it they exist, human triggering of large and destructive avalanches may be likely.

Be aware of the potential for large, deep avalanches due to the presence of buried surface hoar.>Be alert to conditions that change with elevation.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

2 - 5

Deep Persistent Slabs

Deep persistent releases have been most common in the southeast corner of the region. Watch for triggering on unsupported terrain with smooth ground cover.

Choose well supported terrain without convexities.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

1 - 6

Valid until: Dec 6th, 2012 2:00PM