Avalanche Forecast

Issued: Jan 3rd, 2012 9:14AM

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

swerner, Avalanche Canada

Purcells.

Summary

Confidence

Fair - Timing, track, or intensity of incoming weather is uncertain

Weather Forecast

A small "Pineapple" (or maybe we should refer to it as a Papaya due to lack of moisture in the Interior) is pushing through the Southern ranges. Tuesday night snow amounts 5-10cms. Strong SW winds. Wednesday: Light snow amounts up to 5cms. Wind 60-90km/hr from the SW. Freezing levels rising to 2000m through the day. Wednesday night snow amounts 10-15cms. Freezing levels fall to 1000m. Thursday: Snow amounts 5-10cms. Freezing levels falling from 1000m to valley bottom by Thursday afternoon. Friday: Mix of sun and cloud. Light winds from the SE. Freezing levels valley bottom.

Avalanche Summary

One natural size 3 avalanche reported. This was at 2900m on a North aspect, and failed down to glacier ice. Explosive avalanche control missions on Monday reported one size 3 that was remotely triggered from 500m away. This was at 2800m on a NW aspect. Natural activity has tapered off, but I'd still be suspicious of the mid-December layer susceptible to rider triggers (especially sledders) please don't rule this out yet; especially with rising freezing levels and a bit of snow on it's way.

Snowpack Summary

The Purcell region has seen up to 140cms since the Christmas holiday. The new snow has settled into a cohesive slab, and the average total snowpack depth is near 200cms. The ridgetop winds have consistently blown from W-SW and have redistributed the storm snow into wind slabs. Wind slabs are widespread and found in exposed alpine and treeline locations. Even open areas below treeline are suspect for wind affected snow. In sheltered areas the snow surface is starting to form surface hoar crystals (feathery), and surface facets (sugary crystals). These crystals may create a weak layer in the snowpack once buried. Below the surface down 80-140cms lurks the mid-December surface hoar/ crust/ facet layer. This is a layer of concern. Test results on this are still variable in the moderate to hard ranges with sudden planar characteristics. It continues to be sensitive to large triggers like cornice fall and explosives. Between this layer and the bottom of the snowpack sits a well settled mid pack. At the bottom of the snowpack basal facets and depth hoar crystals live. These become a concern in thinner snowpack areas, and have become reactive under the new load of the post Christmas storms.

Problems

Wind Slabs

In the alpine and at treeline winds continue to redistribute storm snow into wind slabs. I'd be suspect of any open area, even below treeline. Wind slabs may be stiff, and have a hollow drum like sound beneath you.

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

Elevations: Alpine, Treeline.

Likelihood

Likely - Very Likely

Expected Size

1 - 5

Persistent Slabs

A cohesive slab sits over a surface hoar weakness buried in mid-December. This layer is still touchy and susceptible to rider triggers. Avalanches on this layer will be large, destructive and may fracture across surprisingly far distances.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

3 - 7

Cornices

Cornices may become weak with rising alpine temperatures. The load on the slope below may trigger the weak mid-December surface hoar layer initiating large to very large destructive avalanches.

Aspects: North, North East, East.

Elevations: Alpine.

Likelihood

Possible - Likely

Expected Size

2 - 6

Valid until: Jan 4th, 2012 8:00AM