Avalanche Forecast

Issued: Dec 2nd, 2013 9:26AM

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

jabbiss, Avalanche Canada

Purcells.

Significant variation in storm snow totals have occurred within the region with the south receiving the most. The danger rating will be HIGH on Tuesday in the alpine for the south.

Summary

Confidence

Fair - Intensity of incoming weather systems is uncertain

Weather Forecast

Overnight and Tuesday: The low pressure system stalls over south-eastern British Columbia tonight bringing light to locally moderate precipitation in the southern Kootenays before the arctic front moves further south tomorrow. Mainly dry conditions for Tuesday with a chance of light snow over the southern portion of the region. The winds building light to moderate from the north-east and freezing levels to valley bottom.Wednesday: Outbreak conditions continue; cold and dry. The winds will be light to moderate northerly.Thursday: The arctic air persists over the interior, cold and dry.

Avalanche Summary

Numerous size 1.5 avalanches have been reported as a result of explosive work. With the forecast precipitation and winds to continue in the southern portion of the region tonight, I suspect the chance for avalanche activity to be heightened through Tuesday.

Snowpack Summary

The snow distribution varies greatly across the region but average depth of snow at treeline is approximately 60-80cm.The storm over the weekend deposited 10-20cm of storm snow in the northern portion of the region while the southern reaches of the forecast region received 30-50cm. These storm slabs are sitting on-top of weak surface layers of facetted crystals and melt/freeze crusts. In areas that received more snow, there is likely a cohesive storm slab developing that may overload these weak layers or may make them primed for human triggering. To complicate the scenario, north winds in the forecast will likely deposit wind slabs on south facing slopes.The deeply buried early season crust near the base of the snowpack continues to be a concern. There have been reports of weak sugary crystals at the crust interface that may create an easy sliding surface and allow for wide propagations. The weight of new snow may be enough to 'wake up' these deeper layers. If so, they have the potential to produce large, destructive avalanches.

Problems

Storm Slabs

The storm slab problem will be heightened in the southern portion of the forecast region where they received 40cm of new snow. To complicate the scenario, a forecast shift in wind pattern will likely create fresh wind slabs on south facing slopes.

The new snow will require several days to settle and stabilize due to the presence of newly buried weak layers.>Avoid travelling in areas that have been reverse loaded by winds.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

2 - 5

Persistent Slabs

The added load from the new snow may be enough to tip the scale and wake up the deeply buried October crust weak layer. This layer is most likely a problem for northerly aspects in the upper treeline and alpine.

Be aware of the potential for large, deep avalanches due to the presence of a buried crust/facet combo.>

Aspects: North, North East, East.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

3 - 5

Valid until: Dec 3rd, 2013 2:00PM