Avalanche Forecast

Issued: Dec 12th, 2013 8:17AM

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

swerner, Avalanche Canada

Purcells.

Summary

Confidence

Fair - Intensity of incoming weather systems is uncertain on Saturday

Weather Forecast

Friday shows a lull between systems. The Interior will be mainly dry, especially further south. By Saturday another pacific frontal system will spread light-moderate amounts of precipitation accompanied by strong westerly ridgetop winds. Friday: Cloudy with snow amounts 3-5 cm. Alpine temperatures near -9. Ridgetop winds strong from the West. Saturday: Cloudy with snow amounts 5-10 cm. Alpine temperatures -8. Ridgetop winds strong from the West.Sunday: Cloudy with snow amounts near 10 cm. Alpine temperatures -7. Ridgetop winds moderate from the SW.

Avalanche Summary

No recent avalanche activity has been reported. On Tuesday, a wind slab avalanche size 1.5 was easily triggered under the weight of a rider from a NE aspects. Cross-loaded features and the immediate leeward side of ridge crests are likely touchy. Loose dry surface sluffing from steeper terrain is expected, use caution near terrain traps like cliffs, and depressions.

Snowpack Summary

Shifting winds have formed stiff wind slabs on the lee side of ridge lines.Snowpack depths at treeline vary across the region, averaging 70-110 cm. Incremental snowfall amounts are burying surface hoar and a faceted upper snowpack. In the Southern part of the region there is 40-70cm of low density snow sits over the late-November surface hoar/ crust/ facet interface. This layer may become reactive with additional load, and warming temperatures, forming more of a slab.In the Northern part of the region the lower/mid snowpack is weak, composed of an early season crust (more prevalent on north aspects) weak faceted crystals and depth hoar above and below the crust. This persistent slab may be stubborn to trigger, but if triggered have high consequences.

Problems

Wind Slabs

Shifting winds have redistributed snow, forming wind slabs in lee terrain that may be more reactive above a weak layer. Recent new snow may have a poor bond to old surfaces causing loose dry avalanches from steep, sheltered terrain.

Whumpfing is direct evidence of a buried instability.>Be cautious as you transition into wind affected terrain.>

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

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 3

Persistent Slabs

In deeper snowpack areas, it may be possible to trigger an avalanche on a layer of buried surface hoar. In shallower ones, watch for a weak sugary facet layer near the base. The most likely place to trigger them is on steep, convex slopes.

Caution on open slopes and convex rolls at and below treeline where buried surface hoar may be preserved.>Dig down to find and test weak layers before committing to a line.>Whumpfing is direct evidence of a buried instability.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Unlikely - Possible

Expected Size

2 - 5

Valid until: Dec 13th, 2013 2:00PM