Avalanche Forecast

Issued: Dec 24th, 2017 4:42PM

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.

jmcbride, Avalanche Canada

Kootenay Boundary.

Watch for areas of stiffer snow, where a cohesive soft slab has formed above the buried crust/surface hoar layer. A prime combination for human triggered avalanches.

Summary

Confidence

Moderate - Due to the number of field observations

Weather Forecast

Trace new snow expected overnight Sunday and Monday morning.MONDAY: Mix of sun and cloud. Ridge wind light from the northwest. Temperature -13. Freezing level surface.TUESDAY: Mix of sun and cloud. Ridge wind light from the southwest. Temperature -14. Freezing level surface.WEDNESDAY: Cloudy, light flurries. Accumulation trace. Ridge wind light from the southwest. Temperature -12. Freezing level surface

Avalanche Summary

Saturday several explosives triggered slab avalanches to Size 2 were reported on northwesterly to southeasterly aspects in the alpine as well as sloughs to Size 1 in steep terrain. On Friday storm storm was sloughing naturally in steep terrain with loose snow releases to Size 1, while skier traffic produced small slab releases to Size 1 in areas where the storm snow had formed into a slightly stiffer, cohesive slab above the December 15th layer.Earlier in the week numerous Size 1-2 natural, skier, remote, and explosives triggered slab avalanches were reported on all aspects, releasing both within the storm snow and on the December 15th crust/surface hoar layer; including a Size 1.5 skier accidental.Also several classic signs of instability (whumpfing, shooting cracks, small slab releases) have been reported since Monday. See the Mountain Information Network (MIN) for great recent reports here, here and here, as well as a video here.Check out the Forecasters' Blog here for some great terrain advice.

Snowpack Summary

Storm snow totals over the past week average between 50-90 cm across the region. This new snow has now settled to 30-80 cm and overlies the "December 15th crust/surface hoar layer", a variety of old snow surfaces depending on aspect and elevation. On solar aspects a thin melt-freeze crust has been buried. On northerly aspects, feathery, surface hoar crystals (10-60 mm in length) have been buried below treeline as well as protected areas at treeline. In the alpine the old surface consisted of sugary snow (facets) and hard, wind scoured snow. Recent snowpack tests have produced very easy to hard, sudden and resistant results on the December 15th crust/surface hoar layer indicating a high degree of variability and uncertainty in this layer. Below the December 15th layer, the late November crust is now buried 70-100 cm and has been producing hard, resistant results in recent snowpack tests. Beneath this crust the lower snowpack is well settled..

Problems

Wind Slabs

Winds at upper elevations have created soft slabs on alpine leeward and cross-loaded slopes.

Be careful with wind loaded pockets, especially near ridge crests and roll-overs.If triggered the wind slabs may step down to deeper layers resulting in large avalanches.

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

Elevations: Alpine.

Likelihood

Possible

Expected Size

1 - 2

Persistent Slabs

Recent storm snow has settled into cohesive soft slabs above the December 15th crust/surface hoar layer. Expect these slabs to be easily triggered especially on steep or convex slopes and sheltered areas where surface hoar has been buried.

Use conservative route selection, choose moderate angled and supported terrain with low consequence.Avoid open slopes and convex rolls at and below treeline where buried surface hoar may be preserved.Watch for signs of unstable snow or slab formation such as whumpfing, or shooting cracks.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 2

Valid until: Dec 25th, 2017 2:00PM