Avalanche Forecast

Issued: Dec 25th, 2017 4:01PM

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

jmcbride, Avalanche Canada

Kootenay Boundary.

Watch for areas of stiffer snow, where a soft slab has formed above the buried crust/surface hoar layer. A prime combination for human triggered avalanches especially on steep, convex, or wind loaded terrain.

Summary

Confidence

Moderate - Forecast snowfall amounts are uncertain on Thursday

Weather Forecast

TUESDAY: Mix of sun and cloud. Ridge wind light from the southwest. Temperature -13. Freezing level surface.WEDNESDAY: Mostly cloudy, flurries. Accumulation trace. Ridge wind light from the west. Temperature -12. Freezing level surface.THURSDAY: Snow. Accumulation 5 - 15 cm. Ridge wind moderate from the southwest. Temperature -8. Freezing level surface.

Avalanche Summary

On the weekend, several explosives triggered slab avalanches to Size 2 and loose snow avalanches to Size 1 were reported on northwesterly to southeasterly aspects in the alpine. Sloughing in steep terrain and a skier controlled Size 1.5 wind slab avalanche on a loaded feature at 2100 m on a northeast aspect were also reported.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; see the Mountain Information Network (MIN) for great recent reports here, here and here. Check out the Forecasters' Blog here for some great terrain advice.

Snowpack Summary

Last week's storm snow has now settled to 30-80 cm of soft slab overlying 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 have been buried below treeline as well as protected areas at treeline. And in the alpine, the old surface of of sugary snow (facets) and hard wind-scoured snow lies below the storm snow.Recent snowpack tests have produced very easy to hard, sudden and resistant results on this December 15th crust/surface hoar layer indicating a high degree of variability in this layer. Below the December 15th layer, the late November crust is now buried 60-100 cm and has been producing hard, resistant results in recent snowpack tests. Beneath this crust the lower snowpack is well settled.

Problems

Persistent Slabs

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 in sheltered areas below treeline where surface hoar may be buried.

Watch for stiff or hollow feeling snow, whumpfing, or shooting cracks.Avoid open slopes and convex rolls at and below treeline where buried surface hoar may be preserved.Use conservative route selection, choose moderate angled and supported terrain with low consequence.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 2

Wind Slabs

Variable winds have created wind slabs on most aspects in the alpine and exposed treeline areas. These slabs may be easily triggered by a skier or rider and have the potential to step down to the December 15th layer.

If triggered the wind slabs may step down to deeper layers resulting in large avalanches.Be careful with wind loaded pockets, especially near ridge crests and roll-overs.Watch for whumpfing, stiff or hollow feeling snow, shooting cracks or recent avalanches.

Aspects: All aspects.

Elevations: Alpine.

Likelihood

Possible

Expected Size

1 - 2

Valid until: Dec 26th, 2017 2:00PM