Avalanche Forecast

Issued: Dec 21st, 2017 3:50PM

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

jmcbride, Avalanche Canada

Kootenay Boundary.

The recent storm snow overlies a variety of old surfaces; including crusts and surface hoar, and has proven to be easily triggered by light loads such as a skier or rider. Conservative terrain choices are the key for a few more days.

Summary

Confidence

High - The weather pattern is stable

Weather Forecast

FRIDAY: Mix of sun and cloud, light flurries. Accumulation trace. Ridge wind light from the northeast. Temperature -9. Freezing level surface.SATURDAY: Mostly sunny. Ridge wind light from the northwest. Temperature -12. Freezing level surface.SUNDAY: Cloudy, light flurries. Accumulation trace. Ridge wind light from the southwest. Temperature -12. Freezing level surface.

Avalanche Summary

On Wednesday observers reported evidence of a natural avalanche cycle up to Size 2 that occurred during the storm event; as well as, numerous Size 1-2 skier, remote, and explosives triggered slab avalanches on primarily north to west aspects between 1800-2100m, failing both within the storm snow and on the December 15th crust/surface hoar layer.On Tuesday, several natural and skier controlled storm slab avalanches to Size 1.5 were observed on a wide variety of aspects and elevations, while explosive control produced numerous storm slab avalanches to Size 2.5-3 on southeast to southwest aspects.On Monday, numerous storm slabs up to Size 2 were reported in the Nelson area: These were either easily triggered by skiers or ran naturally. Also since Monday, several classic signs of instability (whumpfing, shooting cracks, easy sudden results in snowpack tests) have been reported. See here and here for the great MIN reports and video.

Snowpack Summary

Storm snow totals since Sunday average between 30-60 cm across the region, with the highest amounts in the Nelson area. This new snow has fallen on the "Decemebr 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 results on the 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 70-100 cm. Beneath this crust the lower snowpack is well settled.

Problems

Storm Slabs

Recent storm snow and wind have created widespread, soft slabs above the December 15th crust/surface hoar layer. Expect these slabs to be easily triggered by light loads such as a skier or rider.

Caution on open slopes and convex rolls below treeline where buried surface hoar may be preserved.Watch for signs of instability such as whumpfing, or cracking. Use conservative route selection, choose moderate angled and supported terrain with no consequence.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

1 - 3

Wind Slabs

Winds are redistributing the recent storm snow building fresh wind slabs in leeward areas.

Use caution in lee areas. Recent wind loading has created wind slabs.

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

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 2

Valid until: Dec 22nd, 2017 2:00PM