Avalanche Forecast

Issued: Dec 28th, 2017 5:01PM

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.

mbender, Avalanche Canada

South Columbia.

There is uncertainty as to the intensity of a storm impacting the region Thursday night into Friday. Danger ratings and problems reflect the upper end of forecast precipitation which is concentrated in the south of the region.

Summary

Confidence

Moderate - Timing, track, or intensity of incoming weather system is uncertain on Friday

Weather Forecast

FRIDAY: Flurries or periods of snow, accumulation 5-10cm overnight Thursday into Friday / Light south southeast wind / Alpine temperature -10 SATURDAY: Flurries, accumulation 5-10cm / Light to moderate southwest wind / Alpine temperature -12 SUNDAY: Mix of sun and cloud / Light northwest wind / Alpine temperature -11

Avalanche Summary

Recent reports are limited to both natural and human triggered loose dry avalanches in steep terrain to size 1 in areas that have not been affected by wind. A report from just northeast of Nakusp on Monday shows a size 2 persistent slab release on a north aspect at 1800 metres. Another report from Sunday showed a skier-triggered size 1 persistent slab releasing from a north aspect at 2200 metres. Expect continued potential for triggering loose dry avalanches in steep terrain and consider the potential to trigger persistent slabs as the load continues to increase on top of the mid December persistent weak layer.

Snowpack Summary

15-30cm of low density new snow has buried a recently formed layer of weak feathery surface hoar and/or sugary facets. A layer buried mid-December that consists of surface hoar, sun crust and/or sugary facets is now down approximately 60-100cm.The bond at this old snow interface is of critical importance in areas where the overlying snow has consolidated into a slab. The most concerning areas are those that saw pronounced surface hoar development before the storm, such as sheltered areas at and below treeline, sheltered northerly aspects in the alpine, or anywhere the surface hoar formed on top of a sun crust. Recent snowpack tests show wide ranging reactivity on this weak layer but suggest that it may be most reactive on northerly aspects.A rain crust that formed in late November is now buried 100-140cm deep. Recent evidence from the North Columbia region suggests it may be reactive in steep, variably loaded terrain features in the alpine.

Problems

Storm Slabs

Recent low density snowfall has been concentrated in the south of the region. Where accumulations are higher expect the new snow to sluff and gather mass in steep terrain. West and northwest winds have created soft windslabs in alpine lees.

Be cautious as you transition into wind affected terrain.Sluffs may trigger deeper instabilities.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 2

Persistent Slabs

A weak layer of surface hoar buried 50-80 cm deep has potential to produce large avalanches where the overlying snow has settled into a cohesive slab.

Buried surface hoar is more likely to be preserved in sheltered openings at and below tree line.Watch for signs of slab formation, such as wind effect, whumpfing, or shooting cracks.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 3

Valid until: Dec 29th, 2017 2:00PM