Avalanche Forecast

Issued: Dec 7th, 2016 3:48PM

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

jfloyer, Avalanche Canada

Northwest Coastal.

We are in an active wind slab cycle. The cold temps are prolonging availability of snow for wind transport and keeping the buried surface hoar layer reactive.

Summary

Confidence

Moderate - Wind effect is extremely variable

Weather Forecast

Thursday: Dry, some high cloud and cold. Treeline temperatures around -18 C. Winds easterly 40-50 km/h.Friday: Increasing cloud, but staying dry. Treeline temperatures around -17 C. Winds easterly 50 km/h, diminishing during the day.Saturday: Flurries. Temperatures around -18 C. Light winds.

Avalanche Summary

Several very large (size 3) avalanches were noted from Bear Pass due to wind loading late on Tuesday or early on Wednesday. Wind slab avalanches are a big concern in all areas through the forecast period, and could easily step down to the surface hoar layer below. Previously, a widespread natural avalanche cycle peaked on Friday and Saturday with avalanches noted to size 3 running on a variety of aspects and elevations from 1100-1800 m. There were a couple of close calls with people being caught out by the touchiness of the buried surface hoar layer.

Snowpack Summary

The storm from last weekend brought 30-80 cm of cold powder. This new snow has been blown around by the wind - first by southwesterly winds during the storm, then by northerly winds with the onset of arctic air, and now by easterly outflow winds. As a result, expect wind slabs in a wide variety of locations. Unless you're really certain about the recent local wind history, the safest bet is to assume wind slabs could be present on all aspects in exposed alpine, treeline, and even some below treeline locations. The recent snow buried a surface hoar layer, which now lies 60-120 cm in many parts of the region. This layer was reactive during and after the storm and is likely still reactive to human triggers. A thick rain crust exists 20-30 cm below the surface hoar layer, with isolated reports of weak facets (sugary snow) forming above the crust; however, that interface hasn't become a concern as of yet. Treeline snow depths are around 140-200 cm in the Terrace and Stewart areas, but substantially less further north. Concerns in the north are basal weaknesses that may exist in the shallower snowpack areas especially on smooth alpine features like glaciers.

Problems

Wind Slabs

Winds have switched from southeasterly, then northerly, now easterly. Outflow winds are blowing at low elevations as well as high. Touchy wind slabs exist on a wide variety of exposed slopes.

Avoid areas of wind slab. Switching winds may have reverse loaded slopes and may surprise you.If triggered the storm/wind slabs may step down to deeper layers resulting in large avalanches.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 3

Persistent Slabs

A persistent weak layer mostly comprising of touchy surface hoar is buried 60-120 cm deep throughout the region, and is currently reactive to human triggers.

Dig down to find and test weak layers before committing to a line.Avoid slopes where buried surface hoar is preserved. Remote triggering from a distance is possible.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

2 - 3

Valid until: Dec 8th, 2016 2:00PM