Avalanche Forecast

Issued: Dec 20th, 2016 4:42PM

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

jlammers, Avalanche Canada

Northwest Coastal.

Extreme winds and heavy snowfall on Tuesday night will push the avalanche danger to HIGH.

Summary

Confidence

Moderate - Intensity of incoming weather systems is uncertain on Wednesday

Weather Forecast

Tuesday night and Wednesday: Up to 45cm of new snow in the overnight period with an additional 7-15cm during the day / Extreme southwest winds / Freezing level at 650mThursday: 5-20cm of new snow / Light and variable winds / Freezing level at 300mFriday: Light flurries / Light and variable winds / Freezing level at surface

Avalanche Summary

Observations were highly limited on Monday, but I suspect there was an impressive natural avalanche cycle on Sunday night in response to heavy storm loading. More snow on Tuesday night will promote ongoing storm slab activity. There is some uncertainty regarding the reactivity of the buried weak layers below Sunday's storm snow. Surprisingly large avalanches at this interface remain possible and may remain sensitive to human triggering or a surface avalanche in motion.

Snowpack Summary

On Sunday night the region got pummeled by heavy snowfall and extreme westerly winds. By Monday morning, 24 hour totals were around 40cm in Bear Pass, and between 40 and 85 cm along the Hwy 16 corridor west of Terrace. With heavy snowfall and extreme winds forecast for Tuesday night, the developing storm slab will increase in depth and destructive potential. The new snow sits over a highly variable interface resulting from last week's cold snap. This mid-December interface consists of hard wind slabs in wind exposed terrain, widespread faceting of the upper snowpack, and 5-10 mm surface hoar in sheltered areas. The new snow may bond poorly to this interface. A layer of surface hoar which was buried mid-November was down 50-80 cm in many parts of the region prior to the storm. This layer has been dormant recently but may increase in reactivity with storm loading. In many areas in the region, the snowpack is reported to be very thin and faceting has been reported through the entire snowpack. In these areas, old rain crusts from November are developing weak facets. Concerns in the north of the region are basal weaknesses that may exist in the shallower snowpack areas.

Problems

Storm Slabs

Heavy snowfall and extreme winds on Tuesday night are expected to add size and destructive potential to a developing storm slab. The new snow sits over a weak interface which may prolong the reactivity of these slabs.

Good day to make conservative terrain choices.The new snow buries a weak layer which may increase the reactivity of new slabs.The new snow may require several days to settle and stabilize.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely - Very Likely

Expected Size

1 - 3

Persistent Slabs

A layer of surface hoar down 80-120 cm and/or a weak crust layer near the bottom of the snowpack may wake-up as a result of storm loading. Storm slab avalanches may step down to these layers resulting in very large and destructive avalanches.

Storm slab avalanches may step down to deeper layers resulting in very large avalanches.Avoid exposure to overhead avalanche terrain, large avalanches may reach the end of run out zones.Be aware of the potential for large avalanches due to the presence of buried surface hoar.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

2 - 4

Valid until: Dec 21st, 2016 2:00PM