Avalanche Forecast

Northwest Coastal

Date issued

Dec 29, 2017 - 3:45 PM

Valid until

Dec 30, 2017 - 2:00 PM

Prepared by

jmcbride

Pockets of reactive wind slab remain a concern for wind-exposed alpine and treeline areas. New snow and wind on Monday will heighten avalanche danger.

Regular season
Friday - December 29, 2017
Alpine

2 - Moderate

Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Natural avalanches unlikely; human-triggered avalanches possible.

Small avalanches in specific areas; or large avalanches in isolated areas.

Treeline

1 - Low

Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

Natural and human-triggered avalanches unlikely.

Small avalanches in isolated areas or extreme terrain.

Below Treeline

1 - Low

Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

Natural and human-triggered avalanches unlikely.

Small avalanches in isolated areas or extreme terrain.

Saturday - December 30, 2017
Alpine

2 - Moderate

Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Natural avalanches unlikely; human-triggered avalanches possible.

Small avalanches in specific areas; or large avalanches in isolated areas.

Treeline

1 - Low

Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

Natural and human-triggered avalanches unlikely.

Small avalanches in isolated areas or extreme terrain.

Below Treeline

1 - Low

Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

Natural and human-triggered avalanches unlikely.

Small avalanches in isolated areas or extreme terrain.

Sunday - December 31, 2017
Alpine

2 - Moderate

Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Natural avalanches unlikely; human-triggered avalanches possible.

Small avalanches in specific areas; or large avalanches in isolated areas.

Treeline

2 - Moderate

Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Natural avalanches unlikely; human-triggered avalanches possible.

Small avalanches in specific areas; or large avalanches in isolated areas.

Below Treeline

2 - Moderate

Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Natural avalanches unlikely; human-triggered avalanches possible.

Small avalanches in specific areas; or large avalanches in isolated areas.

Avalanche Problem 1: Wind Slabs

What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?

Wind slabs are likely lingering around ridge crests, exposed gullies and cross-loaded slopes. These wind slabs may be poorly bonded to the buried mid-December crust layer and reactive to human triggers such as a skier or rider.

Travel and Terrain Advice
  • Be cautious as you transition into wind affected terrain.
  • Watch for whumpfing, stiff or hollow sounding snow, and shooting cracks.

Avalanche Summary

On Thursday an isolated wind slab release (size 1.5) was observed on an actively wind loading, steep terrain feature in the northern part of the region. And on Sunday, a skier remotely triggered a large wind slab (size 2) near the Shames area. The avalanche was triggered on a steep south-facing rocky ridgetop at 1400 m. Check out the Mountain Information Network (MIN) report here for details. And please post your own observations to the MIN!

Snowpack Summary

The Arctic high remains in place and strong outflow winds have affected the surface snow over the past week, scouring some slopes down to an old crust and forming hard deposits in many other areas. Winds have come from a variety of directions over the past several days and have built wind slabs in much of the alpine and exposed treeline terrain. These wind slabs rest on a variety of old surfaces including a thick melt-freeze crust that formed in mid-December. In some sheltered locations, soft feathery surface hoar crystals may exist on top of this crust. In non-wind affected terrain, 30-40 cm of unconsolidated snow overlies the mid-December crust layer.

Beneath the mid-December crust the snowpack is reported to be generally strong with the possible exception of areas around Stewart and northern parts of the region where the late-October, basal crust can be found near the base of the snowpack. Weak sugary snow may exist around this basal crust and could be triggered from shallow snowpack areas.

Weather Forecast

SATURDAY: Mostly sunny. Ridge wind moderate west. Temperature -15. Freezing level surface.

SUNDAY: Mostly cloudy. Ridge wind light northwest. Temperature -12. Freezing level surface.

MONDAY: Cloudy, flurries. Accumulation 5-10 cm. Ridge wind moderate southwest. Temperature -5. Freezing level 400 m.

Danger Ratings Explained

1 - Low

Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

Natural and human-triggered avalanches unlikely.

Small avalanches in isolated areas or extreme terrain.

2 - Moderate

Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Natural avalanches unlikely; human-triggered avalanches possible.

Small avalanches in specific areas; or large avalanches in isolated areas.

3 - Considerable

Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

Natural avalanches possible; human-triggered avalanches likely.

Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas.

4 - High

Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

Natural avalanches likely; human-triggered avalanches very likely.

Large avalanches in many areas; or very large avalanches in specific areas.

5 - Extreme

Avoid all avalanche terrain.

Natural and human-triggered avalanches certain.

Large to very large avalanches in many areas.

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