Avalanche Forecast

Northwest Coastal

Date issued

Feb 10, 2018 - 5:00 PM

Valid until

Feb 11, 2018 - 2:00 PM

Prepared by

mgrist

Strong winds are creating reactive slabs at higher elevations. Human triggered avalanches will remain likely in wind-loaded areas. Choose conservative terrain and be aware of overhead hazard, especially on sunny aspects.

Regular season
Saturday - February 10, 2018
Alpine

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.

Treeline

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.

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.

Sunday - February 11, 2018
Alpine

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.

Treeline

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.

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.

Monday - February 12, 2018
Alpine

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.

Treeline

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.

Below Treeline

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.

Avalanche Problem 1: Wind Slabs

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

Northerly winds have produced reactive wind slabs in exposed areas at tree line and above. Be aware that these slabs overlie a crust from early-February and may also step down to a deeper weak layer.

Travel and Terrain Advice
  • Be careful with wind loaded areas, especially near ridge crests and roll-overs.
  • Watch for whumpfing, hollow sounds, shooting cracks or recent avalanches.
  • Use ridges or ribs to avoid pockets of wind loaded snow.

Avalanche Problem 2: Persistent Slabs

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

A weak layer exists 120-150 cm below the surface. It may still be possible for humans to trigger persistent slab avalanches on this layer, or for a storm slab release to step down to this layer resulting in a very large avalanche.

Travel and Terrain Advice
  • Caution around convexities or areas with a thin or variable snowpack.
  • Minimize overhead exposure during periods of heavy loading from new snow, wind.
  • Avoid lingering in runout zones. Large avalanches may run long distances.

Avalanche Summary

On Friday, explosives control work produced a few size 1.5 storm slab avalanches lower elevations near Terrace.

A widespread natural avalanche cycle occurred in the Terrace and Shames areas during Wednesday's storm with reports of numerous storm slab avalanches up to size 2 running far and fast in the new snow at all elevations.

A week ago, persistent slab activity was reported north of Meziadin Junction where a natural size 4 and a natural size 3.5 were observed on a southwest and northeast aspect at 1800-2000 m and are suspected to have failed on the mid-January layer.

Snowpack Summary

Strong north west winds picked up on Saturday, redistributing last week's storm snow and creating fresh wind slabs at higher elevations.

Wednesday's storm delivered another 30-50 cm of new snow, and a mix of rain and freezing rain for areas west of Terrace, bringing storm snow totals of over 100 cm in the last 2 weeks for the Terrace area. Northern areas have seen about 50-70 cm of recent storm snow.

Beneath the storm snow lies a 5-20 cm thick crust that was buried early February and can be found 60-110 cm below the surface on all aspects up to 1400 m.

A weak layer buried in mid-January is 120-150 cm below the surface and remains a concern. In most areas this layer is a crust, but it may also be surface hoar in sheltered areas at tree line and below. This layer produced a few human triggered avalanches throughout the region over the past week, and may remain reactive as it adjusts to the weight of the new snow.

In thinner snowpack areas (north and inland), deeper crust / surface hoar layers that were buried in December and early January may still be a concern. They are buried over 150 cm below the surface, but could still be reactive and triggered from shallow spots.

Weather Forecast

Strong northerly winds and sun effect are the main weather factors on Sunday. A proper storm arrives on Tuesday bringing significant snowfall.

SUNDAY: Mostly sunny. Ridge wind moderate to strong from the north / east. Temperature -7. Freezing level valley bottom.

MONDAY: Increasing cloud in the afternoon. Ridge wind strong from the northwest. Temperature -6. Freezing level valley bottom with an above freezing layer between 1400m and 1900m.

TUESDAY: Snow (15-25cm). Ridge wind strong, southwest. Temperature near 0. Freezing level 500 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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