Avalanche Forecast

South Coast

Date issued

Dec 19, 2017 - 3:26 PM

Valid until

Dec 20, 2017 - 2:00 PM

Prepared by

mgrist

Fresh storm slabs may bond poorly to the latest melt-freeze crust in the North Shore mountains - watch for how the new snow is bonding to the old surface. Farther north, be alert for signs of instability in the alpine.

Regular season
Tuesday - December 19, 2017
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

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

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.

Wednesday - December 20, 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.

Thursday - December 21, 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.

Avalanche Problem 1: Storm Slabs

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

Tuesday's storm snow will be falling on a widespread melt-freeze crust that exists to upper tree line elevations. Keep a close eye on how the new snow is bonding to the old surface.

Travel and Terrain Advice
  • Avoid steep convex slopes.
  • Watch for signs of instability such as whumpfing, cracking or recent natural avalanches.
  • Use caution in down wind (lee) areas. Wind loading may create pockets of wind slab.

Avalanche Summary

Rain on Sunday night and subsequent cooling on Monday 'locked up' the upper snowpack and avalanche activity tailed off.

Avalanche activity may increase with 15-20cm snow forecast to fall on Monday's melt-freeze crust. Winds blowing snow around can significantly increase snowfall amounts on down wind (lee) features, so keep a close eye on storm snow amounts as you travel.

Snowpack Summary

Temperatures got quite warm at the tail end of Sunday's storm, with up to 30mm rain falling on 15-20cm snow. Clearing overnight into Monday morning resulted in a widespread melt-freeze surface crust up to the highest treeline elevations.

Since then, we've had 20cm snow by Tuesday afternoon. Some key questions to consider are "How well is the new snow bonding to the old surface?" and "Are there areas with deep wind-loaded pockets?"

Below tree line the snow pack is thin and there are many early season hazards. Snowpack depths range from 40 cm at 800 m elevation to 200 cm at 1220 m.

Weather Forecast

After Tuesday's storm it will clear up and we'll have dry weather with cool temperatures for the foreseeable future.

WEDNESDAY: Sunny with occasional cloudy periods. Ridge wind moderate northerly. Freezing level 600 m.

THURSDAY: Some clouds and isolated flurries. Ridge wind moderate from the north west. Temperature near -3. Freezing level 400 m.

FRIDAY: Sunny. Ridge wind moderate from the north east. Temperature near -4. 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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