Avalanche Forecast

South Coast

Date issued

Dec 21, 2017 - 3:09 PM

Valid until

Dec 22, 2017 - 2:00 PM

Prepared by

jmcbride

Winds are redistributing the recent storm snow at upper treeline and alpine elevations. Watch for areas of wind-affected "chalky" or "pillowy" snow.

Regular season
Thursday - December 21, 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.

Friday - December 22, 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.

Saturday - December 23, 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

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: Wind Slabs

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

Recent storm snow is now being redistributed by wind at upper treeline and alpine elevations and lies on a widespread melt-freeze crust that has been showing poor signs of bonding.

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

Avalanche Problem 2: Loose Dry

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

Storm snow has been showing signs of poor bonding to the most recent crust. Use caution on steep or convex terrain.

Travel and Terrain Advice
  • Be cautious of sluffing in steep terrain, particularly where the debris flows into terrain traps.

Avalanche Summary

On Wednesday a natural Size 2 slab avalanche was reported on a southeast aspect at 1400 m on the North Shore mountains, as well as a Size 1 on a west aspect at 1200 m. Ski cutting however produced no results.

On Tuesday a few natural, loose dry avalanches were observed in the recent storm snow. Ski cutting also produced storm slab and loose dry avalanches to Size 1 running far on the December 18th crust. And explosive control work produced storm slab avalanches up to Size 1.5, propagating about 40 m wide with crowns 20-25 cm deep, failing on the December 18th crust.

Snowpack Summary

Northerly winds have redistributed the recent storm snow scouring windward, northwesterly slopes and loading leeward southerly slopes in the alpine and exposed treeline areas. Approximately 20-30 cm of storm snow sits on top of the most recent December 18th melt-freeze crust which formed following Sunday's rain event to upper treeline elevations. Below this crusts lies 20 cm of wet snow which appears to be well bonded to the lower December 16th crust. Below this second crust the snowpack is well settled to ground. Snowpack depths are about 125 cm at 1100 m elevation, with many early season hazards present at lower elevations.

Weather Forecast

FRIDAY: Mix of sun and cloudy, light flurries. Accumulation trace. Ridge wind light from the northwest becoming southwest. Temperature -3. Freezing level 400 m.

SATURDAY: Mostly sunny. Ridge wind light from the east. Temperature -4. Freezing level 400 m.

SUNDAY: Cloudy, flurries. Accumulation 5-10 cm. Ridge wind light from the southeast becoming south. Temperature -5. Freezing level surface.

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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