Avalanche Forecast

Sea To Sky

Date issued

Dec 20, 2017 - 4:45 PM

Valid until

Dec 21, 2017 - 2:00 PM

Prepared by

jmcbride

Strong northerly winds have redistributed the recent storm snow scouring windward areas and loading lee slopes.  These wind slabs may be easily triggered by a skier or rider.

Regular season
Wednesday - December 20, 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

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.

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

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.

Friday - December 22, 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?

Winds during and since the storm have been from the northwest - which is not the usual pattern.

Travel and Terrain Advice
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.

Avalanche Summary

Up to 20 cm of low-density snow fell on Tuesday, accompanied by moderate to strong northerly winds building reactive soft slabs 20-40cm thick. Numerous natural, skier controlled and skier remote storm slab avalanches to Size 1.5 were reported on various aspects at treeline and below running both within the storm snow as well as on the Dec 15th melt-freeze crust. See here for details in the Mountain Conditions Report. Ski cutting also produced a Size 2 storm slab avalanche in the alpine on Tuesday.

And on Monday, we had reports of reactive wind slabs on steep north-facing slopes. See here for the MIN report.

Snowpack Summary

There is now 30-50cm of snow sitting on a melt-freeze crust that formed on all aspects and elevations in mid-December (December 15th layer). On high north aspects this crust is thin with dry, sugary snow crystals (facets) below.  Below the upper December 15th crust, the late November crust is now buried 70-100 cm with a thin layer of sugary snow (facets) above.  Beneath this crust the lower snowpack is well settled.

Weather Forecast

THURSDAY: Cloudy, flurries. Accumulation up to 5 cm. Ridge wind moderate from the west. Temperature -4. Freezing level 400 m.

FRIDAY: Mix of sun and cloud. Ridge wind light from the northeast. Temperature -8. Freezing level 400 m.

SATURDAY: Mainly sunny. Ridge wind moderate from the east. Temperature -8. 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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