Avalanche Forecast

Northwest Coastal

Date issued

Dec 14, 2017 - 3:13 PM

Valid until

Dec 15, 2017 - 2:00 PM

Prepared by

swerner

Snowfall amounts vary across the region. New storm slabs and dry loose surface avalanches are the name of the game.

Regular season
Thursday - December 14, 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

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 15, 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

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.

Saturday - December 16, 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

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: Storm Slabs

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

New storm slabs will likely build through the forecast period on all aspects and elevations. Dry loose surface avalanches may occur from steeper terrain features.

Travel and Terrain Advice
  • Use ridges or ribs to avoid pockets of wind loaded snow.
  • Be alert to conditions that change with elevation.

Avalanche Problem 2: Deep Persistent Slabs

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

Weak layers buried near the base of the snowpack can't be dismissed while new snow or rain load the surface. I'd limit my exposure to large overhead slopes, especially when the sun makes an appearance on Monday.

Travel and Terrain Advice
  • Take extra caution around areas with a thin or variable snowpack.
  • Avoid areas with overhead hazard.

Avalanche Summary

On Thursday, numerous loose wet avalanches size 1-2 were reported from below treeline elevations. Avalanche activity will likely increase through the forecast period with new rain, snow and wind.

I'd also remain suspect of the deep persistent layers. On December 7th and 8th a few deep persistent slabs to size 3 out of steep south facing alpine terrain in the Skeena corridor west of Terrace were reported. Basal facets were thought to be the culprit in these events. The deep persistent slab is something to keep on your radar, especially with additional stress of the new storm snow.

Snowpack Summary

On Thursday the region received another 10-20 mm of rain at most elevations and some new snow at high alpine elevations. With freezing levels dropping the remainder of the precipitation may fall as snow during the forecast period and likely have a poor bond to the old snow surfaces. The alpine and treeline has seen significant wind effect on all aspects including wind slab and wind pressed snow.

Below the surface sits a well-settled snowpack overlying several layers of interest. Down 70-100 cm you'll likely find a 5-10 cm thick crust which was buried on November 23 and in some areas, a feathery surface hoar layer down 50-60 cm producing moderate snowpack test results. The widespread crust/facet interface that was buried at the end of October can now be found 100-200 cm deep and exists region wide.

Weather Forecast

Friday: Cloudy with possible sunny periods. Alpine temperatures near -2 and freezing levels 700 m. Ridgetop winds strong from southwest.

Saturday: Snow amounts 20-40 cm with the heavier accumulations in the northern part of the region. Alpine temperatures -2 and freezing levels 800 m. Moderate to strong ridgetop winds from the southwest.

Sunday: Snow amount 5-10 cm. Alpine temperatures near -3 and freezing levels 600 m.

Check out the Mountain Weather Forecast for more detail.

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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