Avalanche Forecast

North Columbia

Date issued

Dec 26, 2017 - 4:42 PM

Valid until

Dec 27, 2017 - 2:00 PM

Prepared by

cgarritty

Cold temperatures will keep persistent slab problems in play for longer than usual. Be especially wary of wind-affected snow at lower elevations.

Regular season
Tuesday - December 26, 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

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.

Wednesday - December 27, 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

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 28, 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

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.

Avalanche Problem 1: Persistent Slabs

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

A weak layer of surface hoar buried 30-50 cm deep has potential to produce large avalanches in areas where the overlying snow has settled into a cohesive slab. The greatest caution is needed around steeper, sheltered slopes at treeline and below.

Travel and Terrain Advice
  • Distribution of buried surface hoar is highly variable. Avoid generalizing your observations.
  • Buried surface hoar is more likely to be preserved in sheltered openings at and below tree line.
  • Watch for signs of slab formation, such as whumpfing and shooting cracks

Avalanche Problem 2: Wind Slabs

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

Shifting winds have been redistributing last week's storm snow into wind slabs on a variety of aspects. Ridge crests and crossloaded areas will be the most likely trigger points. Use extra caution at treeline where wind slab may overlie surface hoar

Travel and Terrain Advice
  • Be aware of highly variable recent wind loading patterns.
  • Use ridges or ribs to avoid pockets of wind loaded snow.

Avalanche Summary

Recent reports show continued observations of wind slabs releasing to Size 1 with skier traffic and ski cutting, primarily on south aspects around exposed terrain features in the alpine. One naturally triggered Size 1.5 was also observed on Sunday. Skier traffic has also been producing small dry loose sluffs from steep, sheltered terrain.

Persistent slab avalanche danger remains a primary concern in areas where last week's storm snow consolidates into a slab above the December 15 interface. In areas where the storm snow has not yet consolidated into a slab, expect continued potential for triggering loose dry avalanches in steep terrain.

Snowpack Summary

Last week's storm brought 40-60cm of new snow to the region. Shifting winds (predominantly from the north) have since redistributed this storm snow into wind slabs on a variety of aspects at higher elevations while cold temperatures have inhibited slab formation elsewhere. Cold has also worked to decrease the cohesion and reactivity of older wind slabs.

The storm snow lies over a wide variety of old surfaces, including large surface hoar (weak, feather-like crystals), hard crusts formed by sun or wind, and sugary facets. The bond at this old snow interface is of critical importance in areas where the overlying snow has consolidated into a slab. The most concerning areas are those that saw pronounced surface hoar development before the storm, such as sheltered openings at and below treeline. Recent snowpack tests show wide ranging reactivity at this interface but suggest that it may be most reactive on northerly aspects.

A crust which was formed by rain in late November is another major feature in the snowpack and is down approximately 80-110cm at treeline elevations. Recent evidence suggests it may be coaxed into reacting in steep, variably loaded terrain features in the alpine.

Weather Forecast

Wednesday: Mainly cloudy with isolated flurries and a trace of new snow. Light west winds. Alpine high temperatures of -17.

Thursday: Mainly cloudy with isolated flurries and a trace to 5 cm of new snow. Light west winds. Alpine high temperatures of -17.

Friday: Cloudy scattered flurries bringing 2-5 cm of new snow, increasing overnight. Light east winds. Alpine high temperatures of -15.

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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