Avalanche Forecast

North Columbia

Date issued

Dec 21, 2017 - 3:53 PM

Valid until

Dec 22, 2017 - 2:00 PM

Prepared by

mbender

Choose conservative terrain in the coming days due to the presence of a persistent slab problem.

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

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.

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

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.

Saturday - December 23, 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.

Avalanche Problem 1: Wind Slabs

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

Recent west and northwest winds switching to north and northeast have formed windslabs in the lee of terrain features on a variety of aspects.

Travel and Terrain Advice
  • Use ridges or ribs to avoid pockets of wind loaded snow.
  • Be cautious as you transition into wind affected terrain.
  • Be aware of areas that have been reverse loaded by winds

Avalanche Problem 2: Persistent Slabs

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

Expect persistent slabs to be become more reactive as recent new snow settles into a slab. Likely spots are sheltered open areas at and below tree line or sheltered northerly aspects in the alpine.

Travel and Terrain Advice
  • Watch for whumpfing, shooting cracks, or signs of recent natural avalanches.
  • Use conservative route selection, choose moderate angled and supported terrain with low consequence.

Avalanche Summary

Reports from the past few days include several natural storm and wind slab avalanches to size 2.5 in the alpine and tree line. There have also been a few reports of skier accidental and remote (triggered from a distance) avalanches to size 1.5 and 2 at and below tree line. These avalanches are failing at the interface between the recent storm snow and the old snow surface. As snow settles and consolidates in the coming days expect to see in human triggered avalanche activity continue even as the stormy period ends.

Snowpack Summary

30-50cm of new snow accumulating over the last week sits on a wide variety of old surfaces including large surface hoar (weak, feather-like crystals), hard crusts formed by sun or wind, and sugary facets. As the snow load builds and slab properties develop, it will be important to monitor how the new snow is bonding to the old surfaces. Most concerning would be areas that have seen previous surface hoar development, such as sheltered areas at and below tree line and sheltered northerly aspects in the alpine. Additionally, any steep solar aspects where the surface hoar may be sitting on sun crust should be considered areas where this weak layer may be very likely to trigger.

A crust which was formed by rain in late November is a major feature in the snowpack and is down approximately 80-110cm at tree line elevations. Snowpack tests suggest the snow above is currently bonding well to it.

Snowpack depth decreases rapidly below tree line. Look out for early season hazards such as rocks, stumps, and open creeks.

Weather Forecast

FRIDAY: Mix of sun and cloud / wind light north / alpine temperature -14

SATURDAY: Mainly sunny / wind moderate northwest / alpine temperature -15

SUNDAY: Mix of sun and cloud / wind light to moderate north / alpine temperature -11

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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