Avalanche Forecast

South Columbia

Date issued

Jan 11, 2018 - 4:53 PM

Valid until

Jan 12, 2018 - 2:00 PM

Prepared by

cgarritty

Very dangerous avalanche conditions have developed in the Columbias. Travel in avalanche terrain is not recommended.

A Special Public Avalanche Warning is in effect in this region.

Regular season
Thursday - January 11, 2018
Alpine

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.

Treeline

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.

Below Treeline

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.

Friday - January 12, 2018
Alpine

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.

Treeline

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.

Below Treeline

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.

Saturday - January 13, 2018
Alpine

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.

Treeline

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.

Below Treeline

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.

Avalanche Problem 1: Storm Slabs

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

Many avalanches have been human-triggered in the recent 50 to 70 cm of storm snow. The snow is sitting on variable surfaces and it is not bonding well to them. The new snow is likely to be especially reactive where it has been affected by the wind.

Travel and Terrain Advice
  • The new snow sits on a persistent weak layer and will require more time than usual to stabilize.
  • If triggered, the storm slabs may step down to deeper layers resulting in large avalanches.

Avalanche Problem 2: Persistent Slabs

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

Light triggers have released large avalanches on this layer, which is buried around 70 to 110 cm deep in the snowpack. Storm slabs are regularly stepping down to this layer. When triggered, it is producing large, destructive avalanches that run far.

Travel and Terrain Advice
  • Be aware of the potential for large, deep avalanches due to the presence of buried surface hoar.
  • Numerous large avalanches have run on this layer in well supported, treed terrain.

Avalanche Summary

Reports from Wednesday show a continuation of recent heightened avalanche activity involving both storm slabs above the January 5 surface hoar layer as well as the deeper December 15 surface hoar. Avalanche activity on these interfaces continues to be extensive, with natural and remote (from a distance) triggers forming a large percentage of observations. Sizes have ranged from Size 1-3, slab depths have generally ranged from 40-80 cm, and activity was observed on all aspects and elevation ranges. Reports from the previous two days showed similar activity.

Some of the notable themes that are emerging from recent activity include accidental and remote triggering, 'step down' release types, releases on surprisingly low angle, supported terrain, and wide fracture propagations.

Looking forward, expect a continuation of heightened human triggering potential that may increase toward the weekend with forecast warming.

Snowpack Summary

Recent storms brought 50-70 cm of new snow to the region. The new snow sits on surface hoar on sheltered slopes as well as sun crust on steep solar aspects. Numerous storm slab avalanches resulted as the new snow formed a slab before bonding to the surface. Moderate to strong southwest winds accompanied the snowfall at higher elevations, creating wind slabs in lee features.

As the new snow continues to consolidate, it forms a dangerous slab above several persistent weak layers that exist in the snowpack. The first of these is the (January 5) surface hoar that exists at the new snow interface. Recent snowpack tests and avalanche activity show a high degree of reactivity at this layer. Below it, another surface hoar layer from late December 26 is showing increased involvement in 'step' down releases in the region. This layer is found at similar depths, but just below the January 5 interface.

Below these layers, the December 15 surface hoar is now buried 70 to 110 cm deep. This layer has been the failure plane in many recent destructive slab avalanches, with many of these 'stepping down' to this layer from shallower releases. Its reactivity has been most apparent at treeline and below.

Weather Forecast

Friday: Cloudy with scattered flurries bringing 2-5 cm of new snow, mainly in the afternoon and overnight. Light southwest winds. Alpine high temperatures to -5.

Saturday: Mainly cloudy with isolated flurries and a trace of new snow. Light to moderate southwest winds. Freezing level rising to 1400 metres with alpine high temperatures to -2. Mild temperature inversion.

Sunday: Mainly sunny with valley cloud due to temperature inversion. Light southwest winds. Freezing level to 1700 metres with alpine temperatures to +2. Cooler at lower elevations.

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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