Avalanche Forecast

North Columbia

Date issued

Nov 27, 2017 - 5:12 PM

Valid until

Nov 28, 2017 - 2:00 PM

Prepared by

mgrist

New snow and southerly winds will be driving avalanche danger on Tuesday. Avoid exposure to avalanche terrain during periods of rapid loading by rain, snow, or wind.

Regular season
Monday - November 27, 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.

Tuesday - November 28, 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.

Wednesday - November 29, 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: Storm Slabs

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

A natural avalanche cycle is expected on Tuesday with the incoming snow. Be particularly cautious where the November 27th crust is prominent.

Travel and Terrain Advice
  • Avoid exposure to avalanche terrain during periods of rapid loading by rain, snow, or wind.
  • Watch for signs of instability such as whumpfing, cracking, or recent natural avalanches.
  • Use conservative route selection, choose moderate angled and supported terrain with low consequence.

Avalanche Problem 2: Persistent Slabs

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

Rapid loading by rain or snow will increase the likelihood of avalanches occurring on this layer.

Travel and Terrain Advice
  • Avoid exposure to avalanche terrain during periods of rapid loading by rain, snow, or wind.

Avalanche Summary

On Monday, several storm slabs to Size 2.5 were reported near Rogers Pass, on northerly aspects between 2000 - 2700m.

On Saturday we received reports of a skier-triggered Size 1 storm slab on a NW aspect at 2150m elevation in Rogers Pass. See here for the MIN report.

A natural avalanche cycle is expected on Sunday into early Monday in areas where rain falls on dry snow. We currently have very limited observations in this region. Please submit your observations to the Mountain Information Network (MIN).

Snowpack Summary

The warm temperatures and rain on Sunday resulted in a crust (November 27th crust) up to approximately 1900m, which was covered by up to 10cm of snow at higher elevations as temperatures cooled significantly. This November 27th crust could prove to be an excellent sliding layer when additional snow falls on Tuesday.

Previously, 15 to 30 cm of snow over the past few days sits on a 2cm thick crust formed on November 23rd. The heavy rain last week rapidly shrunk the snowpack and transitioned much of the snowpack from dry to moist snow.

The depth of the snowpack varies greatly with elevation. Recent reports suggest the average depth is 200+cm in the alpine, 100-150cm at treeline, and decreasing rapidly below treeline where the primary hazards are rocks, stumps, and open creeks.

A major feature in the snowpack is a 2-5 cm thick crust which was formed around Halloween and can be found approximately 70-100cm down at treeline elevations. We currently have very limited snowpack observations within this region and it is critical to supplement this information with your own observations.

Weather Forecast

The next system arrives on Tuesday with modest accumulations of snow and more seasonal temperatures.

TUESDAY: Snow (5-15cm) during the day with another 5-10cm possible overnight. Moderate southerly ridgetop winds. Freezing level around 1000 m.

WEDNESDAY: Clearing throughout the day. Moderate westerly ridgetop winds. Freezing level around 1200 m.

THURSDAY: Cloudy with isolated flurries. Light to moderate southwesterly ridgetop winds. Freezing level around 1100 m.

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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