Avalanche Forecast

North Columbia

Date issued

Apr 16, 2018 - 4:28 PM

Valid until

Apr 17, 2018 - 2:00 PM

Prepared by

jmcbride

Storm slabs will continue to build at treeline and above. Watch for loose, wet activity on sunny and lower elevation slopes. And be mindful of hazard from large cornices overhead, especially if it's sunny or warm.

Regular season
Monday - April 16, 2018
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 - April 17, 2018
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 - April 18, 2018
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?

Recent widespread storm slabs may be poorly bonded to the crust below. Be very cautious with wind-loaded and cross-loaded slopes at treeline and above where these slabs may be especially deep and reactive.

Travel and Terrain Advice
  • Use caution in lee areas in the alpine and treeline. Wind and storm snow may form deep slabs.
  • Sluffs may be easy to trigger in steep terrain that is sheltered from the wind
  • Choose well supported terrain without convexities.

Avalanche Problem 2: Cornices

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

Large cornice failures have been reported in the past week. A cornice collapse may also have the potential to trigger a deep weak layer resulting in a very large avalanche.

Travel and Terrain Advice
  • Cornices may weaken with sun and daytime heating. Minimize your exposure to large cornices overhead.
  • Give cornices a wide berth when traveling on ridges.

Avalanche Problem 3: Loose Wet

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

Warming temperatures or a bit of sun may destabilize surface snow and initiate loose, wet avalanche activity especially below treeline and on steep south facing features.

Travel and Terrain Advice
  • Avoid sun exposed slopes when the solar radiation is strong, especially if snow is moist or wet.
  • If triggered the loose wet sluffs may step down to deeper layers resulting in large avalanches.

Avalanche Summary

Sunday's warm weather, rain and sunny breaks produced a widespread loose, wet avalanche cycle from size 1.5-3 on steep, solar aspects from 1600-2600 m, in the afternoon.

Saturday there were reports from the southeast corner of the region of natural cornice and storm slab avalanche activity (size 2-3) on north and west aspects from 2500-2700 m.

On Friday there were reports of a skier-triggered wind slab (size 1) on a steep, cross-loaded, southeast-facing slope at 2700 m. As well as a size 1.5, skier-triggered loose, wet release release in moist snow above a crust on a west-facing slope at 2000 m.

Also on Friday a large cornice collapsed on a southeast aspect at 2500 m, with several 'bus-sized' blocks observed in the debris.

Thursday there were reports from Glacier National Park of a size 4 wind slab that occurred overnight and is thought to have been cornice triggered on a north aspect at 2800 m.

Wednesday there was a report of a natural cornice failure that resulted in a size 2.5 wind slab release (50 cm deep and 200 m wide) on a northeast aspect at 2600 m, north of Glacier National Park.

Snowpack Summary

About 30 cm of recent storm snow overlies a crust on all aspects to at least 2300 m (and possibly higher on south aspects). While 40-50 cm of storm snow can be found at higher elevations in the alpine. Below 2200 m this new snow has a surface crust becoming moist or wet by the afternoon.

Within the upper snowpack there are now a few different crusts with only the most-recently buried posing a concern with recent storm snow at upper elevations.

There is some lingering concern around the mid-March Persistent Weak Layer (PWL) in the alpine where it is found 90 to 120 cm below the surface. This buried crust/surface hoar interface was widely reactive but has not produced any avalanche in the past week. It is likely trending towards dormancy, but we need to keep it in the back of our minds as it is capable of producing large avalanches should it be triggered (think step-down from a surface avalanche or a cornice collapse).

Deeper persistent weak layers from December and January are generally considered dormant at this time but may be starting to become reactive to large triggers, such as a cornice collapse.

Weather Forecast

TONIGHT: Cloudy, flurries. Accumulation 10-15 cm. Ridge wind light to moderate, northwest. Alpine temperature near -5. Freezing level 1000 m.

TUESDAY: Mostly cloudy, light flurries. Accumulation 2-4 cm. Ridge wind moderate to strong, northwest. Alpine temperature near -7. Freezing level 1600 m.

WEDNESDAY: Mix of sun and cloud, light flurries. Accumulation 2-4 cm. Ridge wind light, southwest. Alpine temperature near -4. Freezing level 1700 m.

THURSDAY: Increasing cloud. Ridge wind light, southwest. Alpine temperature near -2. Freezing level 2000 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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