Avalanche Forecast

Kootenay Boundary

Date issued

Feb 5, 2018 - 4:45 PM

Valid until

Feb 6, 2018 - 2:00 PM

Prepared by

mgrist

Storm slabs have been reactive to human triggers, resulting in some surprisingly large avalanches. Several weak layers are lurking deeper in the snowpack: Now is a good time to choose simple terrain, free from overhead hazard.

Regular season
Monday - February 5, 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 - February 6, 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

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.

Wednesday - February 7, 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

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?

Storm slabs remained very touchy on Monday, with human triggered avalanches to size 2.

Travel and Terrain Advice
  • Avoid exposure to overhead avalanche terrain, large avalanches may reach the end of run out zones.
  • The new snow will require several days to settle and stabilize.
  • 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?

The snowpack is complex and continues to produce very large avalanches on several weak layers buried 80 to 150 cm below the surface. Moderate-angled, simple terrain selection remains critical to safe mountain travel.

Travel and Terrain Advice
  • Good day to make conservative terrain choices.
  • Be mindful that deep instabilities are still present and have produced recent large avalanches.
  • Minimize overhead exposure; avalanches may reach the end of run out zones.

Avalanche Summary

On Sunday in the north west part of the region, skiers triggered a size 2.5 avalanche that likely stepped down to the mid January layer. See the MIN report for more details.

On Sunday, ski cutting near Nelson produced many soft slab avalanches to size 1.5 on down wind (lee) features that had seen additional snow loading due to wind.

Looking forward, expect newly formed storm slabs and the persistent slabs beneath them to remain reactive to human triggers with the potential for very large and destructive avalanches.

Snowpack Summary

Each of the past three days has seen locally heavy snowfall (eg: 15cm in 5 hours at Kootenay Pass on Sunday) coupled with moderate to strong winds at upper elevations. Storm snow totals vary from 25-50 cm and the winds created reactive wind slabs at tree line and above (see Avalanche Summary above).

Critical instabilities are buried well below the surface: See this video for a summary of conditions near Nelson.

A mixture of weak surface hoar and/or a crust from mid-January is buried beneath all the storm snow at 90-120cm deep. Numerous recent avalanches to size 2.5 have been reported on this layer and large, destructive avalanches at this interface remain a concern.

Below this layer lies a second crust/surface hoar interface buried early-January that is now 120-140 cm deep. Several recent avalanches have stepped down to this layer.

The mid-December surface hoar layer is buried 130-160 cm below the surface. This layer has continued to produce step down releases and "sudden" test results. It is most pronounced at tree line, but is also present below tree line .

Weather Forecast

Tuesday: Isolated flurries (5 cm possible) / Moderate west winds / Freezing level around 900m

Wednesday: Snow (10-15cm) / Moderate west winds / Freezing level near 1200m

Thursday: Snow (10-15cm) / Moderate westerly winds / Freezing level around 1400m

Note: Confidence is low for forecast wind values and precipitation amounts on Wednesday and Thursday.

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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