Avalanche Forecast

South Columbia

Date issued

Apr 21, 2018 - 3:55 PM

Valid until

Apr 22, 2018 - 2:00 PM

Prepared by

cgarritty

New snow accumulations are likely to become increasingly reactive as warming and solar radiation increase over the day. Increase your caution around sun-exposed slopes as well as wind loaded areas.

Regular season
Saturday - April 21, 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.

Sunday - April 22, 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.

Monday - April 23, 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.

Avalanche Problem 1: Wind Slabs

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

New snow and strong winds will have formed new wind slabs in the lee of wind-exposed terrain. Slabs are likely to remain reactive over the near term and reactivity is likely to increase with sunshine and daytime warming on Sunday.

Travel and Terrain Advice
  • Minimize your exposure to any sun exposed slopes.
  • Be especially careful with wind loaded pockets near ridge crests and roll-overs.

Avalanche Problem 2: Cornices

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

Large cornice failures have been reported in the past week and cornices will weaken with daytime warming on Sunday. Cornice releases may have enough force to trigger a deep weak layer and cause a very large and destructive avalanche.

Travel and Terrain Advice
  • Cornices become weak with daytime heating. Minimize your overhead exposure.
  • Give cornices a wide berth when travelling on or below ridges.

Avalanche Problem 3: Loose Wet

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

Daytime warming and sun exposure will promote natural loose wet avalanche activity on Sunday - especially around steep sun-exposed terrain.

Travel and Terrain Advice
  • Be cautious of sluffing in steep terrain, particularly where the debris flows into terrain traps.
  • Minimize exposure when the solar radiation is strong, especially if snow is moist or wet.

Avalanche Summary

A few natural storm and wind slab releases were observed on Thursday, ranging from size 1-2.5. One of these was a size 2 slab triggered by a loose wet avalanche while the 2.5 wind slab was triggered by natural ice fall on a north aspect at 2200 metres. It featured a 45 cm crown fracture.

Reports from Wednesday included an observation of a natural size 2 cornice release from a north aspect in the north of the region. Numerous natural loose wet avalanches were also observed on sun-exposed aspects in the Monashees.

Tuesday's reports showed numerous small (size 1) storm slabs releasing both remotely and with ski cutting. Crown fractures were in the 35-40 cm range and activity was observed at all elevations and on all but west aspects.

In addition to heightened storm slab activity following regular snowfalls, observations from late last week also showed a pattern of heightened cornice failure activity.

Looking forward, sunshine and warming temperatures are expected to promote both natural and human-triggered avalanches limited to the new snow. The same warming may also cause natural cornice collapses.

Snowpack Summary

About 15-20 cm of new snow has accumulated above a melt freeze crust found on the previous surface on all but north aspects above about 2200 metres. Below it, About 70-100 cm of rapidly settling storm snow overlies another supportive crust which shares similar distribution to the upper crust.

Within the upper snowpack there are now a few different crusts with the shallowest of these (down 30-50 cm) now a limited concern as warm temperatures have promoted settlement and bonding in the upper snowpack.

With warm daytime temperatures and poor overnight recovery of cold temperatures, there remains some lingering concern around the mid-March persistent weak layer in the alpine where it is found 80 to 120 cm below the surface. This buried crust/surface hoar interface was widely reactive but is likely trending towards dormancy. It should be kept in the back of our minds for its capability to produce large avalanches should it be triggered with a heavy load such as a snowmobile or cornice collapse.

Deeper persistent weak layers from December and January are considered dormant at this time.

Weather Forecast

Sunday: A mix of sun and cloud. Light northwest winds. Freezing level to 1700 metres with alpine high temperatures around -5.

Monday: Sunny. Light south winds. Freezing level to 2400 metres and climbing overnight. Alpine high temperatures around +1.

Tuesday: Mainly sunny. Light south winds. Freezing level to 3000 metres with alpine high temperatures around +5.

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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