Avalanche Forecast

Sea To Sky

Date issued

Apr 16, 2018 - 3:57 PM

Valid until

Apr 17, 2018 - 2:00 PM

Prepared by

mconlan

Watch out for new wind slabs as well as thick, old wind slabs. They have been reactive to natural and human activity. Don't be complacent with the weak layer lurking in our snowpack, as it has produced large avalanches recently.

Regular season
Monday - April 16, 2018
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.

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

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.

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

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: Wind Slabs

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

North winds redistributed recent snow, producing wind slabs in lee features. Previous wind slabs from the weekend are 1 m thick from strong south winds. Watch your overhead exposure, as cornices could trigger slabs on slopes below.

Travel and Terrain Advice
  • Avoid steep slopes when air temperatures are warm or the solar radiation is strong.
  • Use ridges or ribs to avoid pockets of wind-loaded snow.
  • Use extra caution around cornices: they are large, fragile, and can trigger slabs on slopes below.

Avalanche Problem 2: Persistent Slabs

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

Professionals are still travelling cautiously due to a weak layer buried 60 to 140 cm. Be particularly cautious around upper treeline and alpine terrain on shady aspects, where this layer is most likely to be a problem.

Travel and Terrain Advice
  • Pay attention to overhead hazards like cornices, which could trigger persistent slabs.
  • Be aware of the potential for wide propagation due to the presence of buried weak layers.
  • Avoid steep terrain or areas with a thin or variable snowpack.

Avalanche Summary

On Sunday, a large (size 3) persistent slab avalanche was remotely triggered on a north aspect in alpine terrain after substantial air temperature and solar warming over the day. The avalanche likely released on the weak layer described in the Snowpack Summary.

Otherwise, loose wet avalanches were easy to initiate in the afternoon with solar warming. Evidence of large slab avalanches was observed, likely occurring early in the weekend.

Snowpack Summary

10 cm of new snow has been redistributed into lee terrain features from strong north to northeast winds. This overlies variable surfaces, including wind slabs at high elevations, a sun crust on south aspects, and a temperature crust on all aspects up to 1900 m. Previous wind slabs are up to 1 m deep from strong south winds. Ridgelines are heavily corniced and if they fail they could trigger slabs on slopes below.

A weak layer consisting of surface hoar, facets, and/or a melt-freeze crust from late March is now buried about 60 to 140 cm. This layer is spotty in its distribution and has recently produced large and destructive avalanches. It is mostly likely to be problematic on west, north, and east aspects between 1900 m and 2250 m. Professionals are treating it seriously and avoiding terrain because of it.

Below this, the snowpack is well-settled and strong.

Weather Forecast

TUESDAY: Cloudy with light snowfall, accumulation 2 to 5 cm, light to moderate west winds, alpine temperature -7 C, freezing level 1200 m.

WEDNESDAY: A mix of sun and cloud, light southeast winds, alpine temperature -6 C, freezing level 1300 m.

THURSDAY: Morning sun and afternoon clouds, light southwest winds, alpine temperature -5 C, freezing level 1400 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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