Avalanche Forecast

Sea To Sky

Date issued

Apr 17, 2018 - 4:01 PM

Valid until

Apr 18, 2018 - 2:00 PM

Prepared by

mconlan

A warming trend is expected on Wednesday with possible sunny periods. This recipe recently produced large, destructive avalanches on a buried weak layer. Don't be complacent -- these are not typical spring snowpack conditions.

Regular season
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

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.

Thursday - April 19, 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: Persistent Slabs

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

Professionals are travelling cautiously due to a weak layer buried 60 to 140 cm. The layer is found in upper treeline and alpine terrain on shady aspects. Recent avalanches were triggered during sunny and warm weather, which may occur Wednesday.

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

Avalanche Problem 2: Loose Wet

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

The sun packs quite a punch at this time of year. Be cautious if the sun comes out or if you feel warm air temperatures, as this will weaken the snow surface and increase the likelihood of loose wet avalanches.

Travel and Terrain Advice
  • Use extra caution on slopes if the snow is moist or wet.
  • Avoid steep slopes when air temperatures are warm, it is raining, or the solar radiation is strong.
  • Use caution above cliffs and terrain traps where small avalanches may have severe consequences.

Avalanche Problem 3: Wind Slabs

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

Lingering wind slabs exist near ridges. They are found on all aspects due to winds starting from the north and then shifting to the south. Watch your overhead exposure, as cornice falls could trigger slabs on slopes below.

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

Avalanche Summary

On Monday, small slabs and loose wet avalanches were triggered by skiers and the sun.  The slabs were most often in lee terrain features near ridges.

On Sunday, a large (size 3) persistent slab avalanche was remotely triggered by a snowmobile 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.  Another large avalanche was observed close to Whistler, on a north aspect around 2000 m, with a depth of about 1 m.  It was suspected to have released during last weekend’s storm followed by sun and likely on the same weak layer.  Be particularly cautious with this layer on Wednesday, as the sun may periodically shine along with rising freezing levels.

Snowpack Summary

10 cm of new snow has been redistributed from strong north winds followed by southwest winds.  This overlies variable surfaces, including old wind slabs at high elevations, a sun crust on south aspects, and a temperature crust on all aspects.  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

WEDNESDAY: Mostly cloudy with light snowfall and afternoon clearing, accumulation 1 to 3 cm, light southeast winds, alpine temperature -6 C, freezing level 1400 m.

THURSDAY: Mostly sunny, light southwest winds, alpine temperature -5 C, freezing level 1600 m.

FRIDAY: Cloudy with light snowfall, accumulation 2 to 5 cm, light to moderate southwest winds, alpine temperature -5 C, Freezing level 1500 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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