Avalanche Forecast

Sea To Sky

Date issued

Apr 18, 2018 - 4:21 PM

Valid until

Apr 19, 2018 - 2:00 PM

Prepared by

jmcbride

Warming and sun are forecasted for Thursday. This combination recently produced large, destructive avalanches on a persistent weak layer. Don't be complacent, these are not typical spring snowpack conditions for this region.

Regular season
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

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.

Friday - April 20, 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.

Avalanche Problem 1: Persistent Slabs

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

Professionals are traveling 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 on Thursday.

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

Avalanche Problem 3: Wind Slabs

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

Lingering wind slabs exist near ridges. Warming on Thursday may make these slabs easier to trigger. Watch your overhead exposure, as cornice falls could trigger slabs on slopes below.

Travel and Terrain Advice
  • Caution in lee and cross-loaded terrain near ridge crests.

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 with substantial air temperature and solar warming over the day. The avalanche likely released on the late March weak layer. Also on Sunday a 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 weekends storm followed by sun and likely on the same weak layer.

Snowpack Summary

About 10-20 cm of new snow has been redistributed by strong northwesterly winds. This overlies variable surfaces, including old wind slabs at high elevations, and crusts on most aspects.

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 most likely to be problematic on west, north, and east aspects between 1900 m and 2250 m. Professionals are treating this layer very cautiously and avoiding terrain because of it. Read Conditions report here.

The mid and lower snowpack are well-settled and strong.

Weather Forecast

TONIGHT: Mostly clear. Ridge wind light, south. Alpine temperature near -4. Freezing level 1000 m.

THURSDAY: Mostly sunny, increasing cloud in the afternoon. Ridge wind light, southwest. Alpine temperature near -3. Freezing level 1700 m.

FRIDAY: Mostly cloudy, flurries beginning in the evening. Accumulation 5-10 cm. Ridge wind light to moderate, southwest. Alpine temperature near -4. Freezing level 1600 m.

SATURADY: Decreasing cloud, flurries ending. Accumulation 2-5 cm. Ridge wind light, west. Alpine temperature near -5. 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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