Sea to Sky Avalanche Forecast

Issued: Feb 15th, 2021 4:00PM

Tue Feb 16th Current Conditions
Alpine Considerable Treeline Moderate Below Treeline Moderate
Wed Feb 17th 2 Day Outlook
Alpine Considerable Treeline Moderate Below Treeline Low
Thu Feb 18th 3 Day Outlook
Alpine Considerable Treeline Moderate Below Treeline Low

The alpine rating is considerable, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Wind Slabs.

This is an unusual snowpack for these mountains and while the wind slabs that have caused so much damage are slowly healing, they may remain sensitive to human triggering. A fresh round of wind slabs may form Tuesday afternoon as northwest wind picks up.

Summary

Confidence

Moderate - Uncertainty is due to the extreme variability of wind effect on the snowpack.

Weather Forecast

Cloudy Tuesday, bluebird Wednesday & then another storm approaches Thursday.

MONDAY NIGHT: Freezing level lowering to valley bottom, light northwest wind, trace of snow possible. 

TUESDAY: Overcast, freezing level rising to 1000 m, light west/northwest wind for most of the day with potential for moderate wind in the afternoon, trace of snow possible.

WEDNESDAY: Clear skies, freezing level rising to about 1000 m, light west/northwest wind at most elevations with moderate northwest wind at the highest elevations, no snow expected.

THURSDAY: A few clouds at dawn building to scattered cloud cover in the afternoon, freezing level rising to about 1000 m, moderate southeast wind initially with wind speed ramping up through the day. Strong to extreme southerly wind expected Thursday afternoon, no snow expected during the day. 10 to 20 cm of snow expected Thursday night.

Avalanche Summary

Sunday was a much quieter day for avalanche activity. Avalanche control work produced very small avalanches with the exception being a single size 2.

Another fatal avalanche involvement occurred in the region on Saturday in the Brandywine valley. The person caught was carried several hundred metres over steep, rugged terrain and through treed slopes below. The avalanche is described as a size 1 (small) wind slab on a southwest aspect at 1700 metres. The incident report can be seen here.

On Friday, a large (size 2.5), fatal avalanche was triggered by skiers at 2200 metres on a west-facing slope on Phalanx Mountain. The avalanche is described as a wind slab that formed to the lee of recent strong east winds. Two people out of a group of three were involved in the avalanche and one person was killed. The incident report can be seen here. A second, smaller (size 1.5) wind slab was triggered by skiers on a nearby slope, again causing injury to the person involved.

A bout of strong northerly winds on Thursday caused conditions in the region to change rapidly, with new and touchy wind slabs forming in unusual places. Numerous natural releases from size 1 to size 2 were observed in the Whistler area above about 1900 metres on and since Thursday. 

It is important to note wind has not affected elevation bands uniformly across the region, with heavy wind effect and slab formation noted even below treeline in some areas. This is a critical piece of information as new snow obscures existing wind slab formations.

Two more skiers were involved in an avalanche in the Supercouloir feature of Mamquam Mountain on Thursday. This avalanche was a natural wind slab release and again resulted in serious injuries and an urgent evacuation by helicopter.

Snowpack Summary

As of Monday afternoon, 5 to 15 cm of new snow has accumulated with moderate southeast wind. This adds to the existing wind slab problems brought on by powerful east and northeast winds that have been redistributing loose snow into wind slabs. Winds have been circling the compass expanding problems to all aspects. The images in this MIN from Rainbow show the extensive wind effected snow.

Monday's snow adds to the 15 cm of older low density snow that can be found in shaded, sheltered areas and to more widespread wind-affected snow. This previous surface has been transforming into weak, faceted grains under recent cold temperatures. On solar aspects, a thin sun crust may be found near the new snow interface or beneath recently wind transported snow. Both of these interface types are contributing to the reactivity of new wind slabs.

Below the evolving surface, 50-100 cm of settled storm snow sits on a persistent weak layer from late January that consists of facets at upper elevations, surface hoar in sheltered areas, a melt-freeze crust below 1900 m, and a sun crust on south-facing slopes. There could be more than 100 cm on this layer in wind loaded areas. Although this structure is suspect, we have no recent reports of avalanches failing at this interface within the region.

A crust from early December, currently considered dormant, may be found around 200+ cm deep in the snowpack.

Terrain and Travel

  • Pay attention to the wind, once it starts to blow fresh sensitive wind slabs are likely to form.
  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Be aware of highly variable recent wind loading patterns.
  • Pay attention to cornices and give them a wide berth when traveling on or below ridges.

Problems

Wind Slabs

An icon showing Wind Slabs

Likelihood

Possible-Likely

Expected Size

1 - 2.5

5 to 15 cm of light density snow fell Sunday night into Monday accompanied by moderate southeast wind which has created a very soft and thin slab.  

This slab rests on top of stiff wind slabs up to 40 cm in depth that have been most problematic on southeast, south and southwest facing slopes. These were created by strong outflow winds and have been the key element in a string of recent serious avalanche accidents. They may remain sensitive to human triggering.

Give cornices a wide berth from above and below, recent faceting and loss of cohesion in cornices can make them brittle and prone to fail. 

Aspects:

All aspects.

Elevations:

All elevations.

Valid until: Feb 16th, 2021 4:00PM

Forecast Trend

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