Sea to Sky Canadian Avalanche Advisory - Archived, Expired

Date Issued: Saturday, February 13, 2021, 04:00
Valid Until: Sunday, February 14, 2021, 04:00
Prepared by: cgarritty

This is an archived avalanche bulletin.

Dangerous avalanche conditions have resulted from a recent wind event that redistributed loose snow into reactive wind slabs at higher elevations. Increase caution as you gain elevation and watch for slabs in unusual locations as a result of reverse loading from northeast wind.

Danger ratings

Saturday
Alpine: 3 - Considerable
Treeline: 2 - Moderate
Below Treeline: 1 - Low

Travel and Terrain Advice

    Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
    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.
	
Avalanche Problem 1: Wind Slab

What Elevation? Alpine, Treeline
Which Aspects? All
Chances of Avalanches? Likely
Expected Size? Small to Large

An uptick in northeast winds on Thursday formed new wind slabs, especially across south to southwest aspects in the alpine. These more recent slabs may remain sensitive to human triggers on Saturday. Older, more stubborn wind slabs may be found on a wide range of aspects.

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

Avalanche Summary

We have preliminary information about a large (size 3) avalanche occurring in the vicinity of Blackcomb Mountain on Friday. We will update this summary once we have more details.

A bout of strong northerly winds on Thursday caused conditions in the region to change rapidly, with new and touchy wind slabs forming in many leeward terrain features. Numerous natural releases from size 1 to size 2 were observed in the Whistler area above about 1900 metres. We are awaiting further detail about a skier-triggered avalanche that occurred in the Mamquam Glacier area on Thursday.

Notably, a size 3 (very large) persistent slab was remotely triggered (from a distance) by a group of skiers in the McGillivray Pass area (northern South Coast Inland region) on Monday. This occurred on a southwest aspect at 2400 metres. It was described as a hard wind slab formed over the facet layer from late January detailed in our snowpack discussion.

Snowpack Summary

Ongoing elevated winds from the east and northeast have been redistributing loose snow into reactive new wind slabs in the lee of exposed terrain features at higher elevations.

The snow surface otherwise is composed of up to 15 cm of low density snow in shaded, sheltered areas (now likely a bit more scarce), along with more widespread wind-affected surfaces, including older wind slabs that have been gradually losing cohesion and reactivity under prolonged cold temperatures. On solar aspects, a thin recent sun crust may be found on or right near the surface or beneath recently wind transported snow, potentially contributing to the reactivity of new slabs on sun-exposed aspects.

Below these variable surface conditions, 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 a suspect, we have no recent reports of avalanches failing at this interface within the region. Details of the failure planes involved in two recent avalanches are still pending.

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

Weather Summary

Friday night: Becoming cloudy. Moderate east winds.
Saturday: Cloudy with isolated flurries and a trace of new snow, continuing overnight. Light to moderate east or northeast winds. Alpine high temperatures around -12.
Sunday: Cloudy with easing isolated flurries leaving us with about 5 total cm of new snow, beginning again overnight. Light south or southwest winds. Alpine high temperatures around -10.
Monday: Cloudy with easing flurries from the overnight period leaving us with up to another 5 cm of new snow. Light variable winds. Alpine high temperatures around -7.

Confidence: Moderate