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Avalanche Forecast

Archived

Feb 22nd, 2019–Feb 23rd, 2019

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

South Coast.

Human triggered avalanches are likely this weekend. Fresh snow and a buried weak layer have created dangerous avalanche conditions.

Confidence

Moderate - Forecast snowfall amounts are uncertain on Saturday

Weather Forecast

FRIDAY NIGHT: Cloudy with isolated flurries and possible accumulations of 2-5 cm, light to moderate southwest wind, alpine temperatures drop to -4 C.SATURDAY: Scattered flurries with 5-20 cm of snow, light southwest wind, alpine temperatures around -4 C.SUNDAY: Mix of sun and cloud, light southeast wind, alpine high temperatures around -1 C.MONDAY: Sunny, light northeast wind, alpine high temperatures around -1 C.

Avalanche Summary

On Monday, search and rescue responded to an avalanche in the Mount Seymour backcountry, which resulted in a fatality (see here for incident report). The avalanche occurred on a steep feature at treeline elevation. Avalanche professionals involved in the rescue indicated wide propagation consistent with a persistent slab problem. The crown depth was variable - 40 to 100 cm, indicating there was also wind loading in that area.Another persistent slab avalanche was reported on Wednesday (see MIN report here), and occurred naturally on a south-facing feature at treeline elevation.A widespread avalanche cycle occurred a week ago. These large avalanches (up to size 2.5) showed impressive propagation, with some being triggered remotely (from a distance) and in some occasions stepping down to the crust and releasing persistent slab avalanches.

Snowpack Summary

Roughly 20 cm of new snow sits above a mix of sun crusts and possibly some weak faceted snow and surface hoar. A widespread crust layer is buried 50-100 cm deep with weak snow above it that has produced large natural and human-triggered avalanche over the past week. These videos from Wednesday clearly demonstrate how easy it is to trigger this weak layer (here and here). The reactivity of this layer appears to be worse in the south of the region (i.e. the North Shore Mountains) since this part of the region has seen more snow and it has consolidated into more of a slab. Unusually for this region, we expect this layer to remain reactive for some time into the future. The lower snowpack is settled and strong.

Problems

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.