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Avalanche Forecast

Archived

Feb 21st, 2019–Feb 22nd, 2019

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.
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 unlikely, human triggered possible.

Regions

South Coast.

Avoid avalanche terrain on Friday. Large storm slab and persistent slab avalanches are very likely.

Confidence

Moderate - Forecast snowfall amounts are uncertain on Friday

Weather Forecast

THURSDAY NIGHT: Flurries start overnight with 5 cm accumulations by the morning, moderate west wind, alpine temperatures drop to -5 C.FRIDAY: Short period of intense snowfall with accumulations of 20-30 cm, strong southwest wind, alpine high temperatures around -2 C.SATURDAY: Cloudy with sunny breaks and isolated flurries with 2-5 cm new snow, light southwest wind, alpine high temperatures around -4 C.SUNDAY: Sunny with cloudy periods, light southeast wind, alpine high temperatures around -2 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.Last Friday and over the weekend, there was evidence of a widespread avalanche cycle up to size 2.5. These avalanches showed impressive propagation, with some being triggered remotely (from a distance) and some occasions stepping down to a 50 cm deep crust layer.

Snowpack Summary

On Friday, 20-30 cm of new snow will form reactive storm slabs above new sun crusts and weak faceted snow. 40-80 cm of recent storm snow lies above a crust. The snow has bonded poorly to this crust and natural and human-triggered avalanche have been noted on this interface. These videos 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.