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Avalanche Forecast

Archived

Mar 9th, 2017–Mar 10th, 2017

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

Regions

Northwest Coastal.

It's time to dial back terrain choices at all elevation bands. A buried persistent weak layer has created the potential for surprisingly large avalanches.

Confidence

High -

Weather Forecast

Friday: Mainly clear skies / Moderate easterly winds / Alpine temperature of -15Saturday: Overcast skies with light flurries / Moderate easterly winds / Alpine temperature of -12Sunday: 15-30cm of new snow / Moderate southerly winds / Alpine temperature of -5

Avalanche Summary

On Monday two avalanches were simultaneously remotey-triggered below treeline west of Terrace. See here for the excellent details in the Facebook post. We had reports of two rider-triggered wind slab avalanches (Size 1.5 and 2.0) southwest of Terrace on Sunday. In both cases the crown height was 50cm and shooting cracks were also reported in mellow terrain. There was a remote-triggered Size 1 in the Shames backcountry, also with a 50cm crown, running on a weak facet layer.  These avalanches speak to the touchy persistent avalanche problem in the region.Strong winds on Thursday should promote a new round of wind slab activity. The persistent slab problem is expected to remain touchy as the recent storm snow settles into a cohesive slab over top of the late February persistent weak layer.

Snowpack Summary

On Tuesday night the region received between 5-40cm of new snow with the greatest accumulations occurring in the south of the region. Southwest winds and more recent easterly winds are expected to have redistributed this new snow forming fresh wind slabs on a variety of aspects. 70-150 cm below the surface you'll find a variety of old surfaces which were buried in late February. These weak surfaces, which remain the primary concern in the region, include surface hoar, facets, stiff wind slabs and a melt-freeze crust below 1600 m. The overlying slab remains reactive on this interface, with ongoing reports of whumpfing in flat terrain. Moreover, we've had reports of sudden, propagation-likely test results on this layer. Below this interface the snowpack is generally settled and strong, with the exception being shallow snowpack areas around Bear Pass and Ningunsaw where basal facets remain an ongoing concern.

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.

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.