Avalanche Forecast

Archived

Mar 8th, 2017–Mar 9th, 2017

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.

Alpine

Natural avalanches possible, human triggered probable.

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

Moderate -

Weather Forecast

Expect a mix of sun and cloud for the forecast period. Ridgetop winds should remain generally light to moderate from the east with alpine temperatures sitting at about -15.

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. These avalanches speak to the touchy persistent avalanche problem in the region.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.Conditions will 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 are expected to have redistributed this new snow forming fresh wind slabs in exposed lee terrain. 70-120 cm below the surface you'll find a variety of old surfaces which were buried in late February. These surfaces include surface hoar, facets, crusts, stiff wind slabs and a melt-freeze crust below 1600 m. The overlying slab remains very reactive on this interface which was buried in late February, resulting in widespread whumpfing and cracking in flat terrain. Moreover, we've had reports of sudden, propagation-likely test results on weak facets or the late-February crust / surface hoar.Below this interface the snowpack is generally settled and strong with the exception in 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.