Avalanche Forecast

Issued: Apr 3rd, 2018 4:53PM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is low. Known problems include Persistent Slabs and Wind Slabs.

cgarritty, Avalanche Canada

Purcells.

Weak layers have been preserved within and below our recent storm snow. The depth of these layers varies throughout the region. We're still managing a winter snowpack that requires thoughtful terrain selection.

Summary

Confidence

Moderate -

Weather Forecast

Wednesday: A mix of sun and cloud. Light west winds. Freezing level to 1500 metres with alpine high temperatures around -5.Thursday: Mainly cloudy with light flurries beginning in the afternoon and continuing overnight. Light southeast winds. Freezing level to around 1500 metres with alpine high temperatures around -5.Friday: Mainly cloudy. Light south winds. Freezing level to 1700 metres with alpine high temperatures around -3.

Avalanche Summary

Another recent storm slab release was reported from the north of the region on Sunday, with a depth of 50 cm and the late March layer from our snowpack discussion identified as its failure plane. Avalanches that release over this layer are now being labeled 'persistent slabs'.On Saturday a skier triggered a large slab avalanche (size 2.5) on a south aspect at 2600 m. The avalanche failed on a buried sun crust 150-200 cm below the surface. Another large (size 2.5) wind slab released naturally from a steep southeast-facing alpine slope in the St. Mary's area. Explosives control in the northern half of the region produced numerous storm slabs ranging from size 1.5-2.5 at treeline and alpine elevations. Crown fractured ranged from 30-100 cm. All aspects were represented in results, with the majority of activity focused on northeast to northwest aspects in the alpine.Reports from last week showed steady activity with several size 1-2.5 natural storm slabs and wind slabs being reported on a range of aspects each day. Storm slabs predominantly failed on the late-March interface, with crown depths from 20-50 cm deep and up to 100 cm in some areas.

Snowpack Summary

Storms over the past week brought variable snowfall totals of 20-80 cm to the region, with areas to the south of the region seeing the deepest accumulations. This storm snow sits on a persistent weak layer buried in late March that consist of crusts below 1900 m and on south aspects, and surface hoar on shaded aspects at higher elevations. Other slightly deeper surface hoar/facet layers have been reported on shaded aspects at higher elevations and may be found approximately 40-90 cm below the surface.Deeper persistent weak layers from mid-late February as well as January and December layers, are still being monitored by professional observers, but are generally considered dormant. Facets also linger at the base of the snowpack.

Problems

Persistent Slabs

A weak layer of crust and surface hoar has been buried by a variable 20-80 cm of storm snow and has shown continued reactivity since the last storm. Manage the uncertainty around triggering this layer by sticking to lower angle, supported slopes.

Use caution around sheltered steep or convex slopes where buried surface hoar may be preserved.Minimize exposure to steep south-facing alpine slopes, where buried crust may be preserved.Use conservative route selection, choose moderate angled and supported terrain with low consequence.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 2.5

Wind Slabs

Light new snow has been redistributed by variable winds into thin wind slabs on a variety of aspects. Slabs have also been noted forming above weak layers like crust and surface hoar. Deeper slabs are likely to be found in the south of the region.

Be especially careful with wind loaded pockets near ridge crests and roll-overs.Wind slabs bay be touchier in areas where they overlie crust or surface hoar.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 2

Valid until: Apr 4th, 2018 2:00PM