Avalanche Forecast

Issued: Apr 2nd, 2018 5:24PM

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

cgarritty, Avalanche Canada

Purcells.

Recent snow and lingering problems have maintained heightened avalanche conditions. Our consistently active storm slabs have graduated to 'persistent slab' status and still need to be managed with conservative terrain selection.

Summary

Confidence

Moderate -

Weather Forecast

Tuesday: Mainly cloudy with isolated flurries and a trace of new snow. Light to moderate southwest winds. Freezing level to 1200 metres with alpine high temperatures around -8.Wednesday: A mix of sun and cloud. Light southwest winds. Freezing level to 1100 metres with alpine high temperatures around -9.Thursday: Cloudy with flurries bringing about 5 cm of new snow, continuing overnight. Light southeast winds. Freezing level to 900 metres with alpine high temperatures around -9.

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

Variable snowfall over Sunday night brought a trace to 25 cm to the region, burying previously wind-affected surfaces as well as a new sun crust on sun-exposed aspects. Storm snow totals from the past week are now a highly variable 20-80 cm. This storm snow sits on an 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 40-80 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

Wind Slabs

Light new snow has been redistributed by variable winds into thin new 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.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 2

Persistent Slabs

Weak layers of crust and surface hoar have been buried by a variable 20-70 cm of storm snow and have shown continued reactivity since the last storm. Manage the uncertainty around triggering these layers by sticking to lower angle, supported slopes.

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

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 2.5

Valid until: Apr 3rd, 2018 2:00PM