Avalanche Forecast

Issued: Mar 9th, 2017 4:28PM

The alpine rating is high, the treeline rating is considerable, and the below treeline rating is moderate. Known problems include Storm Slabs and Persistent Slabs.

cam_c, Avalanche Canada

Purcells.

Snow and wind will drive the danger up for Friday, then warming and sun-exposure, coupled with the potential for cornice triggers, will increase the likelihood of massive deep persistent slabs for the weekend.

Summary

Confidence

Moderate - Freezing levels are uncertain on Saturday

Weather Forecast

FRIDAY: Cloudy with 10-15cm of fresh snow accompanied by moderate SW winds. Freezing levels as high as 1500m for southern areas, but remaining near valley bottoms in the north.SATURDAY: A mix of sun and cloud with localized periods of intense sun-exposure and isolated light flurries. Light to moderate SW winds. Freezing levels again rising to 1500m for southern areas, but remaining near valley bottoms in the north.SUNDAY: Mainly cloudy with 3-5cm of fresh snow, light to moderate westerly winds and freezing levels again rising to 1500m for southern areas, but remaining near valley bottoms in the north.

Avalanche Summary

Reports from Wednesday include a 100 cm deep natural Size 3.5 slab avalanche on a SW aspect and a large cornice failure that entrained quite a bit of mass. Explosives control produce wind slab avalanches up to Size 2.5 on north through east aspects. Reports from Tuesday include one natural Size 2.5 on a southeast facing alpine slope in response to intense cross-loading as well as numerous other natural dry loose and soft slab avalanches up to Size 1 on lee aspects. Explosives control produced wind slab and persistent slab avalanches up to Size 3. One of which was 100-300cm deep, failing on the deep persistent facet/crust weakness buried in November.

Snowpack Summary

Around 50-100cm of settled storm snow is bonding poorly to weak faceted snow and small surface hoar on sheltered shady slopes, and/or a thin crust on southerly aspects. Southerly winds have formed touchy slabs at all elevations with multiple weaknesses within and under this recent storm snow. The persistent weakness buried mid-February is now down 60-120 cm and composed of a thick rain crust as high as about 2000 m, sun crusts on steep southerly aspects, and spotty surface hoar on shaded aspects. This layer has produced easy results in recent snowpack tests and has proven especially reactive on steep southerly aspects. Several deeper persistent weaknesses also remain a concern, including surface hoar buried early-February (80-140 cm deep), and mid-January (over a metre deep primarily in the northern Purcells ). Basal facets may still be reactive in shallow, rocky start zones.

Problems

Storm Slabs

Weaknesses within and under the 40-100 cm of recent storm snow are susceptible to human triggering. These storm slabs are particularly deep and touchy on slopes loaded by southerly winds.

If triggered, storm slabs may step down to deeper layers resulting in large avalanches.Be alert to conditions that change with elevation.Be increasingly cautious as you transition into wind affected terrain.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

2 - 3

Persistent Slabs

Various persistent weaknesses strewn throughout the snowpack create the potential for large step-down avalanches. Warming and sun-exposure is expected increase the likelihood of these massive avalanches, especially with cornice-fall triggers.

Be aware of overhead hazards and avoid lingering in runout zones.Avoid steep convexities or areas with a thin or variable snowpack.Be aware of the potential for large avalanches due to the presence of deeply buried weak layers.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

2 - 4

Valid until: Mar 10th, 2017 2:00PM