Avalanche Forecast

Issued: Dec 29th, 2017 4:37PM

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

jmcbride, Avalanche Canada

South Rockies.

With a buried weak layer in the snowpack, recent storm snow and more snow in the forecast, conservative terrain choices are the key for safe skiing and riding this weekend.

Summary

Confidence

Moderate - Due to the number of field observations

Weather Forecast

FRIDAY NIGHT: Snow. Accumulation 10-25 cm. Ridge wind light east, possibly strong at times. Temperature -25. Freezing level surface.SATURDAY: Mostly cloudy, flurries. Accumulation up to 5 cm. Ridge wind light northeast, possibly strong at times. Temperature -17. Freezing level surface.SUNDAY: Mostly sunny. Ridge wind light southwest. Temperature -15. Freezing level surface.MONDAY: Mix of sun and cloud. Ridge wind light southwest. Temperature -12. Freezing level surface.

Avalanche Summary

On Friday several natural storm slab avalanches to size 1 were reported as well as skier and explosive triggered releases to size 1. One ski cut result also produced an isolated size 2 loose, dry avalanche which gathered a large volume of new storm snow.Ski cutting on Thursday produced sluffing and small storm slabs up to size 1 in steep terrain.Limited reports suggest triggering slab avalanches remains possible. Two skier triggered avalanches have been reported east of the divide the past few days. One was a small wind slab triggered in a steep gully and the other was a larger persistent slab (size 2) triggered low on a slope that propagated up and across the slope on a 50 cm deep crust. The later highlights the potential for persistent slab avalanches to have wide propagations.

Snowpack Summary

Up to 20 cm of snow fell overnight with generally light winds however but some areas of the region (Crowsnest Pass) saw strong winds while others (Elk valley) saw lower snowfall amounts.In wind exposed areas this new snow overlies variable wind slabs up to 80 cm thick in some areas. In sheltered areas, 30-60 cm of unconsolidated old storm snow has been covered.The main question in the snowpack surrounds the mid-December layer sitting beneath the recent snow. This layer includes crusts on solar aspects and feathery surface hoar in sheltered terrain at and below treeline, and has produced whumpfing as well as moderate, sudden results in recent snowpack tests. The lower snowpack is composed of mostly soft sugary snow and a few early season crusts that have not produced significant test results recently.For an update on recent conditions check out the Mountain Information Network (MIN) report here. And please post your own reports!

Problems

Storm Slabs

Expect sluffing in steep terrain and touchy storm slabs especially in areas that have been loaded by wind. These slabs may be easily triggered by both human and natural triggers.

If triggered, storm slabs may step down to deeper layers resulting in large avalanches.Be careful with wind loaded pockets, especially near ridge crests and roll-overs.Watch for whumpfing, hollow sounds, shooting cracks or recent avalanches.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 2

Persistent Slabs

A weak layer composed of surface hoar and/or crusts is buried 40-80 cm below the surface.

Watch for signs of instability such as whumpfing, or cracking. Use caution on open slopes and convex rolls at treeline where buried surface hoar may be preserved.Choose well supported terrain without convexities.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

1 - 3

Valid until: Dec 30th, 2017 2:00PM