Avalanche Forecast

Issued: Dec 28th, 2017 4:49PM

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

Lizard-Flathead.

As much as 60 cm of low density storm snow could arrive by Friday morning. Expect both natural and human triggered avalanches in steep terrain and where wind loading has occurred.

Summary

Confidence

Moderate - Intensity of incoming weather systems is uncertain

Weather Forecast

THURSDAY NIGHT: Snow. Accumulation 15-25 cm. Ridge wind light northeast, possibly strong at times. Temperature -18. Freezing level surface.FRIDAY: Mostly cloudy, snow developing and overnight. Accumulation 5-25 cm. Ridge wind light northeast possibly strong at times. Temperature -17. Freezing level surface.SATURDAY: Mostly cloudy, flurries. Accumulation 5-10 cm. Ridge wind light west. Temperature -16. Freezing level surface.SUNDAY: Mostly sunny. Ridge wind light southwest. Temperature -13. Freezing level surface.

Avalanche Summary

Thursday's storm snow has produced numerous natural, storm slab and loose dry avalanches from Size 1-1.5 in steep terrain on all aspects. Avalanche control work produced similar results with loose, dry and storm slab avalanches up to Size 1.Earlier in the week on Monday, one large naturally-triggered avalanche (size 2) was reported on a north-facing alpine feature that likely failed on the mid-December surface hoar layer. There have been some isolated reports of whumpfs at small avalanches failing on the mid-December surface hoar layer, but in most areas the snow above this layer has not settled into a slab yet, this will change if winds are stronger than forecasted.

Snowpack Summary

Wednesday night and through the day on Thursday approximately 30 cm of low density snow fell with generally light winds.This overlies roughly 50 cm of old storm snow from last week that formed harder wind slabs in exposed terrain near ridge top, but remains low density in sheltered areas. The main question in the snowpack surrounds the mid-December layer now buried 80 cm below the surface. The layer includes crusts on south and west facing slopes and feathery surface hoar in sheltered areas at and below treeline, and has been producing moderate, sudden results in recent snowpack tests.The lower snowpack is generally well settled with a thick prominent crust that was buried near the end of November.

Problems

Storm Slabs

Expect sluffing in steep terrain and soft storm slabs in open areas near ridge lines where additional loading from wind has occurred.

Minimize exposure during periods of heavy loading from new snow and wind.If triggered the storm/wind slabs may step down to deeper layers resulting in large avalanches.Be cautious of sluffing in steep terrain.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Very Likely

Expected Size

1 - 2

Persistent Slabs

A weak layer composed of surface hoar and/or crusts is buried 60-80 cm below the surface. Triggering an avalanche on this layer is possible in areas where the snow has settled into a cohesive slab.

Watch for whumpfing, hollow sounds, and shooting cracks.Choose well supported terrain without convexities.Use caution on open slopes and convex rolls at treeline where buried surface hoar may be preserved.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

1 - 2

Valid until: Dec 29th, 2017 2:00PM