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

Lizard-Flathead.

With a buried weak layer in the snowpack, 30 cm of 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 15-25 cm. Ridge wind light east, possibly strong at times. Temperature -22. Freezing level surface.SATURDAY: Mostly cloudy, flurries. Accumulation up to 5 cm. Ridge wind light west, possibly strong at times. Temperature -17. Freezing level surface.SUNDAY: Mostly sunny. Ridge wind light southwest. Temperature -14. Freezing level surface.MONDAY: Mix of sun and cloud. Ridge wind light southwest. Temperature -15. Freezing level surface.

Avalanche Summary

A natural avalanche cycle occurred overnight Thursday producing storm slab avalanches to size 2 on north to northeasterly, wind loaded, upper elevation start zones.Â Avalanche control work on Friday produced explosive-triggered, storm slab avalanches from size 1.5-2. On Thursday natural and skier triggered storm slabs releases up to size 1.5 were reported including a remotely triggering a size 1.5 slab release on an open, rock slope on a north west aspect below treeline. Whumpfing and cracking in rocky areas was also reported on Thursday. See the report on the Mountain Information Network (MIN) here.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 and small avalanches failing on the mid-December surface hoar layer, which remains a concern with the on-going storm loading.

Snowpack Summary

Thursday night 10-15 cm of storm snow fell on top of 25 cm of very low density storm snow and has been modified by winds creating an "upside-down" storm slab layer with stiffer snow on top of softer snow.This upside-down storm snow layer overlies roughly 50 cm of old storm snow from last week that formed into stiff wind slabs in exposed terrain near ridge top, but remains low density in sheltered areas. The mid-December layer is 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 touchy storm slabs especially near ridge lines and in open areas where wind loading has occurred. These storm slabs are sensitive to both natural and human triggers.

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

Aspects: All aspects.

Elevations: All elevations.

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.

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

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Unlikely - Possible

Expected Size

2 - 3

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