Avalanche Forecast

Issued: Jan 2nd, 2019 4:31PM

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

mbender, Avalanche Canada

Purcells.

There is uncertainty with precipitation amounts Wednesday night through Thursday. This forecast is based on the upper end of estimates.

Summary

Confidence

Moderate - Intensity of incoming weather systems is uncertain on Thursday

Weather Forecast

WEDNESDAY NIGHT: Snow, accumulation 10-15 cm / Moderate southwest wind / Alpine temperature -7 C THURSDAY: Snow, accumulation 10-20 cm / Moderate to strong southwest wind / Alpine temperature -3 C / Freezing level 1300 mFRIDAY: Flurries, accumulation 5-10 cm / Moderate to strong south wind / Alpine temperature -7 C SATURDAY: Isolated flurries / Light to moderate west wind / Alpine temperature -6

Avalanche Summary

Expect to see avalanche activity increase on Thursday as a storm passes through the region Wednesday night through Thursday.Reports from Sunday show a few very large (size 2.5 and size 3) deep persistent slabs running during the last storm. These occurred in alpine terrain and are suspected to have failed on deep weak layers formed early in the season. A convincing pattern of deep persistent slab activity has yet to emerge, but these events highlight lingering hazards in thin or variable snowpack areas in the alpine.

Snowpack Summary

An additional 10-15 cm of new snow falling Wednesday night will add to last weekend's storm snow totals of 20-30 cm.A weak layer that formed during the dry spell in early December is now 80 to 110 cm deep. The layer is composed of facets (sugary snow), surface hoar (feathery crystals), and a sun crust (on south aspects). Another similar weak layer is buried 90 to 150 cm deep. Finally, the base of the snowpack has weak faceted layers at alpine and treeline elevations. These basal weak layers have been producing large avalanches as recently as December 31st. Human triggering any of these layers is most likely on slopes that didn't previously avalanche and on slopes that have variable snowpack depth (such as rocky alpine features).

Problems

Storm Slabs

New snow combined with a rising freezing level and moderate to strong southwest wind will promote storm slab development. Deepest deposits will be found in alpine and treeline lee terrain.

Minimize overhead exposure during periods of heavy loading from new snow, wind.If triggered the storm slabs may step down to deeper layers resulting in large avalanches.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Very Likely

Expected Size

1 - 2

Deep Persistent Slabs

Deep persistent slabs have a low likelihood of triggering, yet would create a high consequence avalanche.

Be wary of large interconnected slopes that don't show signs of a recent avalanche.Avoid convexities or areas with a thin or variable snowpack. Especially around steep alpine slopes.

Aspects: All aspects.

Elevations: Alpine.

Likelihood

Unlikely - Possible

Expected Size

2 - 3.5

Valid until: Jan 3rd, 2019 2:00PM