Avalanche Forecast

Issued: Mar 11th, 2017 4:00PM

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

stephen holeczi, Parks Canada

Banff Yoho Kootenay.

Many cornice failures Saturday. Recent avalanches and the uncertainty around the deep persistent weak layers require a conservative approach to terrain features.

Summary

Weather Forecast

Valley bottom temperatures will be around 0C Sunday, with alpine temperatures staying in -8 to -10C range with moderate W alpine winds and only trace amounts of new snow.Â There will be an increase in solar input and warming temperatures this week (12C by Wed. in Banff) which may again increase the avalanche danger, especially in the front ranges.

Snowpack Summary

50-60cm of storm snow settling with warmer temps has been reactive this past week. Moist snow on solar aspects was observed Saturday below treeline due to warmer temps and a bit of sun. Wind slabs have formed in alpine lee areas. Below this storm snow a more settled slab sits over the weaker facets and depth hoar near the base which is widespread.

Avalanche Summary

Large storm/wind slabs have been reactive this last week up to size 3, triggered by explosives or running naturally. Some avalanches have stepped to the basal layers, especially East of the divide. Less natural activity noted on Saturday of these slabs. The big story are widespread cornice failures up to 300m wide reported in many areas.

Confidence

Freezing levels are uncertain

Problems

Wind Slabs

50-60cm of dry snow from the past week is being blown into cohesive wind slabs at higher elevations. Warmer temperatures will help settle these out and become less reactive, but it may take a few days for the snowpack to fully adjust to the new load.

Be aware of the potential for wide propagations.

Aspects: North, North East, East, South East, South, South West.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 3

Deep Persistent Slabs

The entire snowpack is built upon a structurally weak base, which produces very large avalanches when triggered. There is tremendous uncertainty with this sort of condition, and the only way to hedge against it is to avoid the big avalanche terrain.

Use conservative route selection, choose moderate angled and supported terrain with low consequence.Be aware of thin areas that may propogate to deeper instabilites.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

2 - 4

Cornices

Many observations of new cornices formed this week failing as temperatures rise with wide propagations of up to 300m. In some cases this has triggered slabs below and caused larger avalanches. Expect the same for Sunday.

Extra caution needed around cornices with current conditions.

Aspects: North, North East, East, South East, South.

Elevations: Alpine.

Likelihood

Likely

Expected Size

1 - 3

Valid until: Mar 12th, 2017 5:00PM