Avalanche Forecast

Issued: Mar 18th, 2017 4:14PM

The alpine rating is high, the treeline rating is high, and the below treeline rating is considerable. Known problems include Persistent Slabs, Wind Slabs and Loose Wet.

tim haggarty, Avalanche Canada

Little Yoho.

Avoid exposure to avalanche terrain: the new storm has resulted in another avalanche cycle today.

Explosive Avalanche Control is planned for Mt Bourgeau, Mt Stephen and Mt Dennis. Please no climbing or skiing in these areas Sunday.

Summary

Weather Forecast

A significant storm is underway Saturday with moderate to strong SW winds, moderate precip and freezing levels in excess of 2200m. The storm will taper off Sunday morning and freezing levels are expected to drop back to valley bottom as winds shift back to westerly. A cooling trend will continue with light precip until skies start to clear Monday.

Snowpack Summary

10 to 20cm of new snow, combined with 40 to 80cm from the last 14 days has created a significant load over a fundamentally weak snowpack. Rain to 2200m, warmer overall temperatures, and snow in the alpine combined with strong SW winds are building superficial wind and storm slabs and overloading the deep basal facet layers. Poor travel below 2000m.

Avalanche Summary

Control work yesterday on the 93 S produced surprisingly large results reaching historical runnouts to sz 3.5. Extensive Natural activity has been seen on flights throughout the forecasting region to size 4 with fracture lines longer than 1km in many locations. Reports today indicate new activity to the same scale as the latest storm hits.

Confidence

Timing, track, or intensity of incoming weather system is uncertain on Saturday

Problems

Persistent Slabs

Several buried weak layers in the middle of the snowpack have become reactive again this week and should remain that way through this weekend. Once triggered, these will be very large in magnitude.

Dig down to find and test weak layers before committing to a line.
Avoid exposure to overhead avalanche terrain, large avalanches may reach the end of run out zones.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely - Very Likely

Expected Size

2 - 4

Wind Slabs

Wind slabs over 1m deep exist at higher elevations, and in wind effected areas near treeline. This slab reacts easily to tests and continues to produce avalanches. Correspondingly, cornices have grown significantly and are fragile.

Use caution in lee areas. Recent wind loading have created wind slabs.
Pay attention to overhead hazards like cornices which could easily trigger persistent slabs.

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

Elevations: Alpine, Treeline.

Likelihood

Very Likely - Certain

Expected Size

2 - 3

Loose Wet

With freezing levels in excess of 2200m Saturday, the surface snow at lower elevations has been failing naturally. The entire snowpack has lost strength and will take some time to refreeze. Climbers: these events are very likely in gullies and cliffs

If triggered the loose wet sluffs may step down to deeper layers resulting in large avalanches.
Use extra caution on slopes if the snow is moist or wet.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 2

Valid until: Mar 19th, 2017 4:00PM